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Doctoral Dissertations 1896 - February 2014

1-1-2001

Ultrahydrophobic surfaces : effects of topography on wettability. Ultrahydrophobic surfaces : effects of topography on wettability.

Didem, Oner University of Massachusetts Amherst

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ULTRAHYDROPHOBIC SURFACES-EFFECTS OF TOPOGRAPHY ON WETTABILITY

A Dissertation Presented

by

DIDEM ONER

Submitted to the Graduate School of the

University of Massachusetts Amherst in partial fulfillment

of the requirements for the degree of

DOCTOR OF PHILOSOPHY

September 2001

Polymer Science and Engineering

© Copyright by Didem Oner 2001

All Rights Reserved

ULTRAHYDROPHOBIC SURFACES-EFFECTS OF TOPOGRAPHY ON WETTABILITY

A Dissertation Presented

by

DIDEM ONER

Approved as to style and content by:

Thomas J. McCarthy, Chair

Mark T. Tuominen, Member

Thomas J. McCarthy, Department Head

Polymer Science and Engineering

To Nesli§ah, Liitfi and Kerem Oner, and Sinan Deliormanli with all my heart

ACKNOWLEDGMENTS

I would like to acknowledge my advisor, Dr. Tom McCarthy, for all his

enthusiasm, advice and understanding during the last 4 years while we worked on this

exciting Ph.D. thesis. Thank you Tom, for making this so special. Also, I want to thank

Dr. Tuominen and Dr. Strey for their time and input.

It would not have been possible if the McCarthy group were not there.

Especially, I want to thank: Jack for helping me out with many things in the lab; Chris for

his help, input and his friendship; Jeff for our interesting conversations (from nutrition to

ultrahydrophobicity); Xinqiao (a great office mate); Margarita for the wonderful

memories we shared in such a short period of time; Misha for his sincere help; Chuntao;

Terry for our discussions in teaching me how to become an 'engineer'; Kevin; Ebru; like

and Wei (was always so supportive). Special thanks to the PS&E faculty and staff for all

the things they have done- from teaching in class to the kind support they provided to

make me feel like home. I sincerely thank Ho-Cheol, Robin, Firat, Elif, Sabri and Serkan

for making this journey so enjoyable. Also, I will never forget the support from the very

special people in my life, Mutlu, Kutay, Iskender and Emel Yilgor.

I want to thank my brother, Kerem, for our unforgettable memories and for his

patience during our 'Amherst years' together. Also, I am always grateful to Sinan's

family for their sincere support and love.

And to Sinan Deliormanli, my other half. It was him who always believed in me,

filled me with his endless love and care. Thank you, hayatku§um.

Most of all, I feel so blessed to have the everlasting love of the most wonderful

parents in the universe, Nesli§ah and Liitfi Oner. Thanking them will never be enough.

V

ABSTRACT

ULTRAHYDROPHOBIC SURFACES-EFFECTS OF TOPOGRAPHY ON WETTABILITY

SEPTEMBER 2001

DIDEM ONER, B.S., KOC UNIVERSITY

M.S., UNIVERSITY OF MASSACHUSETTS AMHERST

Ph.D., UNIVERSITY OF MASSACHUSETTS AMHERST

Directed by: Professor Thomas J. McCarthy

The overall objective of this Ph.D. thesis is to control the wetting behavior of

surfaces by exploring the effects of topography on wettability, and ultimately make

ultrahydrophobic surfaces. Tliree different approaches were taken in preparing rough

surfaces with controlled wettability. The first approach involved the use of

photolithography that resulted in a series of silicon surfaces with different post size,

shape and separation (Chapter 2). The second approach was the surface modification of

low density polyethylene (Chapter 3). The last one was to adsorb polystyrene colloids

with different diameters onto polyelectrolyte multilayers (Chapter 4).

The wettability of the patterned silicon surfaces prepared by photohthography and

hydrophobized using reactive silane chemistry was explored. Surfaces containing square

posts with X-Y dimensions of 2 )iim-32 )im exhibited ultrahydrophobic behavior with

high advancing and receding contact angles. The contact angles were independent of the

post height and surface chemistry. Surfaces with larger posts were not ultrahydrophobic-

vi

water droplets pinned on these surfaces, hicreasmg the separation between the posts

caused increases in receding contact angles up to the point that water intruded between

the posts. Changing the shape of the posts also increased the receding contact angles due

to the more contorted contact lines.

The oxidative etching of low density polyethylene films followed by uniaxial or

biaxial tension resulted in the formation of micron size fragments. 5 minutes oxidized

films had smaller islands than the 1 5 minutes oxidized ones. The fragments became

smaller and more distant from each other with increase in strain that affected the

wettability of the surfaces. At 400%, the films exhibited ultrahydrophobic behavior. At

a higher strain, the islands were very small and apart from each other, the receding

contact angle dropped significantly.

Submicron and micron scale rough surfaces were prepared by adsorbing

polystyrene colloids onto polyelectrolyte muUilayers. The negatively charged colloids

were efficiently adsorbed onto the outermost cationic polyelectrolyte surface, showing no

aggregation. The advancing water contact angle increased and the receding contact angle

decreased as the surface coverage increased, resulting in a remarkable hysteresis of

-122°. Thus, hydrophobic surfaces could not be achieved by making rough surfaces by

colloidal adsorption.

vii

TABLE OF CONTENTS

Page

ACKNOWLEDGMENTSV

ABSTRACTVI

LIST OF TABLESXI

LIST OF FIGURES •••

Xlll

CHAPTER

1 . INTRODUCTION1

OverviewI

Wetting Behavior of Surfaces 3

Contact Angle Hysteresis 8

Topography of Roughness 10

Three-Phase Contact Line...: 12

Surface Characterization Techniques 14

Dynamic Contact Angle 14

Atomic Force Microscopy 15

X-Ray Photoelectron Spectroscopy 17

Notes and References 20

2. MODEL LITHOGRAPHIC SURFACES TO EXPLOREULTRAHYDROPHOBICITY 22

Introduction 22

Photolithography 23

Dry Etching 25

Reactive Silane Chemistry 29

Experimental 31

Materials 31

Preparation of Silicon Substrates 32

Reaction of Silicon Substrates with Organosilanes 33

Results and Discussions 34

viii

Reaction of Silicon Substrates with Organosilanes 34Critical Size Scale of Roughness for Hydrophobicity 35Structure of 3-Phase Contact Line

'""^

4jLaplace Pressure

Critical Tilt Angle

Conclusions

Notes and References

SURFACE MODIFICATION OF LOW DENSITY POLYETHYLENE 55

Motivation

Surface Treatments ofLow Density Polyethylene 56Deformation of Thin Film Coatings Upon Uniaxial and Biaxial Tension 59Experimental

Materials 64General Methods 64Preparation of Polyethylene Substrates 65Oxidation of Polyethylene Substrates 65Uniaxial Streching of LDPE 65Biaxial Streching of LDPE 66Self-Assembled Monolayers of 1-dodecanethiol 66

Results and Discussions 67

Oxidation of Polyethylene Substrates (o-LDPE) 67

Uniaxial Deformation 69

Change in Surface Roughness of o-LDPE with Strain 72

Biaxial Deformation of o-LDPE 76

Deformation of Gold Coating Bonded to LDPE 78

Conclusions 80

Notes and References 82

4. ADSORPTION OF POLYSTYRENE COLLOIDS ONTOPOLYELECTROLYTE MULTILAYER ASSEMBLIES AND WETTINGBEHAVIOR OF THESE ROUGH SURFACES 86

Introduction 86

Polyelectrolyte Multilayer Assemblies 87

Adsorption of Colloids onto Polyelectrolytes 89

Self-Assembled Monolayers of Alkanethiols on Gold 91

Experimental ^5

ix

Preparation of Silicon Substrates c)y

Pretreatment of Silicon Substrates 9-7

Reaction of Silicon Substrates with

(4-aminobutyl)dimethylmethoxysilane 97Layer-by-Layer Deposition of Polyelectrolytes '95

Adsorption of Polystyrene Colloids onto Polyelectrolyte Multilayers 98

Kinetic Study of the Adsorption Behavior of Colloids 99

Solution Casting Polystyrene Colloids onto Polyelectrolyte

Multilayers 99Gold Coating and Self-Assembled Monolayers (SAMs) of1-dodecanethiol jqq

Results and Discussions1 qq

Pretreatment of Silicon Substrates1 00

Adsorption of Polystyrene Colloids onto Polyelectrolyte Multilayers.... 102

Kinetic Study of the Adsorption Behavior of Colloids 104

Summary of the Rough Surfaces Prepared 114

Solution Casting Polystyrene Colloids onto Polyelectrolyte

Multilayers 118

Conclusions 120

Notes and References 122

BIBLIOGRAPHY 126

LIST OF TABLES

TablePage

2.1

Atomic concentration (XPS) of silicon/silicon oxide surfaces reacted withDMDCS, ODMCS and FDDCS at 15° and 75° takeoff angles 35

2.2 Water contact angle data for silane-modified hexagonally arrayed 40 -

high square post surfaces described in figures 2.7 and 2.8 38

2.3 Water contact angle data for ODMCS-modified surfaces containing 1 6 x16 ^im, 32 x 32 |im, 64 |im x 64 |im, 128 x 128 ^im square posts ofdifferent height 4q

2.4 Water contact angle data for silane-modified surfaces containing different

shaped posts and 8 |im x 8 |nm square posts spaced at various distances(described in figures 2.10 and 2.12) 45

2.5 Laplace pressure calculated using equation 4.1 48

2.6 Tilt angle and hysteresis exhibited by the DMDCS-modified silicon surfaces 50

3.1 Tensile failure modes for thin brittle films^^ 60

3.2 XPS atomic concentration data (at 1 5° and 75° takeoff angles) for LDPE andoxidized LDPE (o-LDPE) with chromic acid for 1, 10, 30 and 50

minutes at 75 °C 68

3.3 Dynamic water contact angles for oxidized LDPE (o-LDPE) substrates that

were then gold-coated and hydrophobized by SAMs of 1-DCT(o-LDPE/Au/l-DCT) 69

4.1 XPS atomic concentration data for a clean silicon substrate, an ABDMS-modified silicon substrate and after building 10 polyelectrolyte layers

composed of PSS and PAH, PAH as the outermost layer 102

4.2 XPS atomic concentration data for an ABDMS-modified silicon substrate,

and after building 10 polyelectrolyte layers composed of PSS and PAH,

PAH as the outermost layer which were then hydrophobized using

self-assembled monolayers of 1-DCT 1 1

1

4.3 Dynamic contact angles for surfaces with 1 fxm colloids, before and after

hydrophobizing with self-assembled monolayers of 1-DCT, using water

and hexadecane (HD) as probe fluids, (time ^ 0 is for polyelectrolyte

multilayers with PAH as the outermost layer) 1 13

xi

4.4 Dynamic contact angles for surfaces with 0.35 |im, before and afterhydrophobizing with self-assembled monolayers of 1-DCT, using waterand hexadecane (HD) as probe fluids

'

j j 3

4.5 Dynamic contact angles for surfaces prepared by adsorbing polystyrenecolloids with the indicated diameters, before and after hydrophobizing withself-assembled monolayers of dodecanethiol (Au/l-DCT), using water andhexadecane (HD) as probe fluids, r is the roughness ratio calculated fromthe AFM height image with 40 ^im x 40 ^m scanned area 115

4.6 Dynamic contact angles for surfaces prepared by solution casting polystyrenecolloids and hydrophobizing with self-assembled monolayers of1 -dodecanethiol (Au/l-DCT), using water and hexadecane (HD) as probefluids, r is the roughness ratio calculated from the AFM height image with40 [im X 40 |um scanned area

1 1

9

xii

LIST OF FIGURES

FigurePage

1 .1 Schematic representation of Young's equation 4

1 .2 Cassic's analysis at high roughness ratios, showing that the water droplet is

mostly in contact with air rather than the solid surface'^' 5

1 .3 (a) Wenzel's, Cassie's, and Johnson and Dettre's analyses on contact angle for

different roughness ratios. The smooth surface had Bvoung = 120";

(b) Johnson and Dettre's analysis was done using sinusoidally roughsurfaces with different roughness ratios'^ 6

1 .4 Water droplets behave differently on tilted surfaces due to the contact anglehysteresis 9

1 .5 Possible three-phase contact line structures on a real surface (—) and an ideal

one(-) 12

1 .6 2-Dimensional (X-Y) representations of two surfaces with the same f| and f2

values, but very different contact line structures. The dark lines are meantto represent possible contact lines 13

1 .7 Schematic representation of dynamic contact angle measurement 15

1 .8 An illustration of an AFM 1

7

1 .9 Illustration of the photoelectric effect"*^' 18

2. 1 Main lithographic steps in preparing the patterned silica substrates 25

2.2 Cross sections of photoresist etched by plasma or liquid etchants 26

2.3 The reactive ion etcher 28

2.4 The reaction of octyldimethylchlorosilane (and other monofunctional

organosi lanes) with silicon dioxide surfaces 30

2.5 The possible reactions of dimethylchlorosilane (and other di functional

organosi lanes) with silicon dioxide surfaces 31

2.6 XPS survey spectrum of silicon/silicon oxide surfaces reacted with FDDCS at

75° takeolT angle 34

xni

2.7 2-D,mensional (X-Y) representation of a series of silicon surtiices containinghcxagonally arrayed square posts, X 2, 8, 16, 32, 64 and 128 am Surfaceswith post heights (Z) of 20, 40, 60, 80, 100 and 140 jim were prepared 35

2.8 SEM images of the surfaces described in figure 2.7. The water contact anglesshown are for DMDCS-modificd surfaces

3(3

2.9 2-dimensional (X-Y) representations of surfaces containing different geometryposts

2. 10 SEM images of the surfaces described in figure 2.9. The water contact anglesshown are for DMDCS-modified surfaces 42

2.11 2-Dimensional (X-Y) representation of a series of silicon surfaces containinghexagonally arrayed square posts, X = 8 ^m and heights (Z) = 40 ^im. Postseparations were varied as 32 \im x 32 ^m, 32 |im x 80 |im,

80 fxm X 80 |im 43

2.12 SEM images of surfaces containing 8 )im x 8 |Lim square posts with different

spacings as described in figure 2.11. The water contact angles shown are

for DMDCS-modified surfaces 44

2.13 SEM images of surfaces containing (a) 32 ^m x 32 |im square posts; (b) 32 ).im

X 32 |im hollow squares. The water contact angles shown are for DMDCS-modified surfaces 46

2. 1 4 Snapshot of a water droplet on the substrate with star-shaped posts,

hydrophobized with DMDCS 47

2. 1 5 Optical micrographs of the Woods metal - silicon - air 3-phase contact line on

the (a) silicon substrate; surfaces named (b) 21 pmSP^" and (c) ,2 ^niSP^"

in Table 2.4. The contact line is clearly contorted by the square posts on the

surface 47

2.16 The apparatus used to measure the critical tilt angle at which the water droplet

starts to roll off the surface 49

2. 1 7 Critical tilt angle as a function of hysteresis exhibited by DMDCS-treated

silicon surfaces. As hysteresis decreased, the critical tilt angle decreased as

expressed in Furmidge's equation 50

3.1 Chromic acid oxidation of LDPE 57

3.2 SEM micrographs of (a) LDPE; and LDPE oxidized with chromic acid

solution for (b) 60 seconds; (c) 6 minutcs^"^ 58

xiv

3.3 Possible tensile failure mechanisms for thin coatings bonded to a substrate-la) through-thickness cracking; (b) through-thickness cracking followed bysubstrate failure; (c) through-thickness cracking followed by interfacialdelamination

60

3.4 Schematic representation of distribution of stress throughout the metal coatingbonded to a polymer support. Stress is maximum at the center of thecoating. This grows with stretching and ultimately there is a crack formingat the center of the coating^^

^

3.5 SEM micrograph of a deformed 3.8 nm platinum coating on PET support. Thestrain was not reported. The strain rate was 1 mm/min. The bar is 5 \im^^ ....62

3.6 Crack density and crack onset strain as a function of coating thickness." Thethicker the coating was, the lower the onset strain and smaller crack densityat saturation 53

3.7 SEM micrographs of LDPE substrates that were oxidized with chromic acid

for 5, 15, 30 and 50 minutes at 75 °C 68

3.8 SEM micrographs taken in the necking region of unoxidized LDPE substrates

that were drawn uniaxially at strains (e) of 260% (extension = 80 mm),314% (extension = 120 mm) and 570% (extension = 200 mm). The surface

microstructure did not change with strain 70

3.9 Strain as a function of extension for LDPE films that were oxidized for (a) 5

minutes and (b) 15 minutes. Strain was calculated by measuring the

distance between two marked points on the LDPE film before and after

necking.Taking the ratio of the difference between the final and the initial

distance, over the initial one gives the strain. Strain = s = (final distance -

initial distance)/initial distance 71

3.10 SEM micrographs ofLDPE substrates that were oxidized with chromic acid for

5 minutes and then drawn uniaxially at strains (s) of 140% (extension = 60

mm), 314%) (extension = 120 mm), 320% (extension = 130 mm), 400%(extension = 150 mm). The surface microstructure changed with strain in

the necking region of the polymer. The water contact angles shown were

after gold coating and reacfing with 1-DCT (Au/l-DCT) 73

3.11 SEM micrographs of LDPE substrates that were oxidized with chromic acid

for 15 minutes and then drawn uniaxially at strains (e) of 30% (extension = 30

mm), 1 80% (extension = 60 mm), 260% (extension = 80 mm), 250%

(extension = 120 mm), 400% (extension = 150 mm) and 570% (extension =

200 mm). The surface microstructure changed with strain in the necking

region of the polymer. The water contact angles shown were after gold

coating and reacting with 1-DCT (Au/l-DCT) 75

XV

3.12 SEM micrographs ofLDPE substrates that were oxidized with chromic acid for5 minutes and then biaxially streched. The surface microstructure changedwith strain. Part 1 had the lowest and Part 4 had the highest strain Thestrain increased in the order: Part 1 < Part 2 < Part 3 < Part 4 > Part 5 Thewater contact angles shown were after gold coating and reacting with 1-DCT(Au/l-DCT), except for Part 4 whose contact angles could not be measureddue to the wrinkled necked region yy

3.13 SEM micrographs of virgin LDPE substrates that were coated with gold andthen necked at strains (e) -140% (extension = 60 mm), 310% (extension =120 mm), 320% (extension = 130 mm), 400% (extension = 150 mm). Thedistance between the gold stripes increased with strain. The water contactangles shown were after gold coating and reacting with 1-DCT(Au/l-DCT)

79

4.1 Dependence of the surface coverage on adsorption time at a nanosphere

concentration of 3.8 x lO'" mL"' at 10 °C onto (PAH-PSS)3-PAH surfacethat was prepared in the presence of 2 M NaCl: (a) 780 nm; (b) 548 nm-(c) 84 nm''

\ 90

4.2 Time courses for polystyrene adsorption (diameter: 548 nm) at 10 "C onto(PAH-PSS)3 - PAH surfaces prepared in the presence of NaCl: (a) 2 M;(b) 0.2 M; (c) 0.02 M and (d) 0 M^'

4.3 Schematic illustration of SAMs of thiols on gold substrate 92

4.4 STM image of an SAM of dodecanethiol on Au (111). Intermolecular spacings

are 0.50 nm which is V3 times larger than the atomic spacing of gold atoms

(0.288 nm) in Au (1 1 1) planes^^ 93

4.5 Kinetics of adsorption of octadecanethiol (solid symbols) and decanethiol

(open symbols) from ethanol onto gold, (a) EUipsometric thickness, (b)

Advancing contact angles of water and hexadecane (HD)^^ 95

4.6 XPS survey of silicon substrate at 15*" takeoff angle (a) reacted with ABDMSfor 3 days in toluene; (b) with 10 polyelectrolyte layers, PAH as the

outermost layer 101

4.7 AFM section analysis was performed using AFM height images to calculate

the roughness ratios of the substrates. The software provided both the

surface distance (peak to valley distance) and the horizontal distance. The

roughness ratio was calculated using the following formula: r = (surface

distance)^ / (horizontal distance)^ 1 03

XVI

4.8 The change in roughness ratio, calculated from the AFM height images withadsorption time of polystyrene colloids (1 ^im) onto the polyelectrolytemultilayers

4.9 AFM height (left) and phase (right) images of the surfaces obtained when thetime of adsorption of polystyrene colloids (1 ^m diameter) ontopolyelectrolyte multilayers was for (a) 10, (b) 30, (c) 60 minutes; (d) 2,(e) 4, (0 6, (g) 8, (h) 10 and (i) 24 hours, (continued next page).'. .'

105

4. 1 0 The change in roughness ratio, calculated from AFM height images, withadsorption time of polystyrene colloids (0.35 ^m) onto the polyelectrolytemultilayers

jQg

4. 11 AFM height (left) and phase (right) images of the surfaces obtained when thetime of adsorption of polystyrene colloids (0.35 \im diameter) ontopolyelectrolyte multilayers was for (a) 1/2, (b) 2, (c) 4, (d) 6 and (e)

1 0 hours, (continued next page)1 09

4. 1 2 The advancing () and the receding () water contact angle with time ofadsorption of (a) 1 |im, (b) 0.35 |im colloids after the substrates werehydrophobized with self-assembled monolayers of 1-DCT 112

4. 1 3 AFM height (left) and phase (right) images of the surfaces obtained using

(a) 0.1, (b) 0.35, (c) 0.5, (d) 1, (e) 2, (f) 3.3 and (g) 4.5 |iim diameter

polystyrene colloids that were adsorbed onto the polyelectrolyte

multilayers for 6 hours, (continued next page) 116

4. 14 AFM height (left) and phase (right) images of the surfaces obtained by using

0.35, 0.5, 1, 2 and 3.3 |um diameter polystyrene colloids that were solution-

casted onto polyelectrolyte multilayers, and dried for 3 days 118

xvii

CHAPTER 1

INTRODUCTION

Overview

The wetting behavior of surfaces is an important property that has been studied

from both fundamental and practical perspectives. Controlling the wettability of surfaces

has been the target in practical applications such as printing plates, anticorrosion paints,

cookware, textiles and biomedical devices. There has been renewed interest in this

phenomenon and numerous recent reports have been published on preparing water-

repellent surfaces.'"'' However, there are critical factors that determine hydrophobicity

that have not been taken into consideration in these papers such as the topological nature

of the surface roughness, the 3-phase (solid-liquid-air) contact line structure (shape,

length, continuity of contact, amount of contact), and the contact angle hysteresis

exhibited by the surface. The primary goal of this thesis work is to examine the

conditions necessary to observe non-wetting behavior of surfaces. This will ultimately

enable the prediction and tuning of wettability. Three different approaches have been

taken in designing and preparing model hydrophobic surfaces with different roughness

topographies.

The first chapter begins with an introduction to the theoretical and experimental

studies reported on wettability of surfaces. Here, hydrophobicity is considered from a

dynamic point of view and dynamic hydrophobicity is redefined as the minimum force

1

needed to move a droplet across a surface. The critical parameters mentioned above in

determining hydrophobicity are discussed in detail. Also, surface characterization

techniques such as x-ray photoelectron spectroscopy, dynamic contact angle analysis and

atomic force microscopy are reviewed.

The second chapter involves the design and preparation of a series of silicon

surfaces with various topographies by photolithography and reactive ion etching

techniques. The surfaces contain posts of different size, shape and separation and are

hydrophobized by using reactive silane chemistry to achieve alkyl-, fluoroalkyl- and

silicone-like surfaces. The wetting behavior of these surfaces is then explored. The

primary characterization techniques are scanning electron microscopy and dynamic

contact angle analysis.

The motivation behind the approach explored in chapter 3 is to prepare

ultrahydrophobic surfaces via a practical route. Surface modification of low density

polyethylene (LDPE) is carried out to achieve ultrahydrophobic surfaces. LDPE film is

first treated with chromic acid followed by uniaxial or biaxial stretching. The strained

and necked polymer film turns out to have a special surface microstructure with feature

size and shape that can be controlled by the extent of strain. The wetting behavior of

these surfaces is analyzed after gold coating and treating with alkanethiols.

In the last chapter, the wetting behavior of a series of rough surfaces prepared by

adsorbing polystyrene colloids onto polyelectrolyte multilayers is investigated. The

2

flexibility of altering the surface roughness is studied by varying both the diameter of the

colloids and the time of adsorption. In-depth understanding of the adsorption behavior of

colloids onto polyelectrolyte multilayers is crucial and this is probed by using atomic

force microscopy in TappingMode'^ The wetting behavior of these surfaces is assessed

by dynamic contact angle measurements after gold coating and hydrophobizing the

surfaces using self-assembled monolayers (SAMs) of alkanethiols.

Wetting Behavior of Surfaces

Wettability of surfaces has been investigated both theoretically and

experimentally since the 19"' century. There is an enormous amount of literature from

the past 60 years directed at studying and controlling the interaction between fluids and

solids.

Thomas Young was the first scientist to investigate the wetting behavior by

measuring the static contact angle. ' According to his argument, wettabihty of an ideal-

homogeneous, smooth, rigid and insoluble solid can be expressed as in figure 1.1 and

equation 1.1. Gvoung is the equilibrium (or static) contact angle and it is the balance

between Ysv, Ysi and yiv , the interfacial tensions of solid-vapor, solid-liquid and liquid-

vapor interfaces, respectively. The interfacial tension is defined as the force per unit

length on a surface or an interface with the units ofmN/m or dyn/cm. Young's equation

gives a single value for the contact angle exhibited by a given solid-liquid-vapor system.

However, as will be pointed out later, real surfaces cannot be characterized by only one

3

contact angle.

Figure 1.1. Schematic representation of Young's equation.

Wenzel introduced an equation to explain the effect of surface roughness on

wettability.''*'"'^

He derived the relation between surface roughness and contact angle for

uniformly rough surfaces:

cos 9w = r (ysv- ysi )/yiv = r cos Bvoung ( 1 .2)

where 9w (Wenzel's angle) and Bvoung are the equilibrium contact angles for the rough

and the smooth surface, respectively. He introduced the term called the roughness factor,

r, which is the ratio of the actual surface area to the geometric projected (or apparent)

surface area. He proposed that for a smooth non-wetting (Gvoung > 90°) surface, the rough

surface is more water-repellent than the smooth one. Inversely, for a smooth hydrophilic

surface (Gvoung < 90°), the rough surface is more hydrophilic than the smooth one. Thus,

since the roughness ratio is always greater than one, depending on the equilibrium contact

angle of the smooth surface, the surface roughness can increase, or decrease the

wettability of the surface. Wenzel's analysis starts to fail at high rougliness ratios when

water droplets rest mainly on air and the surface is so called a 'composite' one.

4

Cassic and linxtcr ' cxlciulcd Wcn/crs analysis lo porous siii laccs hy iho

following equation:

cos OCussie = I'l COS 0 - h

llus equation relates the eontaet angle (Oew) ofa liquid on a composite (air-solid

nuxture) surlaee to the conlacl angle on a smooth surface of the same solid (0), and the

fraction of wnlcr-solid interface area (fi) to water-air interface area ((2), (fi + h =1).

According to this analysis, a Teflon surface with a sialic conlacl angle of 110"

would exhibit a conlacl angle of 1 73" if it was made into a 99% porous Tcllon surface.

They proposed thai at high levels of roughness, the probe liquid (lor example, water)

would jirimarily sit on air contained in the jiores of the rough matei iaf as shown in figure

1.2.

Liquid /-Air

Figure 1 .2.( 'assie's analysis at high roughness ratios, showing that the water ilroplel is

mostly in contact with air rather than the solid surface.

These analyses and many more put emjihasis on the static conlacl angles when

describing Ihe welting behavior of surfaces. Our argument is that the static contact angle

has no practical im|)orlance and one should focus on examining Ihe dynamic contact

angle behavior when assessing the surface's true wettability. I lence, the advancing and

5

receding contact angles, measured while the probe lluid is advanced or receded across the

surface should be important. The dilTerence between the advancing and the receding

contact angles is called the contact angle hysteresis.

Johnson and Dettre studied the wettability ol" sinusoidal surfaces with dilTerent

degrees of roughness and tried to explain dynamic contact angle behavior.'^ For a

hydrophobic surface having GYoung^ 120°, the hysteresis increases with the roughness

ratio up to a critical roughness ratio. At this point, the receding angle (Or) increases to

high values. The drops can no longer intrude between the pores and no hysteresis is

observed.

Figure 1 .3. (a) Wenzel's, Cassie's, and Johnson and Detlrc's analyses on contact angle

for different roughness ratios. The smooth surface had OYmmg = 120"; (b) Johnson and

Dettre \s analysis was done using sinusoidally rough surfaces with different roughness

ratios.

6

According to Johnson and Dcllrc's analysis on geometrically rough surfaces,

contact angle hysteresis can be qualitatively explained by assuming that the advancing

and the receding angles are determined by the balance between the macroscopic

vibrational energy of the drop and the heights of energy barriers. For non-composite

surfaces (roughness ratio < 2) the energy barriers are high between the metastable slates

leading to surfaces with high hysteresis. That is, the drop wants to pin at those

metastable states resulting in a high advancing but a low receding contact angle. As

surface roughness increases and passes beyond the critical value, the receding angle

jumps up resulting in low hysteresis surfaces. The vibrational energy of the drop is now

high compared to the height of the barrier. Hence the drop does not remain pinned at this

position of metastable equilibrium, rather it moves spontaneously. The key point

Johnson and Dettre missed in their investigation is the parameters that have crucial

control on the wettability of surfaces, such as the effect of different topographies of

roughness and the structure of the three phase contact line.

In addition to these theoretical studies, there are numerous experimental reports

published on surfaces exhibiting high water contact angles, as discussed in the following

sections. Each paper reported a single water contact angle, not the advancing or the

receding one. Even though they called these surfaces hydrophobic, in reality it was

impossible to assess the surfaces' true hydrophobicity with only one contact angle value.

7

Contact AiiKle Hysteresis

We believe that dynamic hydrophobicity should be defined as the minimum force

needed to move a water droplet across the surface.'' A hydrophobic surface is one on

which the water droplet camiot pin when the surface is tilted at an angle. For the water

droplet to start rolling off the tilted surface, the downhill side ofthe drop must be at its

advancing angle while the uphill side of it must be at its receding angle. According to

this argument, figure 1 .4 demonstrates the scenarios where water droplets behave

differently on tilted surfaces due to the differences in the contact angle hysteresis.

The surfaces in the cases (a) and (b) exhibit water contact angles of 0a/ Or = 120°

/ 80° and the surface in case (c) exhibits Ba / Or = 70° / 70°. In case (a), a 5 |iL droplet

was increased in volume to 10 ^L, thus it intersects the surface at its advancing angle of

120". hi case (b), a 15 |iL droplet was decreased in volume to 10 f.iL, thus it intersects the

surface at its receding angle of 80". In case (c), a 10 |iL droplet is in equilibrium with 0a/

Or = 70° / 70°.

When surface (a) is tilted, the downhill side of the droplet can advance, but the

uphill side stays pinned until the receding angle is reached. On surface (b), the uphill

side of the droplet can recede, but the downhill side stays pinned until the advancing

angle is reached. On surface (c), the droplet can advance and recede simultaneously.

8

I I I

-

^ ^(a) (b) (c)

Figure 1 .4. Water droplets behave differently on tilted surfaces due to the contact anglehysteresis.

In 1962, Furmidge derived an equation relating the minimum tilt angle at which

the droplet starts rolling/sliding off the surface to the contact angle hysteresis:'^

(mg) sin e / w = ylv (cos Gr - cos Ga) (1 .4)

It is clear from these arguments that the difference between the advancing and the

receding angles (hysteresis) and not the absolute values of the contact angles is important

for hydrophobicity.

There are various parameters that affect the contact angle hysteresis. One of them

is the heterogeneity of surfaces that are composed of domains of different molecules.

Also, if a surface component is soluble or if there is an interaction between the surface

9

and the liquid such as the adsorption of the liquid onto the surface, these will change the

free energy of the surface and hence result in hysteresis. The parameter that we are

concerned about here is the effect of surface roughness on contact angle hysteresis.

Topography of Roughness

Nature has provided us with remarkable examples of the relation between surface

roughness and wettability. Barthlott and his coworkers, as well as others found that

wetting of a leaf''^'^° or an insect wing^' was related primarily to the special

microstructures of their surfaces. 'Self-cleaning' or the lotus leaf effect is defined as a

phenomenon that occurs if the surface is non-wettable and there is removal of

contaminating species by for instance, rain.

As mentioned in the previous section, the effects of roughness on wettability have

been studied theoretically and experimentally since Wenzel's and Cassie's initial

publications. Cassie and Baxter prepared rough surfaces by coating wire gratings with

paraffin wax and reported a low hysteresis surface of 0a / 6r = 152° / 148°. Bartell and

Shepard studied the effects of roughness on contact angle and hysteresis by preparing

reproducible rough surfaces via machining pyramid-shaped asperities with various

heights and angles of inclination into paraffin surfaces.^'' They reported water contact

angles as high as Oa / Or = 1 58V 1 25° (the smooth surfaces had Ba / Or = 1 10° / 99°).

They also predicted that if the angle of inclination was 90°, for example surfaces of

feathers, furs, leaves and certain textiles, the advancing contact angle should be 1 80°.

10

Another important work was by Johnson and Dettre who reported water contact angles of

Oa / Br - 1 59V 1 57° and 0A / Gr = 1 58V 1 S?'* for rough fluorocarbon surfaces, and 0a / Gr

= 158V 153° for rough paraffin wax surfaces which were prepared by spraying solutions

of waxes onto glass slides.^^

In the last decade, there has been a renewed interest in achieving non-wettable

surfaces. For instance, Onda and coworkers prepared 'super water-repellent' fractal

surfaces that exhibited water contact angles as large as 174°.''^ 'Ultra water-repellent'

rough films were made by using microwave plasma-enhanced chemical vapor deposition

of organosilane compounds. The maximum contact angle was -160°.^ Porous alumina

thin films with a roughness of 20-50 nm, hydrophobized by fluoroalkyltrimethoxysilane

yielded 'super-water-repellent alumina' coating films with G = 165°.'*'^ The ion-plated

poly(tetrafluoroethylene) coatings that Drelich et al. prepared also had roughness

dimensions of a few nanometers and these had water contact angles of 150-160°.^ PTFE

powders with diameters of 7.6 \im and 0.83 |nm were used as coatings to make water-

repellent materials with static contact angles of -150°. The key point to all these

examples is that almost all the studies reported a single water contact angle, not the

advancing and the receding ones. Thus, it is impossible to assess the surfaces' true

hydrophobicity from the published contact angle values.

There is another important issue in the literature that needs to be resolved: the

issue of the size scale of roughness needed to impart ultrahydrophobicity. The recent

reports of surfaces with high contact angles indicated that micrometer-, submicrometer-,

11

and nanometer-scale roughness imparted this property." ' Older reports Irom the 194()'s,

1950's, and 196()'s, however, suggested much larger features (tens to hundreds of

micrometers) could also function in this manner.^^'^^ By designing and preparing

surfaces with a range of si/e scale of roughness, this issue is mvestigated in chapter 2.

Thrcc-Phase Contact Line

The intersection of the three interfaces, namely solid-liquid-vapor interfaces,

results in the formation of the three-phase contact line. The three-phase contact line is ;

perfect circle if the surface is smooth, homogeneous, planar and rigid. As the droplet

advances (recedes) across the surface, the whole contact line advances (recedes) and it

starts to deviate from its circular shape and becomes contorted ibr real surfaces that are

rough or heterogeneous, as demonstrated in figure 1 .5.

Figure 1 .5. Possible three-phase contact line structures on a real surface (— ) and an ideal

one (-).

12

We argue that the structure cl lhe 3-plu.se contact line is important to dynamic

wettability (Oa, Ok and hysteresis) and C:assie's analysis does not take the 3-phase contact

line structure into account. Surfaces with many dilTcrent topographies can have the same

li and I- values, but can have very dilTerent contact line structures. I'igure 1.6 shows

representations of two surCaccs with the same f, and 0 values (eq 1 .2). Cassie's equation

predicts the .same equilibrium contact angles, but the advancing and receding contact

angles will be very different on these surfaces becau.se in (a), a nearly continuous contact

line can form (pinning the drop), but in (b) the contact line is discontinuous and unstable.

(ii) (b)

I'igure 1.6. 2-Dimensional (X-Y) representations of two surfaces with the same fi and I2

values, but very different contact line structures. The dark lines are meant to representpo.ssible contact lines.

We are aware of very few experimental works in the literature where scientists

have paid attention to the importance of the three-pha.se contact line, its shape and its

26-28input on the wetting behavior ol'surlaces. Drelich el al. prepared model

heterogeneous surfaces with 2..S [un liydrophilic and 2.5 jun hydrophobic stripes that

were alternating and parallel to each other. 1 he gold surface was patternetl with self-

13

assembled monolayers (hydrophobic and a hydrophilic one) via microcontacl prnU.ng

with an elastomer stamp developed by Whitesides and his group."^"^' Their argument

was that the contortion of the three-phase contact line affected the contact angle. When

measuring the contact angle of the surface with strips parallel to the three-phase contact

line, the contact line was not contorted, hence the contact angles (both the advancing and

the receding) were 2-10" higher. However, the contact line was observed to be contorted

when it was perpendicular to the stripes. This led to a significant effect on the advancing

and the receding contact angles. Actually, a new theory was developed describing the

contact angle behavior for such systems."'^^'^^ j^-^ equation was the modification of the

Cassie's equation, incorporating the line/pseudo-line tension term:

cos e = f, cos 9, + f2 cos 02 - + n 5^

y X^ Xi'

where ysLvi = (5 F, / 5 Lj) r, v. Ay, ni is the line/pseudo-line tension, F is the free energy of

the system; yi.v is the surface tension of the liquid and ri is the radius of the three-phase

contact line (in this case, the half width of the stripes) at the i-component of the surface.

Surface Characterization Techniques

Dynamic Contact Angle

Dynamic contact angle measurement is the most effective technique in this thesis

work to characterize the surface wettability. It is a surface-sensitive technique,

measuring the wetting behavior of the outermost few angstroms of the surface. In the

McCarthy group, the measurements are done with a Rame-Hart telescopic goniometer

14

and a Gilmont syringe with a flat-tipped 24-gauge needle. A water droplet (or another

probe fluid) is introduced to the surface tliiough a microsyringe. As the volume of the

droplet is increased and the three-phase contact line of the water droplet starts to advance

across the surface, the contact angle is simultaneously measured which gives the

advancing contact angle (Ba). Likewise, the receding angle (6,0 is measured as the

volume of the droplet is reduced and the three-phase contact line starts to recede. The

contact angles are usually measured for 8-12 areas across the surface and the average

contact angles are reported. The accuracy of the measured values is ± 2". The difference

between the advancing and the receding contact angle is the hysteresis exhibited by the

surface. Milli-Q water and hexadecane are the probe fluids for our experiments.

syrmge

Figure 1 .7. Schematic representation of dynamic contact angle measurement,

Atomic Force Microscopy

Scanning probe microscopy (SPM) is a relatively new surface analysis technique,

characterizing the probe-sample interactions, based on a sharp probe (usually a silicon

tip) scanning across the surface. SPM consists of two different microscopy techniques,

15

one of them is scanning tunneling microscopy and the other one is atomic force

microscopy (AFM), also called scanning force microscopy (SFM). AFM is a powerful

technique for probing the surface topography of a substrate. This technique was

developed in 1986 by Binning, Gerber, Rohrer, and Weibel at IBM in Zurich,

Switzerland."-^'' There arc three modes of operating an AFM: contact mode, non-contact

mode and TappingMode ".

TappingMode'" AFM is used in this work to characterize the surface

microstructure of the substrates. There is a silicon tip that is attached to a cantilever

oscillating at or near its resonance frequency and its amplitude changes between 20-100

nm. The probe is brought closer to the surface so that it can lightly 'tap' on the surface.

As the tip contacts the surface at the bottom of its swing, the oscillation amplitude is

reduced. This amplitude change is detected by the split photodiode via the reflection of

the laser off the tip. An x-y-z piezo moves the cantilever in the z-direction in order to

maintain a constant amplitude and this vertical position at each (x, y) data point is stored

by the computer resulting in the topographical image of the surface. These images are

height and phase images of the surface.

The AFM software lets the user do several different analyses of the image. In this

thesis work, section analysis software is used. Section analysis provides information

about the surface distance and the horizontal distance of a rough substrate when a line is

drawn across the scanned image. Thus, based on 8 different locations over the scanned

area, the ratio of (surface distance) to the (horizontal distance) can be calculated and one

16

can oblain the Wcn/cl's roughness ratio. Th.s calculation is schematically shown

detail in chapter 4.

in

Detector

lilcclronics

i eciihiick I ,oop Mamlains

Constant Oscillution Amplitude

Spill

Pholodiode

Deleclor

Scanner

( antilevLM and Tip

NanoScopc

Controller

Siibslrulc

Figure 1.8. An illustration oTan AVM

X-Ray IMiotoelectron Spectroscopy

X-ray pliotoclectron spectroscopy (XPS), also known as cicclron spectroscopy lor

chemical analysis (luSCA) is a powerful surface analysis technique hased on

photoelectric erfect. It provides a quantitative elemental analysis oflhe surface's

outermost 10- 100 A^\^reven - 200 A^^' depending on the experimental conditions and

the equi|)ment used. 1 he photoelectric effect is the phenemonon where an orhila

17

electron is ejected from an atom upon its interaction with a photon. The electron has a

kinetic energy, Ek, given by the equation:

Eb = hi) - El(1.6)

where Eb is the binding energy of the atomic orbital electron, Ito is the photon energy,

photon

2p

2s

Is

photoelectron

Figure 1 .9. Illustration of the photoelectric effect.^*^

When the X-ray source with monochromatic X-ray photons (in our lab, either Al

Ka or Mg Ka) hits the targeted sample under a high-vacuum environment, photoelectrons

are produced. These photoelectrons are detected by the analyzer that counts the number

of electrons with different kinetic energies. Every element of the surface has a unique

binding energy and hence kinetic energy. Thus, it is possible to identify the elements

according to the peaks in the XPS spectrum that plots photoelectron intensity versus

binding energy. Also, the atomic compositions can be calculated by integrating the area

under the peaks while taking into consideration the atomic sensitivity factors which

depend on the photoelectric cross sections of the atoms.

The sampling depth of the XPS depends on the angle between the sample and the

detector, called the takeoff angle. As the takeoff angle increases, it samples atoms deeper

18

in the surface and as the angle decreases, it samples atoms closer to the surface. The

number of electrons detected (N) is proportional to the number of electrons ejected (N.)

at sampling depth (t) by the following equation:

N = N„exp(—!—

)

XsinO (1-7)

where 9 is the takeoff angle and X is the electron mean free path. According to this

equation, 95% of the photoelectrons detected are from the outermost S^sinB. For

instance, Cis electrons have a mean free path"^^ of 14 A, so at 15" takeoff angle, the

ejected photoelectrons that are from the outemiost -10 A can be detected, whereas at 75"

takeoff angle, this detection depth increases to the outennost -40 A. Hence, XPS is also

useful in quantitatively obtaining the depth profile of a sample by varying the takeoff

angles.

19

Notes and References

(1) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125.

(2) Shibuichi, S.; Onda, T.; Satoh, N.;Tsujii,K. J. Chem. 1996,700, 19512.

(3) Hozumi, A.; Takai, O. Thin Solid Films 1997, 303, 122.

(4) Tadanaga, K.; Katata, N.; Minami, T. J. Am. Ceram. Soc. 1997, 80, 1040.

(5) Tadanaga, K.; Katata, N.; Minami, T. J. Am. Ceram. Soc. 1997, 80, 3213.

(6) Veeramasuneni, S.; Drelich, J.; Miller, J.D.; Yamauchi, G. Prog Ors Coat 199757,265.

(7) Yamauchi, G.; Miller, J.D.; Saito, H.; Takai, K.; Ueda, T.; Takazawa, H.;Yamamoto, H.; Nislhi, S. Coll. and Surf.. A: Physicochemical and Engineering.Aspects 1996, 77^, 125.

(8) Ogawa, K.; Soga, M.; Takada, Y.; Nakayama, I. Jpn. J. Appl. Phys Pt 2 - Lett

1993, 32, L614.

(9) Kunugi, Y.; Nonaku, T.; Chong, Y.B.; Watanabe, N. J. Electroanal. Chem 1993353, 209.

(10) Schakenraad, J.M.; Stokroos, I.; Bartels, H.; Busscher, H.J. Cells and Materials

1992, 2, 193.

(11) Miller, J.D.; Veeramasuneni, S.; Drelich, J.; Yalamanchili, M.R.; Yamauchi, Y.

Polym. Eng. Sci. 1996, 36, 1849.

(12) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 1111.

(13) Young, T. Philos. Trans. R. Soc. London 1805, 95, 65.

(14) Wenzel, R.N., Ind. Eng. Chem. 1936, 27, 105.

(15) Wenzel, R. N., J. Phys. Colloid Chem. 1949, 55, 1466.

(16) Cassie, A. B. D. Discuss. Faraday Soc. 1948, 5, 1 1.

(17) .lohnson, R.E.; Dettre, R.H. Surface Coll. Sci. 1 969, 2, 85.

(18) Funnidge, C. G. L. J. Colloid Sci. 1 962, 7 7, 309.

20

(19) Walanabc, T.; Yamaguchi, \. Jounuil ofPesticide Science 1991, /6,(,51.

(20) Barlhlolt, W.; Nciiihuis, C. PUmta 1997, 202, 1.

(21) Wagner, T.; Ncinhuis, C; BarlhloU, W. Acta Zool. 1996, 77, 213.

(22) Cassie, A.B.D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546.

(23) Barlell, V. C; Shepard, .1. W. J. P/iys. Clwm. 1953, 57, 211.

(24) .lohnson, R. .Ir.; Dcltrc, R. II. Adv. Clwm. Ser. 1963, 43, 136.

(25) Dettrc, R.ll.; Johnson, R.E. in Wetting; S.C.I. Monograph No. 25, Sociely ofChemical Industry, London, 1967, p 144.

(26) Drelich, .1.; Wilbur, .1. L.; Miller, J. D.; Whitcsides, G. M Lanf^nuiir 1996 121913. '

'

(27) Drelich, .1. Ph.D. Dissertation, University of Utah, 1993.

(28) Drelich, J.; Miller, J. D. Langmuir 1993, 9, 619.

(29) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002.

(30) Kumar, A.; Bicbuyck, II. A.; Whitesides, G. M. Langmuir 1994, 10, 1498.

(31) Drelich, J.; Miller, .1. D.; Kumar, A.; Whitesides, G. M. Colloids Surf., A:

Physicochem. Eng. Aspects 1994, 93, 1.

(32) Drelich, .1.; Miller, .1. D. Part. Sci. Tcchnol. 1992, 5, 1 1.

(33) Digital Instruments, Scanning Prohc Microscopy Training Handbook, 1998,

Santa Barbara, CA

(34) Binning, G.; Quate, C. F.; Gcrber, C. H. Phys. Rev. Lett. 1986, 56, 930.

(35) Clark, D. T.; Thomas, II. R. ./. ofPolym. Sci. 1911, 15, 2843.

(36) Andradc, J. D. X-ray Photoelectron Spectroscopy; Andrade, .1. D., Ed.; Plenum

Press: New York, 1985, Vol. I, p 105.

(37) Clark, D. T.; Thomas, 1 1. R. J. ofPolym. Sci. 1977, 15, 2843.

21

CHAPTER 2

MODEL LITHOGRAPHIC SURFACES TO EXPLORE ULTRAHYDROPHOBICF

Introduction

In this phase of the dissertation work, model silica surfaces were designed and

prepared in order to examine the critical parameters, such as the structure and

discontinuity of the three-phase contact line, that control ultrahydrophobicity. The

surfaces containing hydrophobic posts should form discontinuous and unstable 3-phase

contact lines with water droplets. The posts must be close enough together and

hydrophobic enough that water does not intrude between them. Surfliccs such as these

are referred to as composite surfaces as the intersection of the water droplet with the

surface consists of a composite mixture of water-solid interface area and water-air

interface area. The wettability of such surfaces was first addressed by Cassie and

Baxter,' as introduced in the first chapter of the thesis. We also stressed that this

equation and many others were derived for a drop at equilibrium on a surface, thus the

contact angles predicted were "equilibrium contact angles". We argue that the structure

of the 3-phase contact line is important to dynamic wettability (Oa, Or and hysteresis). A

nearly continuous contact line causes the drop to pin on the surface, whereas a

discontinuous contact line leads to an unstable droplet that rolls off the surface with a

small till. By varying the size, shape and the separation between the posts we were able

to make surfaces where the water droplet pinned or rolled off easily.

22

It is well known that photolithography and dry etchnig are the most common

techniques used in the fabrication of integrated circuits and solid-state devices where the

substrate is usually silicon wafer, hi the McCarthy group, the chemistry of reactive

organosilanc reagents on silica surfaces has been investigated for several years now.

Hence, the techniques developed in microelectronics manufacturing and the reactive

silane chemistry were combined in this work to study the effect of surface microstructurc

on wettability.

Photolithography

Photolithography is the most widely used technique in the printing industry as

well as in semiconductor processing.' The ultimate goal of this process is to transfer

three dimensional patterns onto the semiconductor substrate (most commonly silicon

wafer) accurately and precisely. The process first involves designing the targeted

features using computer software (CAD) and preparing the photomask consisting of these

patterns. This is followed by transfering the pattern on the photomask into the

photoresist. This pattern can then be replicated on the underlying semiconductor

substrate by a subsequent etching step. Photolithography uses UV radiation of A, = 360-

410 nm when transfering the patterns from the photomask to the photoresist.

In this work, two different kinds of photolithography technique - contact

lithography and projection lithography - were used. Contact lithography is the oldest

technique developed in this field.

23

Figure 2.1 shows the Hthographic steps involved in the fabrication of a patterned

silicon wafer. The process starts with cleaning the silicon wafer and spni-coating the

primer (if needed) and the photoresist (in our case, a positive photoresist). If contact

lithography^ is used, the mask is then placed close to the wafer and the mask pattern is

aligned with respect to the crystallme planes of the wafer. Next, the mask is brought into

contact with the wafer and is simultaneously exposed to UV light. After that, the sections

of the photoresist that have been exposed to UV light dissolve if the wafer is immersed

into an appropriate solvent called the developer. This results in a pattern of the mask on

the resist. The time spent in the developer will affect the shape and the size of the

resulting patterns on the photoresist. It is very important to make sure that the pattern on

the photoresist has the targeted dimensions of the original mask, since this pattern is

going to be used as the 'mask' in the further step of reactive ion etching. Likewise, if the

photoresist is not developed long enough, then the 'mask' will not be accurate. Thus, it

needs to be examined by optical microscopy before any further steps such as reactive ion

etching.

The contact lithography has some drawbacks such as loss of resolution and

sharpness of the features as the pattern size decreases. Hence, if small and sharp features

(< 8 |j,m) are designed, then one needs to consider a different technique such as projection

lithography (a stepper). In this technique, the mask patterns are projected onto the wafer

and hence, the wafer is no longer in direct contact with the mask. This results in more

defect-free and accurate patterns. It also has the advantage that the mask doesn't wear

out easily.

24

UV light

Silicon wafer

MaskPhotoresist

Develop

Silicon wafer

RIE

IPlasma

Remove photoresist

Figure 2. 1.Main lithographic steps in preparing the patterned silica substrates,

Dry Etching

After the photolithography step, there are two ways of replicating the pattern onto

the substrate, dry or wet chemical etching. Especially for the last couple of decades, the

dry etching, or plasma-enhanced etching has been replacing the conventional wet

chemical etching due to its unique process.'* It resuhs in anisotropic features at low

25

temperatures and in dry environment. Additionally, the technological need to etch

features with < 0.5-1 ^m accurately and precisely requires the use of dry etching.

The conventional wet chemical etching is a well-established technique. However,

the resolution of the features is limited by undercutting. As the solvent etches

downward, it also etches laterally at the same rate resulting in an isotropic etch, as shown

in figure 2.2. This causes changes in pattern size and shape. Also, the use of corrosive

acids and toxic solvents causes the wet chemical etching process to be less attractive.

One of the biggest advantages of the plasma etching technique is that it etches

much faster in the vertical direction than in the horizontal direction resulting in an

anisotropic profile as shown in figure 2.2. It can be performed in full automation and

does not require any harmful chemicals.

Mask > MaskPhotoresist to be etched

Substrate

Dry EtchWet Chemical Etch

> w

Anisotropic Profile Isotropic Profile

Figure 2.2. Cross sections of photoresist etched by plasma or liquid etchants,

26

There are several different dry etching technologies, such as plasma etchuig,

reactive ion etching and ion beam milling. Among these, reactive ion etching (RIE) is

the best-controlled technique offering high selectivity and anisotropy and it was used m

this work.

Usually, radio-frequency (rf) or microwave electric fields are used to excite the

low-pressure glow discharges.' The etching gas (or combination of gases) decomposes

and ionizes into electrons, free radicals and ions due to this electric field. The reactive

species have longer mean free paths due to the lower operating pressures of 1 x 10"^ to 1

X 10"' Torr in RIE. This lets the ions impinge vertically at the substrate's surface leading

to an anisotropic etch.^

The cycle of plasma etching starts with the collision of gas molecules and the

energetic electrons resulting in highly reactive species that are responsible for the etching

reactions even at low temperatures. These are some of the possible reactions:"*'^

1) excitation (rotational, vibrational, electronic)

e' + A2 ^ A2* + e"

2) dissociative attachment

e" + A2 ^ A" + A"" + e"

e" + A2 A" + A^

3) dissociation

e" + A9 -> 2A + e"

27

4) ionization

e" + A2 -> Ai^ + 2e'

5) dissociative ionization

e" + A2 + A + 2e"

Next, these chemical species (for instance free radicals) diffuse and adsorb onto

the surface. It has been stated that the free radicals chemisorb and react with the surface/

Other primary processes include: 1) ion-surface interactions - leading to neutralization

and secondary electron emission, sputtering and ion-induced chemistry; 2) electron-

surface interactions - leading to secondary electron emission and electron-induced

chemistry; and 3) radical-surface interactions - resulting in surface etching as well as film

deposition.

The critical step is the desorption step. The product of the reaction between the

free radicals and the surface should have an appropriate vapor pressure in order to desorb

and diffuse into the gas phase.

Gas Inlet

PlasmaWafer

To Vacuum

PumpElectrode

Figure 2.3. The reactive ion etcher.

28

Reactive Silane Chemistry

Reactive organosilicon compounds have been used to modify metal oxidc*^'^and

polymer surfaces.' ^'^ Due to the covalent attachment of silane monolayers onto silica

surfaces, they are stable and ideal for many applications.'' From enhancmg the

biocompatibility of a surface,"'^ modifying the surface for sensor applications"" to

immobilizing chemically active species,^^ the silane reagents have been used in a variety

of areas.

Recently, in the McCarthy group, there has been an extensive study done on

exploring silica surfaces modified with monofunctional (R.^SiX), difunctional (R2SiX2)

and trifunctional (RSiX.^) organosilanes (X = CI, OR, NMe2).^''" Unlike other groups'

work, in these reports the reaction conditions such as the kinetics and the amount of water

in the system were well optimized in order to have well-characterized surfaces. The

dynamic contact angles changed if the reactions were performed in a solvent (usually

toluene) or in the vapor phase at -75 "C. The vapor phase reaction resulted in surfaces

with higher contact angles than the liquid phase reaction. Si-O-Si covalent bonds are

formed when the monofunctional silanes are reacted with the silica surfaces, as shown in

figure 2.4. The n-alkyldimethylsilane groups have cross-sectional area of 32-38 A^, and

they form densely packed monolayers.*^"' The water does not penetrate the

monolayers, resulting in contact angles of the surfaces that are independent of the chain

length of the alkyl group. They stated that the vapor phase reaction was a convenient

and an easy way of preparing surfaces with high yields.

29

Figure 2.4. The reaction of octyldimethylchlorosilane (and other monofunctionalorganosilanes) with siHcon dioxide surfaces.

In contrast to the monofunctional silanes, the difunctional silanes can react in two

different ways with hydrated sihca. As shown in figure 2.5, they can either covalently

attach to the surface or they can polymerize to form grafted polysiloxanes depending on

the reaction conditions. It was shown that the vapor phase reaction of short chain length

alkylsilanes results in oligomeric structure and in surface-grafted alkylsiloxanes in the

presence of water. The wettability of these surfaces was very impressive. The water

(as well as hexadecane and methylene iodide) droplets slide off the surfaces easily due to

the negligible hysteresis behavior they exhibited when prepared in the vapor phase. For

instance, for dimethyldichlorosilane the dynamic water contact angles were 9a / 6r = 105°

I 104"*. We call such surfaces without any hysteresis 'ultralyophobic'.

30

OH OH OHH3C

CH

Si

CI

CI

H,C^ /CH

Covalent

Attachment

H3C' \ .CH

o o ^ °»

Vertical

Polymerization

HO^/

si-

H,C I

CH, \

CH3

CH

O

Six.

/ ^OHSi

/\

OH OCH

Figure 2.5. The possible reactions of dimethylchlorosilane (and other difunctionalorganosilanes) with silicon dioxide surfaces.

Experimental

Materials

Ethanol, toluene, sulfuric acid and hydrogen peroxide (30%) were used as

received from Fisher. Organosilanes were obtained from Gelest and used as received.

House purified water (reverse osmosis) was further purified using a Millipore Milli-Q®

1

8

system that involves reverse osmosis, ion exchange and filtration steps (10 ohm/cm).

Contact angle measurements were made using a Rame-Hart telescopic goniometer with a

24-gauge flat-tipped needle; dynamic advancing and receding contact angles were

31

were

recorded as the probe fluid, water, purified as described above, was added to and

withdrawn from the drop, respectively. Scanning electron micrographs (SEM)

obtained using an Amray 1 803TC instrument with an accelerating voltage of 25 kV. X-

ray photoelectron spectra (XPS) were obtained using Perkin-Elmer-Physical Electronics

5 1 00 spectrometer with Mg excitation at takeoff angles of 15" and 75". Atomic

concentrations of the substrates were calculated using the sensitivity factors, Si is, 0.270;

Cis, 0.250; Ois, 0.660; Fis, 1.00, obtained from the samples ofknown composition.

Preparation of Silicon Substrates

4-in. and 3-in. silicon wafers were obtained from International Wafer Service

(<100> orientation, P/B-doped, resistivity from 20 - 40 Q-cm, thickness from 450 - 575

|Lim). A contact lithographic mask (with hexagonally arrayed square posts of 16, 32, 64

and 128 length and width) was constructed by Photronics Inc. The preparation of the

silicon substrates, including the mask preparation and patterning of the wafers, were all

performed at the Cornell Nano fabrication Facility. The other masks were designed using

a CAD program and prepared with a GCA PG3600F optical pattern generator.

Photolithography was used to transfer the patterns of the masks onto the silicon wafers.

Subsequent to irradiation, the wafers were etched with a proprietary mixture of gasses

using a Plasma Therm SLR-770 reactive ion etcher for different durations. When the

etching process was complete, the wafers were cleaned oxidatively using a Branson IPC

P2000 Barrel Etcher. Wafers were then placed in a solution of ammonium hydroxide,

hydrogen peroxide and water (4:1:1) for 15 minutes, rinsed with copious amounts of

32

water and spin dned. The wafers were then cut into 1.5 cm x 1.5 cm pieces, placed in a

custom designed (slotted hollow glass cylinder) sample holder and were cleaned by

submersion into a mixture of concentrated sulfuric acid and hydrogen peroxide (30%)

(7:3) overnight. The wafers were then rinsed with copious amounts of purified water and

dried in a clean oven at 120 °C for 5 hours immediately prior to silanization reactions.

Reaction of Silicon Substrates with Organosilanes

The silicon substrates were placed in a custom-designed (slotted hollow glass

cylinder) sample holder and placed in a flask containing 0.5 mL of organosilane reagent:

dimethyldichlorosilane (DMDCS), n-octyldimethylchlorosilane (ODMCS) or

heptadecafluoro- 1 , 1 ,2,2-tetrahydrodecyldimethylchlorosilane (FDDCS). The wafers

were not in contact with the liquid silanes. The vapor phase reactions were carried out

for 3 days at 65-70 °C. The hydrophobized wafers were rinsed with toluene (2 aliquots),

ethanol (3 aliquots), 1 : 1 ethanol/water (2 aliquots), water (two aliquots), ethanol (2

aliquots) and then water (3 aliquots) and were then dried in a clean oven at 120 °C for 30

minutes.

33

Results and Discussions

Reaction of Silicon Substrates with Qrganosi lanes;

The silicon substrates were hydrophobized with dimethyldichlorosilane

(DMDCS), n-octyldimethylchlorosilane (ODMCS) or heptadecafluoro-1,1,2,2-

tetrahydrodecyldimethylchlorosilane (FDDCS). The reactions were carried out in vapoi

phase for 3 days to give dense monolayers. Figure 2.6 shows a representative XPS

survey spectrum for a smooth silicon/silicon oxide surface reacted with FDDCS. Table

2.1 gives the atomic concentration of the surfaces reacted with DMDCS, ODMCS and

FDDCS at 15° and 75° takeoff angles.

10 ] 1 1 ! 1 1 1i H-—h"!—I

i1 1 1 1 1 1

f-

1000.0 900.0 800.0 700.0 600.0 500.0 -tOO.G 300. 0 200.0 lOO.O 0.8

ABIDING ElOGY. eV

Figure 2.6. XPS survey spectrum of silicon/silicon oxide surfaces reacted with FDDCSat 75° takeoff angle.

34

Table 2.1.

Atomic concentration (XPS) ofsilicon/silicon oxide surlaces leictcd will.DMDCS, ODMCS and I'DDCS at 15" and 75" takeolT angles.

DMDCS ODMCS15° 75° 15° 75° 15° 75°

%Si 14.44 39.48 26.82 48.69 14.22 41.92

%C 55.05 29.05 48. 11 14.32 26.91 11.16

VoO 30.51 31.47 25.07 37.00 20.73 32.60

%F38.14 14.32

Critical Size Scale ol'Rouuhness for llvdiophobicitv

Figure 2.7 is a 2-dimensional schematic representation of one series of silicon

oxide surfaces that was prepared using photolithographic techniques. These surfaces are

hcxagonally arrayed square posts varying in size from 2 |im x 2 |am to 128 |im x 128 |luii

X

X D2x

2x

Figure 2.7. 2-l)imensional (X-Y) representation of a series of silicon surlaces containing

hcxagonally arrayed square posts, X - 2, 8, 16, 32, 64 and 128 (.uii. Surfaces with post

heights (Z) of 20, 40, 60, 80, 100 and 140 ^m were prepared.

35

2 |im Ba/Gr^ 176V 14P 8 |.im Oa / Ok - 173V 134"

16 |im eA/eR= IVr/ 144° 32}im eA/eR= 168°/ 142°

64 |am eA/eR= 139°/81° 128 urn Ga/Or^ 116°/80°

Figure 2.8. SEM images of the surfaces described in figure 2.7. The water contact

angles shown are for DMDCS-modified surfaces.

36

Different post heights (20, 40, 60, 80, 100 and 140 ,m) were prepared by vaiying

the etching time for 1 6, 32, 64 and 1 28 size posts. The posts are spaced as indicated

in the figure. Figure 2.8 contains SEM images of the surfaces explained in figure 2.7.

These surfaces were oxidatively cleaned (to remove any of the lithography mask and/or

etching chemicals) and chemically modified by reaction with silanizafion reagents in the

vapor phase.

Table 2.2 shows water contact angle data for surfaces that were etched to give 40

^m high posts and reacted with DMDCS, ODMCS or FDDCS. Data for smooth

silicon/silicon oxide surfaces are shown for comparison. The surfaces are named, ^SP^',

with the superscript X indicating the post length and width and the superscript Y

indicating the post height. The surfaces with post sizes of 32 and less exhibited both

high advancing and receding contact angles. We ascribe the slight differences in contact

angles between the differently modified surfaces to experimental error in the

modification reactions or in the measurement of contact angles and not to the size of the

posts or the identity of the silanization reagents. All of these surfaces are extremely

hydrophobic. Water droplets were unstable on these surfaces. Droplets moved

spontaneously on slightly tilted surfaces. For the surfaces with post sizes 64 |im and

greater, low receding contact angles (lower than those on the smooth surfaces) were

observed. Advancing angles were higher than those on the smooth surfaces, but lower

than those on the surfaces with smaller features. The water drops pinned on these

surfaces and did not rotate nor move spontaneously when the surfaces were slightly tilted.

37

It is apparent that water intruded between the greater spaced posts and that the

contact line was pinned by the greater solid-liquid contact. We note that all of these

surfaces contained 25% solid-liquid interfacial area (f, in Cassie's equation in chapter 1 );

this equation predicted equilibrium contact angles (using the measured advancing contact

angles on smooth surfaces) of 145°, 143° and 150° for the DMDCS, ODMCS and

FDDCS surfaces, respectively. These were in line (between the measured advancing and

receding contact angles) with the surfaces containing features of 32 |im and less.

Table 2.2. Water contact angle data for silane-modified hexagonally arrayed 40 [im -

high square post surfaces described in figures 2.7 and 2.8.

X YSP

; X = post width and length

Y = post height

Silicon SurfaceDMDCS-modified ODMCS-modified

Oa Or Oa Or

FDDCS-modified

Oa Br

smooth 107° 102° 102° 94° 119° 110°

2 nmgp40 |im176° 141° 174° 141° 170° 146°

8 |imgp40 |jin

173° 134° 173° 139° 170° 140°

16 |imgp40 [im171° 144° 174° 134° 168° 145°

32 ^|ngp40 \im168° 142° 170° 132° 170° 146°

64 ).imgp40 |im139° 81° 114° 65° 149° 100°

128 |iingp40 \im116° 80° 95° 58° 131° 93°

38

Surfaces containing 16, 32, 64 and 128 posts with post heights varynig from

20 Mm to 140 were prepared and derivatized using ODMCS. The dynamic contact

angles were measured and are reported in Table 2.3. The contact angles were

independent ofthe height of the posts for 16m and 32 nm posts, ind.catmg that water

did not intrude between them (the probe fluid could not detect the depth ofthe region that

it did not penetrate).

The surfaces described thusfar (square posts) with features 32 ^m and smaller

exhibited very high advancing and receding contact angles, but significant (30-40°)

hysteresis. Water droplets came to rest on these surfaces when the surfaces were

horizontal (but rotated continuously). This behavior differs from the randomly rough

surfaces that we have prepared^^'^^' on which droplets moved spontaneously on horizontal

surfaces. We suspect that the regular hexagonal array of features with regions that can

support a straight contact hne over significant lengths is the source of this hysteresis.

39

Tabic 2.x Walcrconlacl angle data for ODMCS-nrndillal surfaces conlaining K, ,„„ x16 ^m, 32 ,m x 32 ,n,, M x (,4 12« x 128 sc,ua,c posts ol d.ricra'r

^SP^; X post width and length

Y = post height

Silicon Post

Surface Height (|iim)

1 6 |ini^p2() ^im20 173" 138°

1 () 11 Mic^ r )4() II tii1 \ / ^ 1 1 1 1 IV'' Mill

40 174°134"

I 6 linic^ iim60 169° 138°

1 () finiigpXO fini80

169°

136°

1 6 |nnc n 1 00 iim100 173° 138°

16 |jmgp!4() fim140 168° 136°

20 170° 137°32 uiiic n4() mil

40 170° 132°32 ^iniQ T)6f) urn

60 168°139°

32 umc inn80 167° 134°

32 (iiiicj I) 1 00 )i

m

100 173° 134°

32 fiiiigpl4() fiMi140 166° 131°

lit ftiii^iji,\/ |nii20 1

09° 80°

64 timo r^4() mid40 1

14° 65°

(\A IMlinr\^k(l iiitiV 1^ 1 1 1 1 1 1' 1 ^ '" ' 1 1 1 1 i

60 128° 64°

(^A 1 1 iiir~i III!)i/*T LM 1 1^ I J'' " ' n 1 1

1

80 149° 113°

iiiiifiT~\l()(t iirnf

' '' IJ'^'*' Mill

100 145° 57"

64 pmepl4() |im 140 1 A°

1 2X nnn^p2() |mi 20 93° 74°

I2H |iiin^p40 fuii 40 95° 58°

1 28 ^tnigp()0 |im 60 120° 69"

1 28 [ini^ pKO fit}} 80 116° 73°

128 ftnij^jjIOO ftiii 100 126° 65"

128 |iiii(^pl40 (ini 140 118° 15"

40

Structure of 3 -Phase Contact Line

We designed and prepared three different surfaces with the objective of contorting

the 3-phase contact line. These are described in Figure 2.9 and SEM images are shown in

Figure 2. 1 0. These surfaces are named 'RP' for the staggered rhombus-sliaped posts, TP'

for the indented square-shaped posts and 'StP' for the 4-point star-shaped posts. Tlie post

height of these surfaces was maintained at 40 fim.

8 |Lim

< >16 |im

32 |im

IP

8 |im

XX X

16 |im

StP

32 (.im

8 |im<—

>

4 (^O ^ O-

<> 0 ^-32 |im

32 |im

RP

Figure 2.9. 2-dimensional (X-Y) representations of surfaces containing different

geometry posts.

41

Figure 2.10. SEM images ofthc surfaces described in figure 2.9. The water contact

angles shown arc for DMDCS-modillcd surlaces.

Wc also prepared a series of surfaces containing 8 |.mi x 8 |.uii liexagonaily

arrayed square posts (40 \xm high) that were spaced at greater distances than the surfaces

described in figures 2.7 and 2.8. fhese were prepared with the objective of making tlic }-

phase contact line less continuous.

42

4x

X

4x

4x

23 (iinSP40 |.im

lOx

X 6

lOx

56 ^mSP40 |im

Ox

iMgurc 2. 1 1 . 2-Dimcnsional (X-Y) rcprcscntalion of a series of silicon surfaces

containing hexagonally arrayed square posts, X = 8 ^im and heights (Z) = 40 \im. Post

separations were varied as 32 fim x 32 |j.m, 32 |im x 80 |im, 80 |im x 80 |im.

43

Figure 2.12. SEM images of surfaces containing 8 |im x 8 |,im square posts with dilTcrcnt

spacings as described in figure 2.11. The water contact angles shown are for DMDCS-modified surfaces.

Water contact angle data for the silane-modificd surfaces shown in figures 2.10

and 2. 1 2 are reported in Table 2.4. The subscripts in the structure names indicate the

minimum distance between square post centers. The entry labeled i6fimSP'*"^"" is the

same surface as that labeled ^^'"\sp'*** (these squares are spaced by 1 6 |.im) in Table 2.3

and is displayed there for comparison. Changing the shape of the posts from square to

44

indented square, star or staggered rhombus did not affect the advancing contact angles,

but caused significant increases (up to 22°) m receding contact angles and con-esponding

decreases in hysteresis. The receding contact angles were higher for the staggered

rhombus posts than those for the star-shaped posts which were higher than those for the

indented square posts. We interpret these changes as due to more contorted and longer 3-

phase contact lines. Increasmg the spacing between the posts also resulted in no changes

in the advancing contact angles and increases in the receding contact angles (up to 2(r)

for square posts spaced up to 32 ^im. We interpret these increases as due to decreases in

the contact length of 3-phase contact lines. Contact angles decreased significantly for the

substrate with posts spaced by 56 )nm. Water intruded between these posts, increasing

the contact length of the 3-phase contact line.

Table 2.4. Water contact angle data for silane-modified surfaces containing differentshaped posts and 8 |im x 8 |im square posts spaced at various distances (described in

figures 2.10 and 2.12).

e rDMDCS ODMCS FDDCS

Silicon Surface

OA Ba Gk Oa

176° 156" 174° 155° 168° 153°

jp40 ^im 175° 143° 173° 140° 169° 146°

|g|-p4() ^im 175° 149° 174° 147° 170° 148°

Qp4()16 ^m*^^ 173° 134° 173° 139° 170° 140°

Qn4() \im

23 \iuP^ 175° 146° 172° 147° 167° 147°

32 ixmP^ 173° 154° 172° 154° 169° 155°

c p4()56 ^ini'^*^

121° 67° 112° 68° 134° 92°

45

In addition to the surfaces discussed above, in order to observe the pinning effect

of the water droplet upon formation of a continuous contact line of water droplet on the

surface, we prepared two surfaces described in figure 2.13. Both of these surfaces

treated with DMDCS and resulted in high advancing but low receding water contact

angles. Hence, the water droplets were pinning on these surfaces that exhibited high

hysteresis.

were

9A/eR= 161°/ 110° eA/eR= 161° / 137°

^1 llMi^"

i^ffi ^IttHI llMi

(a) (b)

Figure 2.13. SEM images of surfaces containing (a) 32 x 32 |im square posts; (b) 32|im X 32 |im hollow squares. The water contact angles shown are for DMDCS-modifiedsurfaces.

We attempted to view the 3-phase contact line between water and the surfaces

reported here using optical microscopy - with no success, shown in figure 2. 14. The

droplets acted as lenses and we could not focus on the contact lines. To overcome this

problem, we put drops of molten Woods metal on the surfaces and viewed the contact

lines after the metal solidified and was removed from the surfaces. Figure 2.15 shows an

optical micrograph of the Woods metal - silicon - air 3-phase contact line on the silicon

46

substrate and also on the surfaces named 23 ..SP^"^^"^

and 32 ,n,SP^"^'"^

in Table 2.4. The

contact line was clearly contorted by the square posts on the surface.

Figure 2.14. Snapshot of a water droplet on the substrate with star-shaped posts,hydrophobized with DMDCS.

25 |im 25 iim

(a) (b) (c)

Figure 2.15. Optical micrographs of the Woods metal - silicon - air 3-phase contact line

on the (a) silicon substrate; surfaces named (b) 23 nmSP'*^^"^ and (c) 32 pmSP''° in Table

2.4. The contact line is clearly contorted by the square posts on the surface.

47

Laplace Pressure

Walcr starts penetrating between the hydrophobic posts iflhe hydrostatic

pressure, AP, is exceeded. The general equation lor this phenomenon is as follows:"

AP =-(p

y'" cos ()a)/A^4^^

where p is the perimeter ofthe unit cell, A is the area of air space in the unit cell, y'^'is the

surface tension of liquid (in this case, water), Oa is the advancing water contact angle

(greater than 90°). AP was calculated for the hydrophobic surfaces prepared and it is

listed in Table 2.5. 2 square posts had the highest pressure, followed by the 8 |iim

square posts and staggered rhombus-shaped ones, fhc pressure trend was consistent with

the hydrophobicity ofthe surfaces and their dynamic contact angles.

Table 2.5. 1 .aplace pressure calculated using equation 4. 1

.

Silicon Surface AP (dync/^im^) X 10^

2 |ini^p40 jini 72.4

X )itn<^|j'1() |iin 18.0

1 () |iin^p4() fiiii 9.0

32 l^^^gp'^*^ 4.4

6-1 |iiii(^p'U) )ini 1.7

1 2H |im^^ j)'1f) )iin 0.5

23 |mi^<7.2

32 ^m^^'6.6

56 (im'-'*1.9

j^p40 fiin 9.6

48

Critical Tilt An^ ki

was

As the hysteresis decreases, the eritieal tilt angle at which the water droplet starts

rolling oir the surlace should also decrease, as expressed in Furmidge's equation

introduced in the lirst chapter. To observe this, we built an apparatus shown in figure

2. 1 6. A water droplet was put onto the DMnc^S-niodilied substrates and the whee

turned slowly until the droplet started rolling off the surface. The average of 5 tilt angles

for each surface was calculated and is given in Table 2.6. When cos (hysteresis) was

plotted against the critical tilt angle as shown in figure 2.17, an inverse relationship was

observed as predicted in Furniidgc's equation.

Figure 2.16. The apparatus used to measure the critical tilt angle at which the water

droplet starts to roll off the surface.

49

Tilt Angle (degrees)

Figure 2.17. Critical tilt angle as a function of hysteresis exhibited by DMDCS-treatedsilicon surfaces. As hysteresis decreased, the critical tilt angle decreased as expressed i

Furmidge's equation.

Table 2.6. Tilt angle and hysteresis exhibited by the DMDCS-modified silicon surfaces.

Silicon Surface Tilt Angle Hysteresis

smooth 29° 5°

2 |imgp40 jim 20° 35°

8 |.imgp40 jim 17° 40°

16 fimgp40 19° 27°

32 ^mgp40 22° 26°

64 |.imgp40 jim 35° 58°

128 ^mgp40 |im 30° 36°

6° 21°

jp40 19° 32°

g^p40 11° 27°

cp40m23 nmor

14° 29°

32 |imor7° 1

9°

^p40 urn56 iimoi

38° 53°

50

Conclusions

The wettabihty of a number of patterned silicon surfaces that were prepared by

photoHthography and hydrophobized using silanization chemistry was determined.

Surfaces contaimng square posts with X-Y dimensions of 2 ^m - 32 nm and similar

distances between the posts exhibited ultrahydrophobic behavior with high advancing and

receding water contact angles. Water droplets moved very easily on these surfaces and

rolled off of slightly tilted surfaces. Contact angles were independent of the post height

from 20- 140 ^m and independent of surface chemistry (siloxane-, hydrocarbon- and

fluorocarbon-modified surfaces were prepared). Surfaces containing square posts with

X-Y dimensions of 64 ^m and 128 ^m with similar distances between them were not

ultrahydrophobic- water droplets pinned on these surfaces and water apparently intmded

between the posts. Increasing the distance between posts caused increases in receding

contact angles up to the point that water intruded between the posts. This is due to

decreases in the contact length of the 3-phase contact line. Changing the shape of the

posts from square to staggered rhombus, star or indented square also increased the

receding angles due to the more contorted contact lines that form on these surfaces. The

maximum length scale of roughness that imparts ultrahydrophobicity is -32 |im.

Although the surfaces explored here are extremely hydrophobic, they are not as

hydrophobic as many of the surfaces that we have prepared that contain random

roughness (perhaps at multiple length scales). ' We believe that this is due to two

effects: First, on these surfaces that were prepared by photolithography and etching, the

51

3-phase contact line is tortuous only in the X-Y plane of the surface (all of the posts are

the same height). Contact lines on randomly rough surfaces are tortuous m 3 dimensions,

thus the contact lines can be longer and less stable. Second, these lithographed surfaces

are very regular and the contact line can register with the posts in metastable geometries

that will differ around the perimeter of the drop as a function of the angle between the

contact line and the hexagonal spacing direction. We predict that surfaces with randomly

arrayed posts of different heights will be even more hydrophobic than those reported

here.

52

Notes and References

1) Cassic, A. B. D. Discuss. Faraday Soc. 1948, 3, 11.

2) ' k Microlilho^raphv, 2'"'cel., Thompson 1 ,

!•

DC (^aplcM^' "''^ ^'''^ Washmglon"

3) Bowdcn, M. .1. in Introduction to Microlithoi^rapliv, 2"''cd., Thompson 1 F •

Willson, C. G.; Bowdcn, M. .!., luls; American dicmical Society WashinutcinDC, 1994, Chapter 2. ^ ^

4) Mncha, .1. A.; Hess, D. W.; Aydil, E. S. in Introduction to Microlitlio^niphy 2"'

ed., Thompson, L. F.; Willson, C. G.; Bowdcn, M. .1., Hds; American' ChemicalSociety, Washington DC, 1994, Chapter 5.

5) Powell, R.A. in Materials Processing Theory and Practices, Vol. 4: Dry etchingfor Microelectronics, North-Holland Physics Publishing, Netherlands 1984 n

'

119.' 'I

6) Bell, A. T. Solid State Tcchnol. 1978, 21, 89.

7) Winters, H. F. in Plasma Chemistry III; Topics in Current Chemistry Series 94,Springer-Vcrlag, Heidelberg, Germany, 1980, p 69.

8) Leyden, D. E., Ed., Silancs. Surfaces, and Interfaces, Gordon and Breach: NewYork, 1986.

9) Lisichkin, CJ.V., Ed., Chemically Modiped Silicas in Adsorption,

Chromatography and Catalysis, IChimiya, Moscow, 1986.

10) Plueddemann, E. P. in Silane Coupling Agents; 2"'' cd.; Plenum, New York, 1991

11 ) Mittai, K. L., Ed., Silane ami Other Coupling Agents, VSP, Zeist, 1 992.

12) Pesek, .1. J., Leigh, 1. E., Eds., in Chemically Modified Sinfaces; Royal Society of

Chemistry, Cambridge, 1994.

13) Chaudhnry, M. K.; Whitesides, G. M. Langmuir 1991, 7, 1013.

14) Fadeev, A. Y.; McCarthy, T. .1. Langmuir 1998, 14, 5586.

15) IJlman, A. MRS Bull, .lune 1995, 46.

1 6) Andrade, .I.D. in Surface and Interfacial aspects ofBiomedical Polymers,

Andrade, J.D., Ed., Plenum Press,'New York, 1985; Vols. 1 and 2.

17) 1 hoden van Vclzen, E. U.; Engberscn, .1. F. J.; Reinhoudt, D.N. ./. Am. Chem. Soc

1994, 116, 3597.

53

(18) Weiss, T., Scheirbaum, K.D.; Thoden van Velzen, U • Reinhoudt D N • Tnn.iW. Sens. Actuators 1995, B26-27, 203.i^einnouat, U.N., Gopel,

(19) Yang, X; Shi, J.; Johnson, S.; Swanson, B. Langmuir 1998, 14, 1505.

(20) Ingall M.D.K.; Honeyman, C.H.; Mercure, J. V.; Bianconi P A • Kunz R R /Am. Chem. Soc. 1999, 121, 3607. ' '

(21) Fadeev, A. Y.; McCarthy, T. J. Langmuir 2000, 16, 7268.

(22) Fadeev, A. Y.; McCarthy, T. J. Langmuir 1999, 1 5, 7238.

(23) Fadeev, A. Y.; McCarthy, T. J. Langmuir 1999, 15, 3759.

(24) Claudy, P.; Letoffe J. M, Gaget, C; Morell, D.; Serpinet, J. J. Journal ofChromatography 1987, 395, 73.

(25) Chen, W.; Fadeev, A. Y.; Hsieh, M.; Oner, D.; Youngblood, J.; McCarthy T JLangmuir 1999, 15,3395.^^ciuiiy, i

.

j.

(26) Youngblood, J. P.; McCarthy, T. J. Macromolecules 1999, 32, 6800.

(27) Dettre, R H.; Johnson, R. E. in Wetting; S. C. I. Monograph No. 25, Society ofChemical Industry, London, 1967, p 144.

54

CHAPTER 3

SURFACE MODIFICATION OF LOW DENSITY POLYETHYLENE

Motivation

The purpose of this phase of the thesis work was to extend the achievements of

observing ultrahydrophobic behavior discussed in chapter 2 to practical apphcations.

Conventional techniques such as lithography are capable of producing well-defined

structures with desired dimensions and shapes. However, these operations are expensive

and time-consuming and can be used to prepare only small samples. The motivation was

to explore a new practical way to make ultrahydrophobic surfaces. Low density

polyethylene (LDPE) was chosen since it is an inexpensive semicrystalline polymer and

there have been some studies in making its surface rough by various chemical reactions.

In order to impart roughness to LDPE, the following approach was taken.

Chromic acid etching of LDPE, known to form a patchy surface via etching the

amorphous regions prefentially faster than the crystalline regions, was perfomied. Then,

the oxidized polymer was deformed by uniaxial or biaxial tension at a constant rate and

the surface morphology of the necking region was examined by scanning electron

microscopy. The wettability of these rough surfaces was explored after gold coating and

hydrophobization using self-assembled monolayers of 1 -dodecanethiol.

55

Surface Treatments ofLow Denc^i'ty Polyethvlene

Surface modification of polyolefms has been studied over tlie last 40 years in

order to improve adhesion, printabihty and wettabihty properties. The most common

techniques used are plasma,'"' ozone and corona discharge treatments, '" '^chemical

etching,'^"2° photooxidation^'-^^and flame treatment.^^

Our interest was to utilize a surface modification technique to achieve rough

surfaces of LDPE. There are couple of different techniques in the literature where

researchers have observed rough surfaces upon such treatments. For instance, the

commercial pretreatment of corona discharge technique has been shown to cause some

surface roughening as a function of humidity and corona energy. Very recently, Feijen et

al. investigated the effects of CF4 gas plasma treatment on surface properties of

polyethylene (both high density and low density polyethylene).^*^ The LDPE's surface

microstructure changed from a lamellar one to a nanoporous-like one upon plasma

treatment (> 1 5 minutes) whereas this was not observed for HDPE. The etching rate of

CF4 gas was inversely proportional to the crystallinity of the polyethylene substrate used,

indicating that the amorphous phase was etched faster than the crystalline phase. Surface

fluorination occured with a XPS-determined atomic ratio of F:C = 1.4. The interesting

point to make here is that these researchers called the plasma-treated LDPE surfaces

'superhydrophobic' because they exhibited an advancing water contact angle of -140°.

However, they did not comment on the low receding contact angle of -95°.

56

The other effective technique of oxidizing polyolefins is chemical etching using

very strong oxidizing agents. Oxidants such as cliromic acid have been well documented

in the literature.^«-^°-3'^ Whitesides and coworkers have extensively characterized these

'polyethylene carboxylic acid' surfaces by contact angle measurement, XPS and ATR-IR

experiments.'' This technique yields species with hydroxyl groups and then with ketone,

aldehyde and carboxylic acid groups via chain scission.

CrOj/HjSO^/Hp

75 "C

Figure 3.1. Chromic acid oxidation of LDPE.

Whitesides and coworkers were the first ones to publish the surface morphology

of chromic acid-treated LDPE, as shown in figure 3.2." Recently, the surface properties

of the etched LDPE (up to 15 minutes reaction time) was also investigated with an in situ

contact mode scanning force microscopy (SFM) using gold coated and thiol modified

probe tips and also with TappingMode™ SFM. LDPE had an initial morphology

consisting of stacked lamellae/^^"'*^ These workers concluded that the amorphous phase

was etched prefentially by showing that the stacks of lamellae were more exposed to the

surface upon etching and that they started to corrode with etching time. The roughness

value increased from an initial value of -20 nm to --50 nm during 10 minutes of

oxidation.

57

(a) (b) (c)

Figure 3.2. SEM micrographs of (a) LDPE; and LDPE oxidized with chromic acidsolution for (b) 60 seconds; (c) 6 minutes."

Due to environmental considerations, large scale use of chromic acid is

undesirable. Hence, Price and coworkers studied milder oxidizing agents such as

K2Cr207 / H2SO4 , H2O2 and K2S2O8. By using a high-intensity ultrasonicator, they

attempted to promote the oxidation of the polyethylene surface via enhanced

decomposition of the oxidizing agents and production of excited species such as radicals.

The first agent did not give any indication of an oxidized surface, however the other two

resulted in oxidized LDPE surfaces. The paper did not report any surface roughness

analysis using these reagents but only stated that there was no visible change in LDPE

films.

58

Deformation of Thin Film Coatinf^s IJgcmUnuu^^

Deformation of a tiiin Him bonded to a thicker substrate has been the subject of

various pubhcations for the last two decades, mostly due to the concerns of the

electronics industry.^^ When an external force is applied to a substrate with a coating, a

stress is induced within the surface layer. The relaxation of the stresses developed can

lead to cracking^'*-''\ buckling^^'-^^ or delamination.^^'^*^ For instance, if the film is brittle

and the substrate it is bonded to is ductile, upon a tensile stress greater than the fracture

strength of the coating, cracks form.''^ These cracks then tunnel along the film.

'fhcrc are different crack patterns observed for biaxial and uniaxial stresses.

Biaxial stress results in cracks called 'mud cracks' because they are very similar to the

cracks observed upon drying mud."^" On the other hand, upon uniaxial stress long and

parallel cracks with approximately uniform spacings are observed.'''

Strawbridge et al. schematically showed the most common modes of coating

failure upon tensile stress, as shown in figure 3.3."''^ The relative ductility (or britlleness)

of the substrate and the coating, as well as the interfacial bonding are important in

identifying which of these tensile failure modes occur, as compared in Table 3. 1 . One of

the points they made was that the failure occurs in the favor of least resistance or the

greatest motivation. Hence, more irregular cracks would be observed if the stresses are

not distributed evenly in the system.

59

(a)

(b)

(c)

Figure 3.3. Possible tensile failure mechanisms for thin coatings bonded to a substrate

(a) through-thickness cracking; (b) through-thickness cracking followed by substrate

failure; (c) through-thickness cracking followed by interfacial delamination.'^^

Table 3.1. Tensile failure modes for thin brittle films52

Film Substrate Interface Bonding Decohesion Mechanism

Brittle DuctileGood Film cracking-no decohesion

Poor Film cracking-interface decohesion

Ductile BrittleGoodPoor

Film cracking-interface decohesion

Edge decohesion at interface

60

One of the pioneer studies in this field is by Volynskii and coworkers."-^^ They

work on the deformation of a polymer such as PET with thin metallic coatings. In one of

their recent papers, they studied the mechanism of formation of cracks upon tensile

drawing. They coated unoriented PET with a 4-20 mn thick platinum." The coating was

bonded to the sample well so that there was no slip upon stretching. When the polymer

was drawn uniaxially, they believed that the stress was not uniform throughout the

sample, as shown in figure 3.4. The maximum stress was at the center of the coating that

initiated the nucleation of a crack. As the sample was stretched more, the stress grew

bigger and ultimately, a crack formed. In another publication, they stressed on the fact

that the interfacial adhesion was important in forming cracks.^^'* When the energy of

deformation of the coating was greater than the energy of adhesion, the coating started to

peel off.

Figure 3.4. Schematic representation of distribution of stress throughout the metal

coating bonded to a polymer support. Stress is maximum at the center of the coating.

This grows with stretching and ultimately there is a crack forming at the center of the

coating.""

61

They studied the fracture of gold, platinum and carbon coatings (1-15 nm) on a

PET support upon tensile deformation/^ They observed that the coating failed in the

necking region resulting m ribbon-like stripes of platinum coating throughout the sample,

as shown in figure 3.5. The unnecked region remained smooth without any feilure

occurring.

Figure 3.5. SEM micrograph of a deformed 3.8 nm platinum coating on PET support.

The strain was not reported. The strain rate was 1 mm/min. The bar is 5 ^im.^'

On platinum coated PET, they found out that at low strains (5-15%), cracks were

not distributed uniformly across the sample.""^^

At higher strains (-100%), the crack

distribution was regular in the form of stripes aligned with each other. At further

elongation, the distance between the cracks grew larger. Also, the faster the strain rate,

the higher the number of cracks formed at a given strain. An important observation was

that when PET was coated with aluminum which is more ductile than platinum, there

were no cracks formed at low elongations. The thickness of the coating is another

62

parameter that affects the crack formation. The thinner coatn.gs had less cracks than the

thicker ones. Tn other words, they argued that there was a minnnum thickness of a

coating after which it would start fonning cracks.

Letterier and coworkers also studied the fracture failure of silica coatings on PET

films.'' They showed that the coating thickness was an important parameter that affected

the crack density and the cohesive strength of the coating. They observed a higher crack

density at saturation for thinner coatings, as shown in figure 3.6. Also, a thinner coating

started to crack at a higher strain than a thicker one due to its cohesive strength.

5(1 100 150

Coating Thickness (nm)

200

Figure 3.6. Crack density and crack onset strain as a function of coating thickness. ''^ The

thicker the coating was, the lower the onset strain and smaller crack density at saturation.

63

Experimental

Materials

Low density polyethylene film (0.07 mil) was a gift from Flex-O-Film

Corporation. To ensure reproducibility, it was always made sure that the film surl^ice

facing inside the role was reacted and analyzed. Chromium (VI) oxide (Aldrich, 99%),

sulfuric acid (Fisher, Certified ACS Plus grade), methylene chloride (Fisher, Optima

grade), anhydrous ethanol (Aldrich, 99.5%) and 1-dodecanethiol (1-DCT) (Aldrich, 98%)

were used as received.

General Methods

House purified water (reverse osmosis) was further purified using a Milliporc

Milli-Q® system that involves reverse osmosis, ion exchange and filtration steps (1()"^

ohm/cm). Contact angle measurements were made using a Ramc-Hart telescopic

goniometer with a 24-gauge flat-tipped needle; dynamic advancing and receding angles

were recorded as the probe fluid, water, purified as described above, was added to and

withdrawn from the drop, respectively. Scanning electron micrographs (SEM) were

obtained using a JEOL-35CF scanning electron microscope with an accelerating voltage

of 20 kV. X-ray photoelectron spectra (XPS) were obtained using Perkin-Elmer-Physical

Electronics 5100 spectrometer with Mg K„ excitation at takeoff angles of 15" and 75".

Atomic concentrations of the substrates were calculated using the sensitivity factors, Cis,

64

u.2iu; U,„ 0.660; Au,„ 4.950 and 0.540, oblaincd from the samples of known

composition.

Preparation of Polyethylene Substrates

The polyethylene films were eut into 3 x 1 .5 inch pieces, placed in a thimble and

extracted in a Soxhlet extraction apparatus using methylene chloride (or 2-3 days. They

were then dried in yacuum overnight.

Oxidation of Polyethylene Substrates

The polyethylene substrates were placed in a custom designed (slotted hollow

glass cylinder) sample holder and were oxidi/ed by submersion into a stirred mixture of

concentrated sulfuric acid, chromium trioxide and water (29:29:42 by weight) at 75 "C

for 1,5, 1 0, 1 5, 30 and 50 minutes. The oxidi/ed substrates were then rinsed with

copious amounts of purified water and dried in yacuum oycrnight prior to uniaxial and

biaxial deformation.

Uniaxial Streching of LDPE

The 5 minutes and 15 minutes oxidized polyethylene films were drawn uniaxially

at extensions of 60, 120, 130 and 150 mm, and 30, 60, 80, 120, 150 and 200 mm,

respectiyely at a rate of 25 mm/min, using a Instron 441 1 mechanical testing machine.

65

Tlic extension values were the programmed values at wluci, the maehme stopped

streching the fdm. The strains were suhscc,ucntly calculated hy measurmg the distance

between two marked points on the Him before and after necking. Taking the ratio of the

difference between the Tmal and the initial distances, over the initial one gave the strain.

Biaxial Streching of LDIMi

3x3 inch polyethylene lilm oxidi/ed for 5 minutes was biaxially streched using ;

home-built apparatus in Lcsser's laboratory. The dim was constrained at all sides and at

a controlled How rate, it was blown biaxially with flowing water from underneath. The

film was then characterized by SRM.

Self-Assembled Monolayers of 1 -dodecanethiol

The LDPli substrates were coated with 130 A gold using a Polaron Instruments

SEM Coating Unit (E51()()) at 15 niAmps for 2 and a half minutes. They were then

placed in a custom designed (slotted hollow glass cylinder) sample holder and

subsequently immersed into a freshly prepared 10 niM solution of 1 -dodecanethiol (1-

DCT) in anhydrous ethanol for 24 hours. After that, the substrates were rinsed with

anhydrous ethanol twice, and immersed in anhydrous ethanol for 2 hours prior to drying

in vacuum.

66

Results aiul Discussions

Oxidation of Polyethylene Substrates (o-LDPLi)

LDI>1< substrates were oxidized in chromic acid solution and the oxidation

kinetics was studied. Throughout the discussion, the LDPE fdms treated with chromic

acid solution are abbreviated o-LDPE. The reaction was carried out for 1,5,1 0, 1 5, 3()

and 50 minutes. The atomic concentrations were calculated from the XPS spectra as

shown in Table 3.2. As observed from the SEM images in figure 3.7, the chromic acid

solution etched the amorphous regions much faster than the crystalline regions. Thus,

after 5 minutes of oxidation, the bundles of crystalline lamellae were more visible on the

surface. These lamellae started to get corroded at longer oxidation times. I he advancing

and receding water contact angles were Oa / 0|< -116" / 84" for a clean LDPE substrate,

and they decreased to Oa / 0|< 73" / 6" upon oxidizing for I minute. After 10 minutes of

oxidation, the dynamic contact angles stayed almost constant with a high hysteresis. The

oxidized LDPE films were Anther gold coated and hydrophobizcd with self-assembled

monolayers (SAMs) of 1 -DCT. The dynamic water contact angles were about the same

for all these hydrophobizcd surfaces, Oa / 0|< 165" / 90". fable 3.3 siiows the dynamic

water contact angle analysis for the LDPE surfaces that were oxidized and then

hydrophobizcd using SAMs of 1 -DCT.

67

oxicli/cd 1.131 L (o-LI)Pl,) with chromic acid lor 1,10, 30 and 50 minutes at 75 T.

%C

LDPE

15° 75°

98 .88 99.58

o-LUPE

(1 min)

o-LDPE(10 mill)

o-LDPE

(30 min)o-LDPE

(50 min)

91.64 94.25 91.48 93.81 91.84 92.39 91.71 93.07

"-""Q ^-^2 0.42 8.36 5.75 8.52 6.19 8.16 7.61 8.29 6.29

1 5 minutes

Figure 3.7. SEM mierographs of LDPE substrates that were oxidized with ehromie aeid

for 5, 15, 30 and 50 minutes at 75 '^C.

68

Table 3.3. Dynamic water contact angles for oxidized LDPE (o-LDPE) substr ites thatwere then gold-coated and hydrophobized by SAMs of 1-DCT (o-LDPE/Au/l-DCT).

Oxidation Time o-LDPE(minutes)

g

o-LDPE/Au/1-DCTA Oa Gr

1 73" 6" 153"

5 91° 11" 152°

92"

92"

89"

90"

30 94" 9° 160° 90"

10 90" 9" 161"

15 92" 9° 167"

50 88" 8" 167" 106"

Uniaxial Deformation

First of all, for a control experiment, unoxidized polyethylene films were drawn

uniaxially at strains (c) of 260% (extension = 80 mm), 314% (extension = 120 mm) and

570%) (extension = 200 mm) with 25 mm/min strain rate. The surface morphology was

then studied by SEM. Figure 3.8 shows that there was no significant surface roughness

evolving upon necking of the unoxidized polymer. The surfaces were relatively smooth,

with some buckling observed for the surfaces strained at 260%.

The polyethylene films, oxidized for 5 minutes, were uniaxially streched at

extensions of 60 mm, 100 mm, 120 mm, 130 mm and 150 mm, and the ones oxidized for

15 minutes were streched at extensions of 30 mm, 60 mm, 100 mm, 120 mm, 150 mm

69

3KU mm m\ l eu jeol

2eKu Kiseee 0001 i.eu jeol

Figure 3.8. SEM micrographs taken in the necking region of unoxidized LDPEsubstrates that were drawn uniaxially at strains (e) of 260% (extension = 80 mm), 314%(extension = 120 mm) and 570% (extension - 200 mm). The surface microstructure did

not change with strain.

and 200 mm, at a rate of 25 mm/min. The corresponding strains were calculated and

plotted against extension, as shown in figure 3.9. At the first stages of drawing the

polymer, the strain increased linearly up to -300%, then stayed almost constant until it

reached 150 mm extension. After that, due to the strain hardening of the polymer, the

strain started to increase again. We expected to see a surface morphology change

70

between the three distinct regions, that is at low strains, at the pi

high strains.

ateau region and at very

(a)

X/1

80 100 120 140 160

Extension (mm)

(b)600

500

400

• 1—1300

cd

4—

>

200

100

00

a

50 100 150 200

Extension (mm)250

Figure 3.9. Strain as a function of extension for LDPE films that were oxidized for (a) 5

minutes and (b) 1 5 minutes. Strain was calculated by measuring the distance between

two marked points on the LDPE film before and after necking. Taking the ratio of the

difference between the final and the initial distance, over the initial one gives the strain:

Strain = e = (final distance - initial distance)/initial distance .

71

^^^^^^^^^^^^^^^^In the lUerature discussed

above, it was shown that thinner brittle coatings resulted ni smaller fragments after

deformation than the thicker ones. We believe that the oxidation reaction can be

considered as forming a thm brittle coating on the polymer. The thickness of this coating

can be varied by controlling the oxidation time. According to this argument, oxidizing

LDPE for 5 minutes resulted in a thinner brittle layer on the surface than oxidizing it for

15 minutes. The microstructure resulted from the deformation of 5 minutes oxidized

substrate was compared with the 1 5 minutes oxidized one.

Figure 3.10 shows the SEM micrographs ofLDPE substrates that were oxidized

with chromic acid for 5 minutes and then necked at strains of 140, 310, 320 and 400%.

The surface microstructure changed in the necking region with different strain values

which ultimately had an effect on wettability of these surfaces. The water contact angles

shown in the figure were after gold coating and reacting with 1-DCT. The surfaces

consisted of submicron size fragments and the size of the fragments were not observed to

change dramatically with strain. This was the reason why the water contact angles of the

drawn films were very similar to each other in the necking region up to the strain of

320%. These surfaces were hydrophobic with high advancing and receding contact

angles, but still exhibited hysteresis -25". At a higher strain of 400%), the fragments

became smaller and more distant from each other. The receding contact angle increased

to -150° at the strain of 400%, leading to an ultrahydrophobic behavior.

72

Figure 3.1 0. SI<M micrographs of LDPR substrates that were oxidi/cd with chroiuic acid

for 5 minutes and then drawn uniaxially at strains (c) of 140% (extension = 60 mm),314% (extension 120 mm), 320% (extension = 130 mm), 400% (extension - 150 mm).The surface microstructure changed with strain in the necking region of the polymer.

The water contact angles shown were after gold coating and reacting with l-DC"f (Au/1-

DCT).

Figure 3.1 1 shows the SFM micrographs of 15 minute oxidized LDIM' films that

were necked at strains ranging from 30 to 570%. When the o-Ll)PF was drawn to a

small strain of -30%), the formation of small cracks was observed. This type of surface

topography would not have much effect on wettability as proven by the dynamic water

contact angles of Oa / Or - 156" / 95 " after hydrophobi/ation. At strains > 250%, the

73

island formation became very visible in the necking region. These fragments were

considerably bigger in size than the ones formed upon necking the 5 minute oxidized

LDPE. There were couple of differences between the surface topography of the films

that were drawn at strains of-250% and -400%, shown m the figure. First of all, the

distance between the islands started to increase. Also, the island size became smaller

with the increase in stram. These were reflected in the wettability of the surfaces such

that the LDPE film strained at 250% exhibited 0a / Or ~ 1 59V 1 34° whereas the one

strained at 400% had higher advancing and receding water contact angles, 0a / 0r -167" /

150°. The interesting phenomenon was that when the oxidized LDPE was extended more

to a strain of -570%, the surface was no longer hydrophobic. The size of the islands

were < 0.5 ^m, and both the advancing and the receding contact angles decreased to 0a /

0R-158V 12r.

74

e = 260% Oa / Gr = 1 59" / 1 29" e = 250% Oa / 0,, = 1 63^ / \ 340

Figure 3.1 1. SEM micrographs of LDPE substrates that were oxidi/ed with chromic acid

for 1 5 minutes and then drawn uniaxially at strains (e) of 30% (extension = 30 mm),

1 80% (extension ^ 60 mm), 260% (extension = 80 mm), 250% (extension = 120 mm),

400% (extension = 150 mm) and 570% (extension = 200 mm). The surface

microstrueture changed with strain in the necking region of the polymer. The water

contact angles shown were after gold coating and reacting with 1-1)C'T (Au/l-DC f).

75

Biaxial Deformation of o-LDPE

Biaxial deformation was also performed usmg 5 minutes oxidized LDPE films.

An o-LDPE film, constrained at all sides, was blown biaxially by flowing water from

underneath at a controlled flow rate. This resulted in a hemisphere. The strain was

maximum at the top of the hemisphere and decreased going down the hemisphere.

Figure 3.12 shows the change in surface topography across the hemisphere. Part 1 was

the bottom of the hemisphere and hence it corresponds to the lowest strain. Part 2 and 3

were the regions with higher strain and Part 4 was the necked region at the top of the

hemisphere. The SEM micrographs in the figure indicated that the surface topography

changed with strain. As the strain increased from Part 1 to Part 3, the island fomiation

started to be more apparent. These are called 'mud cracks' in the literature.'^" The islands

were very discrete at the necked region. Part 5 was the unnecked region very close to the

necked region and its SEM micrograph shows that the islands were about to be fonned

with cracks in between them.

The wettability of the substrates was studied after gold coating with a thickness of

-130 A and hydrophobizing using self-assembled monolayers of alkanethiol. Figure 3.12

shows the corresponding dynamic water contact angles for each surface. The unnecked

regions with various strain values exhibited low advancing and receding water contact

angles. Since the islands were not discrete, the wettability of the surfaces was not very

interesting. The necked region was predicted to have a hydrophobic behavior, however

because it was a very small wrinkled area, contact angle measurements could not be

performed on that region.

76

Pari 1 eA/eR= 121V69° I>art2

0824 1.0U JE.

Qa/Or = 119"/ 73"

Part 3 0A / Ok - 1 21

" / 77" Part 4 Oa / ()„ = can not be determined

20KIJ xi?0e9 oe24 i.eu jeol

Parts Oa/Gr^ 121V75"

Figure 3.12. SEM micrographs of LDPE substrates that were oxidized with chromic acid

lor 5 minutes and then biaxially streched. The surface microstructure changed with

strain. I^art 1 had the lowest and Part 4 had the highest strain. The strain increased in the

order: Part 1 < Part 2 < Part 3 < Part 4 > Part 5. The water contact angles shown were

after gold coating and reacting with 1-DCT (Au/l-DCT), except for Part 4 whose contact

angles could not be measured due to the wrinkled necked region.

77

Deformation of Gold Coating Bonded to LDPE

were50 nm thick gold film was sputtered coated onto clean LDPE films that

further deformed uniaxially with strains of 140%, 310%, 320% and 400%. Figure 3.13

shows the SEM micrographs of the streched films at the necking region. All of the

substrates except for the last one shown m the figure were stretched at a stram rate of 25

mm/min. It was observed that there were small cracks forming at 140% strain and the

long gold stripes became more apparent at higher strains. The length and the distance of

the stripes did not change between the strains of 300% and 400%. The strain rate also

affected the surface morphology. At 400% strain, the stripes were more separated from

each other if streched at a faster rate.

The wettability of the substrates was studied after gold coating with a thickness of

-100 A and hydrophobizing using self-assembled monolayers of 1-dodecanethiol. In the

figure, the dynamic water contact angles were also shown for each surface. The film

streched at a low strain of 140% exhibited Ga / Or = 150V 1

1

T. Both the advancing and

receding contact angles increased to -168" / 134" at higher strains when the gold stripes

were more distant from each other. The contact angles of the surfaces remained very

similar to each other at 300%) and 400%) strain. The surfaces were ultrahydrophobic

leading to the water droplets rolling off of them at low tilt angles.

78

8 = 400% Oa/ Or - 165V 138"

rate = 15 mm/min

Figure 3.13. SEM micrographs of virgin LDPE substrates that were coated with gold and

then necked at strains (e) -140% (extension = 60 mm), 310% (extension = 120 mm),320% (extension = 130 mm), 400% (extension = 150 mm). The distance between the

gold stripes increased with strain. The water contact angles shown were after gold

coating and reacting with 1-DCT (Au/l-DCT).

79

Conclusions

In this work, our aim was to explore a new practical way of tuning the wettability

of polyethylene surfaces, and ultimately make ultrahydrophobic polyethylene films by

controlling the surface microstructure. We achieved this through an oxidative etching of

LDPE followed by either uniaxial or biaxial deformation. Chromic acid etching of LDPE

led to the formation of a thin brittle surface layer. It etched the amorphous regions

prefentially at first. 5 minutes and 15 minutes oxidized surfaces had different surface

morphologies. Uniaxial deformation of LDPE and oxidized LDPE was perfonned with

strains ranging from 30% to 570%. The effect of oxidadon time and strain on the

formation surface microstructure was studied.

Upon uniaxial tension, the brittle surface of the polyethylene film started to

defomi into islands in the necking region. These fragments became smaller and more

distant from each other with the increase in strain. Also, the surface topography was

different depending on the oxidation time. 5 minutes oxidized LDPE consisted of smaller

fragments at the necking region compared to the 15 minutes oxidized one. When LDPE

was oxidized for 15 minutes, at low strains, the size of the islands were in the order of a

few microns, and the dynamic water contact angles were 6a/ 0r -169" / 122" after

hydrophobization. These contact angles were not significantly different from the

unstreched o-LDPE films with Ga / 6r -167" / 90" after hydrophobization. However, at

higher strains of 400%, the island fragments were further apart from each other and the

substrates started to exhibit ultrahydrophobic behavior with 9a / Or -1 67" / 1 50" after

80

hydrophobization. The strain limit for impartmg hydrophobic behavior was ^570o/o. At

this strain, the islands were very small and apart from each other, hence the receding

water contact angle dropped sigmficantly to 121". They were no longer

ultrahydrophobic.

Biaxial deformation resulted in a microstructure called 'mud cracks', but with

interesting wettability. The water droplets pinned on the samples.

no

Deformation of a gold coating on LDPE resulted in the formation of stripes of

gold perpendicular to the streching direction. The distance between the gold stripes

increased with strain. The surface morphology as well as the wettability of the substrates

were very similar at high strains, with Ga / Qr -168" / 139" after hydrophobizing with an

alkanethiol.

81

Notes and References

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Div. Polym. Chem. 1969, 70, 1271.

Blais, P.; Carlsson, D. J.; Wiles, D. M. J. Polym. ScL, PartA-1 1973, W, 1077.

Fumeaux, G. C; Ledbury, K. J.; Davis, A. Polym. Degrad. Stab. 1981, i, 431.

Adams, J. H. J. Polym. Sci, PartA-1 1970, 8, 1279.

Heacock, J. F.; Mallory, F. B.; Gay, F. P. J. Polym. Sci., PartA-1 1968, 6, 2921.

Davidson, R. S.; Meek, R. R. Eur. Polym. J. 1975, 77, 85.

Owens, K. K. J. Appl. Polym. Sci. 1975, 79, 265.

Olde Riekerink, M. B.; Terlingen, J. G. A.; Engbers, G. H. M.; Feijen, J.

Langmuir 1999, 75, 4847.

Rasmussen, J.R.; Stedronsky, E.R.; Whitesides, G.M. /. Am. Chem. Soc 1977 994736.

Ferguson, G.S.; Whitesides, G.M., in Modern Approaches to Wettability, M. E.

Schrader and G. I. Loeb, Eds., Plenum, New York, 1992, p 143.

Holmes-Farley, S. R.; Reamey, R. H.; McCarthy, T. J.; Deutch, J.; Whitesides, G.

M. Langmuir 1985, 7, 725.

S. Wu, in Polymer Interface and Adhesion, Marcel Dekker, New York, 1982, p

279.

Brewis, D.M.; Birggs, D. Polymer 1981, 22, 7.

Holloway, F.; Cohen, M.; Westheimer, F. H. J. Amer. Chem. Soc. 1973, 9151, 65.

Wiberg, K. B.; Eisenthal, R. Tetrahedron 1964, 20, 1 151.

83

(38) Schonherr, H.; Vancso, G. J. J. Polym. ScL, Polym. Phys. 1998, 36, 2483.

(39) Smith, P. F.; Chun, I.; Liu, G.; Dimitrievich, D.; Rasbum, J.; Vancso G J.Polym. Eng. Sci. 1996, 36, 2129. '

'

'

(40) Kato, K. J. App. Polym. Sci. 1 977, 27,2735.

(41) Bassett, D. C, in Principles ofPolymer Morphology, Cambridge UniversityPress, Cambridge, 1981.

(42) Price, G. J.; Keen, F.; Clifton, A. A. Macromolecules 1996, 29, 5664.

(43) Thouless, M. D. Annu. Rev. Mater. Sci. 1995, 25, 69.

(44) Gill, G. in Current Topics in Materials Science, Kaldis, E., ed., Amsterdam/NewYork, North Holland, 1985, 12:420, p 72.

(45) Thouless, M. D. J. Vac. Sci. Technol 1991, A9, 2510.

(46) Nir, D. Thin Solid Films 1 984, / 72, 4 1

.

(47) Amaratunga, G. A. J.; Welland, M. E. J. Appl. Phys. 1990, 68, 5140.

(48) Kendall, K. J. Phys. D. Appl. Phys. 1971, 4, 1 186.

(49) Thouless, M. D. Acta Metall. 1988, Jd, 3131.

(50) Garino, T. J.; Bowen, H. K. J. Am. Ceram. Soc. 1987, 70, C315.

(51) Nagl, M. M.; Saunders, R. J.; Evans, W. T; Hall, D. J. Corros. Sci. 1993, 35, 965.

(52) Strawbridge, A.; Evans, H. E. Engineering Failure Analysis 1995, 2, 85.

(53) Volynskii, A. L.; Voronina, E. E.; Lebedeva, O. V.; Yaminskii, I. V.; Bazhenov,

S. L.; Bakeev, N. ¥.Vysokomolekulyarnye Soedineniya Seriya A 2000, 42, 262. (in

Russian)

(54) Volynskii, A. L.; Lebedeva, O. V.; Bazhenov, S. L.; Bakeev, N. F.

Vysokomolekulyarnye Soedineniya Seriya A 2000, 42, 665. (in Russian)

(55) Volynskii, A. L.; Voronina, E. E.; Lebedeva, O. V.; Yaminskii, I. V.; Bazhenov,

S. L.; Bakeev, N. F. Vysokomolekulyarnye Soedineniya Seriya A 2000, 42, 658.

(in Russian)

(56) Volynskii, A. L.; Voronina, E. E.; Lebedeva, O. V.; Yaminskii, I. V.; Bazhenov,

S. L.; Bakeev, N. F. Vysokomolekulyarnye Soedineniya Seriya A 1999, 41, \627.

(in Russian)

84

(57) Letterier Y, Andersons, J.; Pitton, Y.; Manson, J.-A. E. J. Polym Sci Pari BPolym. Phys. 1997, 35, 1463. ^ ^

85

CHAPTER 4

ADSORPTION OF POLYSTYRENE COLLOIDS ONTO POLYELECTROI YTFMULTILAYER ASSEMBLIES AND WETTING BEHAVIOR OF THESE ROUGHSURFACES

Introduction

In the literature, one of the ways that the researchers have tried to impart

roughness to the solid surfaces is hy using spherical particles such as PTFE particles,'"^

silica particles^"^ and glass beads.*^ Our attempt is to prepare a series of micron and

submicron scale rough surfaces by adsorbing monodisperse polystyrene lattices and

explore their wettability. The polystyrene colloids with carboxylic acid groups are

adsorbed onto the polyelectrolyte multilayer assemblies that have a cationic

polyelectrolyte as the outermost layer. The kinetic study of the adsorption of colloids

(0.35 |nm and I jum) onto the polyelectrolyte multilayers is performed to have an in-depth

understanding of the adsorption phenomenon and monitored by atomic force microscopy.

The adsorption occurs without any three-dimensional aggregation. The roughness (and

hence the wettability) can be precisely controlled by varying both the diameter of the

polystyrene colloids and the adsorption time. Solution casting of colloids onto the

polyelectrolyte multilayers is also performed. All of the surfaces prepared are then

hydrophobized by gold coating and reacting with an alkanethiol to form self-assembled

monolayers. The wettability is assessed by dynamic contact angle measurement using

water and hexadecane as probe fluids.

86

Polyelectrolytc Multilayer Assnmhli.>Q

In the 1960's Ilcr developed a technique for adsorbing oppositely charged

colloidal particles such as silica and alumina onto a substrate.' About three decades later,

this work was extended by Decher and his coworkers""" to form multilayer assemblies

of polyelectrolytes. Layer-by-layer assembly of polyelectrolytcs has been used to make

ultrathin organic films. The principle of constructing multilayer assemblies is to adsorb

anionic and cationic polyelectrolytes consecutively onto the substrate. The adsorption is

driven by electrostatic attraction between the opposite charges of the polyelectrolytes.

The film thickness can be controlled precisely because it is linearly proportional to the

number of adsorbed polyelectrolytc layers. Depending on the ionic strength of the

polyelectrolytc solution, the thickness of the individual layers can also be modified. For

instance, in high salt concentrations, the electrostatic repulsions between the segments are

screened and the polymer adsorbs with more loops, hence a thicker layer is observed.'

The overall adsorption process has been shown to be independent of substrate size and

topography.

The formation of multilayer assemblies starts with immersing a positively (or

negatively) charged substrate in an anionic (or cationic) polyelectrolytc solution. The

polyelectrolytc adsorbs onto the substrate, and since a high concentration of

polyelectrolytc is used, the surface charge is reversed by the ionic groups remaining at

the interface of the substrate and the solution. Subsequently, the substrate is rinsed with

water and immersed in a cationic (or anionic) polyelectrolytc solution. The reversal of

87

surface charge occurs when the cationic polyelcctrolyle adsorbs onto Ihc anion.e

polyelectrolyte layer. If these steps arc repeated sequentially, n.ulflayer assemblies ar.

formed.

A wide range of substrates from inorganic to polymeric ones, has been used for

multilayer assemblies. For instance, glass can be surface modified by reacting with a

silane coupling reagent containing amine groups onto which anionic

polyelectrolytes are adsorbed. In the McCarthy laboratory, layer-by-layer deposition was

performed by using PMP,^^ 23 p^TFE,'' LDPE2^ and PTFE"''^' as substrates.

In this research, the cationic polyelectrolyte, poly(allylamine hydrochloride)

(PAH) " and the anionic polyelectrolyte, poly(sodium styrenesulfonate) (PSS)

were used since they arc among the most studied systems with well-established results. It

is known that twenty minutes is sufficient time for these polyelectrolytes to adsorb

irreversibly onto the charged substrate. There is one more important point that should be

stated. The complete reversal of surface charge and stratified layers were observed for

PSS-PAH multilayer systems after building 10 layers.'^"'' For the experimental work of

this research, it was crucial that the outermost layer was a positively charged surface so

that the adsorption of carboxylated polystyrene colloids was achieved.

88

Adsorption of ColK)i(ls onto Polyclcctiolvit-^

Solution casting, dip-coaling and laycr-by-laycr deposition arc the methods used

in the Uteraturc to make fihns of organized latex part.cles.^^ '"For instance, Krozcr and

coworkers made aUernative layers of sihea and alumina using a quartz crystal

microbalancc (QCM).'' Meanwhile, Lvov ct al. argued the fact that the use of

polyions as intermediate layers or as 'electrostatic glue' was important in the assembly ol

rigid particles. They made alternating assemblies of silica particles of 25, 45, 75 nm and

poly(diallyldimethylammonium chloride) and followed the mass change during the

assembly process by Q(^M.^" 1 lowcver, Ihey dk\n[ investigate the parameters to control

the number of Si02 layers adsorbed onto the polyelectrolyte lllm, hence they usually

observed three-dimensionally packed spheres. In addition to this, there are other studies

where scientists assembled alternating layers of inorganic colloidal nanoparticles of

CdS,-^"'"' gold,''^'''-^ and TiOVIMiS'*'''''^ with polyelectrolytcs. Although these have

accomplished assemblies in two and three-dimensional arrays, it is not until tiie recent

work of Akashi et al. where they attempted to study the adsorption behavior of colloids

more extensively.''^'

Akashi and coworkers investigated the alternating adsorption of polystyrene

nanospheres (84, 548, 780 nm) onto (he surface ol a polyelectrolyte film thai was

prepared by layer-by-layer assembly.'''' 'fhey believed that the adsorption was based on

the electrostatic interaction and they studied the adsorption behavior by using QCIVI.

They, first of all, assembled 7 alternating layers of PAll and PSS as the precursor film.

89

the top layer bc.ng I>AH. The adsorption of the polystyrene nanospheres was done at 10

"C and for up to 60 minutes, since they were concerned that the aqueous dispersion of the

colloids would aggregate after 60 minutes or at higher temperatures (eg. 15-20 "C). The

maximum surface coverage they achieved was 67% for the colloids with 780 nm and less

coverage for smaller diameters. They concluded that even though small diameter

colloids adsorb onto the surface, the maximum coverage was -55% for 548 nm and

~15"/o for 84 nm, as indicated in figure 4.1.

Figure 4. 1 . Dependence of the surface coverage on adsorption time at a nanosphcre

concentration of 3.8 x lO'" mL'' at 10 °C onto (PAH-PSS).rPAH surface that wasprepared in the presence of 2 M NaCl: (a) 780 nm; (b) 548 nm; (c) 84 nm.'^''

The interesting observation was that there was a critical film thickness of 4 A of

the outermost PAH layer needed for the colloidal adsorption. There was a dependence of

the adsorption behavior of the colloids on the preparation conditions of the

polyclectrolyte multilayers. For instance, as shown in figure 4.2, the surface coverage

90

increased from < 10% to > 55%, depending on if the polyeleetrolyle solutions were

prepared without salt or with 2 M salt, respectively. They also made the point that they

believed the adsorbed colloids did not aggregate or move two-dimensionaliy ui the dried

state since the electrostatic interactions were very strong between the colloids and the

polymer film.

Adsorption time / min

Figure 4.2. Time courses for polystyrene adsorption (diameter: 548 nm) at 10 "C onto

(PAl 1 - PSS), - PAH surfaces prepared in the presence of NaCl: (a) 2 M; (b) 0.2 M; (c)

0.02 M and (d) 0 M.'*''

Self-Assembled Monolayers of Alkanethiols on Gold

Self-assembled monolayer (SAM) systems have been extensively studied over the

last decade due to their potential scientific and technological applications from sensors to

microelectronics.^^ SAMs have the following advantages over the Langmuir-Blodgett

assembly: 1 ) various solvents can be used, and 2) stable films can be prepared. SAMs

91

can be formed either by chemisorption of the molecules with specific sites onto the

substrate, or by physisorption of the molecules driven by the long range forces. The first

paper about SAMs of thiols showed that dialkylsulfides (RS-SR) form oriented

monolayers on gold surfaces.^^ SAMs of alkanethiols on metal substrates are prepared by

the formation of strong bonds between the substrate and the thiol. For instance, the

hemolytic bond strength of methanethiolates on Au(l 11) is -44 kcal mol"'.^'' It has not

been clear how the mechanism of the reaction of thiols with gold surface occurs. The

formation of RS'—Au^ is believed to be either via the loss of the proton as H2 or as

RS-H + Au°n RS -Au" + Y2 H2 + Au°n (4. i

)

RS-H + Au°n + oxidant RS"-Au^ + V2 H2O + Au°n (4.2)

Chemisorption of long chain alkanethiols, HS(CH2)nX, from solution onto the

surfaces of metal substrates such as gold,^' copper,^^'^^ and silver^"* has been a versatile

method of preparing water-repellent surfaces by altering the chemical structure of those

Air-monolayer interface group

Chemisorption

at the surface

Figure 4.3. Schematic illustration of SAMs of thiols on gold substrate.

92

surfaces. For instance, (he surfaces with SAMs of n-alkancth.ols have very low si.rface

free energy(

1<) m.l/ni^). The nu,noh.yers are densely packed and ordered w.th n,c,r I.,

group oriented towards the nionolayer-air or monolayer-liquid uiterlace. SAMs lonu

smooth and homogeneous surfaces regardless of the underlying surface structure. The

evaporation of gold on silicon yields the ( 1 1 1) phase olgold (Au/Cr/Si)" on which the

alkanethiolates self-assemble with an all trans conformation, tilted ^0" IVoni the surface

normal.'"' The S—S spacings are 4.*)7 A and the calculated area per molecule is 21.4 A

resulting in a pseudohexagonal two-dimensional symmetry, as shown in figure 4.4." ''^

figure 4.4. STM image of an SAM of dodecanethiol on Au ( 1 1 1 ). Intcrmolecular

spacings arc 0.50 nm which is V3 times larger than the atomic spacing of gold atoms

(0.288 nm) in Au (1 1 1 )plancs.^'^

In the literature, there are two different scenarios' to explain the thiol-metal

surface interaction resulting in the cry.stalline-like order of alkanethiol monolayers, fhe

93

most likely mechanism is that the alkancthiols (Irst phys.sorb, and then chemisorh on the

gold surface after a lateral motion to form gold mercapl.de units. The other one .s that

the Au-S bond formed is covalent in nature and the R-S-Au assembling molecules have

enough lateral surface mobility. The fact that the longer alkyl chains result in n

ordered self-assembly due to the van der Waals attraction forces than the shorter alkyl

chains support the first scenario, while the second scenario can contribute to the long

time scale ordering.

1 more

The kinetics of the adsorption of alkancthiols were studied by Bain and

coworkers.^' For 1 mM concentration, there were two adsorption steps observed as

shown in figure 4.5. The first one was the fast step where the adsorption of an imperfect

monolayer took place within a few minutes, fhe advancing and the receding contact

angles almost reached their limiting values and the thickness was -80-90% of its

maximum. In the second step, the adsorption was much slower lasting for several hours.

There was a reduction in the defects and enhanced packing was observed. In

addition to these, Ulman found that a concentration of 10 niM can be used for SAMs of

alkancthiols.^"* There was also the ef fect of chain length on adsorption kinetics such that

the longer the alkyl chains, the faster was the adsorption due to stronger van der Waals

interactions.

94

30

^ 20 i

10

a

. • • J• • •

0 •>y/*-r T r— 1 r

120

mCD 60

40

20

0

100 1 o

80

• ~ OoHp

ft fi D O 0

HD

I I

• C10

O CIO

0 10' 10° 10' 10^ 10^ lO"

Time (min)

Figure 4.5. Kinetics of adsorption ofoctadecanethiol (solid symbols) and decanethiol(open symbols) from ethanol onto gold, (a) Ellipsometric thickness, (b) Advancingcontact angles of water and hcxadecane (HD).'''

Experimental

Materials

Si wafers (<100> orientation, P/B dopant, resistivity from 20 - 40 Q-cm,

thickness from 400 - 450 )im) were purchased from International Wafer Service.

Hydrogen peroxide (Fisher, 30%), sodium hydroxide (Fisher, Certified ACS Grade),

hydrochloric acid (Fisher, 1 N), sulfuric acid (Fisher, Certified ACS Plus Grade), ethanol

95

(VWR, 99.5%), (4-aminobulyl)dimcthylmcthoxysilanc (ABDMS) (United Chemical

Technologies), methanol (Fisher, I IPi.C grade), toluene (Aldrich, 99.9%), MnC1,.4l 1,0

(Fisher, Certified ACS Grade), poly(allyhnnine iiydroehlor.de) (PAll) (Aldnch, M„ =

500()()-650()()), poly(sodiLim styrcne suUbnale (I'SS) (Aldrich, M„ 7()()()()), 1-

dodccancthiol (1-DCT) (Aldrich, 98%) were all used as received. C^arboxylated

polystyrene colloids (Polyscicnces, 2.5% aqueous solution) with diameters ofO. 1 , 0.35,

0.5, 1, 2, 3.3 and 4.5 |,im and with concentrations of 3.64 x l()'\4.55x 1()'\ 1.06 x !()'

'

3.64X 10", 4.55 X 10'", 5.68 X 10", 1 .68 x 1 o", 4.99 x 10« particles per ml, respectively,

as estimated by an equation given in the catalog were used after diluting with water and

with no further puriHcalion. The pH values of the polyelectrolyte solutions and the

colloidal suspensions were adjusted with small amounts of cither HCl or NaOll using a

Fisher 825MP pll meter.

General Methods

House purified water (reverse osmosis) was further purified using a Milliporc

Milli-Q® system that involves reverse osmosis, ion exchange and llltralion steps (K)"^

ohm/cm). Contact angle measurements were made using a Rame-I lart telescopic

goniometer with a 24-gaugc flat-tipped needle; dynamic advancing and receding angles

were recorded as the probe fluids, water, purifled as described above, and liexadecane

were added to and withdrawn from the drop, respectively. X-ray photoelectron spectra

(XPS) were obtained using Perkin-Rlmer-Physical Rlectronics 5100 spectrometer with

Mg Ka excitation at takeoff angles of 15" and 75". The .sensitivity factors. Si is, 0.270;

9()

C. 0.250; O,. 0.660; N,. 0.352; Au..„ 4.950 and S,„ 0.540, obtained IVon. ,hc sa.nplcs

ofknown composition were used to calculate the XI>S atomic concentrations. The

colloids tend to aggregate and this was prevented usnig Branson Soniller 450

ultrasonicator. Atomic force microscopy (AI<M) m TappingMode'^^ was performed to

study the surface topography using a Digital Instruments Dimcnsion^^^ 3000. Both the

phase and the height images were recorded. The etched silicon tips on cantilevers

(Nanoprohe^^) have spring constants ranging from 40 to 66 N/m. Nanoscope section

analysis software was used to calculate the roughness ratio of the substrates.

Preparation of Silicon Substrates

The wafers were cut into 1.5x1 .5 cm pieces, placed in a custom designed (slottal

hollow glass cylinder) sample holder and were cleaned by submersion into a mixture of

concentrated sulfuric acid and hydrogen peroxide (7:3) overnight. They were then rinsed

with copious amounts of purified water and dried in a stream of nitrogen immediately

prior to the silani/ation reactions.

Pretreatmcnt of Silicon Substrates

Reaction of Silicon Substrates with (4-aminobutyl)dimelhylmcth()xysilane. The

freshly cleaned silicon substrates were placed in a custom-designed (slotted hollow glass

cylinder) sample holder and placed in a dry schlenk tube. 5% solution of (4-

aminobutyl)dimethylmelhoxysilane (ABUMS) in anhydrous toluene was then cannulated

97

into the schlenk tube. The reactions were carried out for 3 days at 80 °C. The substrates

were then rmsed with toluene (2 aliquots), 1 : 1 methanol/toluene (2 ahquots), methanol (2

aliquots) and Milli-Q water (3 aliquots), each for 3 minutes.

Layer-by-Layer Deposition of PolyelectrolYtes, 0.02 M aqueous solutions of PSS

and PAH (concentration based on repeat units) with 1 M MnCh were prepared and their

pHs were adjusted to 2.2 and 2.9, respectively, with dilute HCl solution. The ABDMS-

modified silicon substrates placed in a custom designed (slotted hollow glass cylinder)

sample holder were subsequently immersed into the PSS solution for 30 minutes. After

the deposition, the substrates were rinsed with three aliquots of water and immersed into

the 0.02 M aqueous solution ofPAH for 20 minutes. This was followed by rinsing the

substrate with water and immersing it into the PSS solution again. Alternating layers of

polyelectrolytes were formed when the substrates were immersed into PSS and PAH

solutions consecutively. A total of 10 layers of PSS and PAH was deposited. The first

PSS deposition took 30 minutes, and the following depositions took only 20 minutes.

The substrates were rinsed with aliquots of Milli-Q water three times between each

polyelectrolyte deposition.

Adsorption of Polystyrene Colloids onto Polyelectrolyte Multilayers

Carboxylated polystyrene colloids with diameters of 0.1, 0.35, 0.5, 1, 2, 3.3 and

4.5 ]xm were used. The latex solutions were diluted using fresh Milli-Q water and pH

was adjusted to 8.8 by adding small amounts of dilute NaOH solution. The substrates

98

soiulion waswith polyclcctrolylc multilayers were immersed into the latex solution. The s,

kept all the time in the ice bath and was sonieated every 10 muu.tes lor 10 seeonds. Alter

6 hours ofadsorption, the substrates were rinsed with Mill,-Q water (3 aliquots), eaeh for

3 minutes. They were dried in vacuum overnight.

Kinetic Study ol'the Adsorption Rdi;.vi,..- nrr.ji..i.|^ 0.35 ,im and 1 ,im

diameter colloids were used to study the kinetics ofadsorption behavior of colloidal

particles onto the polyelectrolyte multilayer assemblies. The adsorption of I ^,m

diameter colloids was carried out for 1/6, 1/2, 1, 2, 4, 6, 10, 24 hours, and that of 0.35 |Lim

for 1/2, 2, 4, 6 and 10 hours. The substrates were rinsed with Milli-Q water (3 aliquots),

each for 3 minutes. They were then dried in vacuum overnight.

Solution Casting Polystyrene Colloids onto Polyelectrolyte Multilayers

10% (by volume) suspensions of polystyrene colloids (0.35, 0.5, 1,2 and 3.3 uni)

in water were prepared, pi! was adjusted to 8.8 by adding small amounts of dilute NaOl I

solution. After buikliiig 10 layers of polyeleclrolyles with PAIl as the outermost layer,

the suspension was poured onto the substrates dropwise with a pipet. They were dried in

air for 3 days.

99

Coating and Self-Assembled Monolayers (S,AM^^ of l-dodecanethiol

A gold film with a thickness of ~1 10 A was thermally evaporated onto the

substrates at pressures < lO"' Torr with deposition rate of -0.2 nm/sec, usmg Thermiomcs

VE-90 vacuum evaporator. The substrates were immediately immersed in a freshly

prepared solution of 10 mM l-dodecanethiol (1-DCT) in anhydrous ethanol. The

reaction was carried out for 24 hours. After that, the substrates were rinsed with ethanol

(3 aliquots) and then were put into ethanol for 2 hours. They were dried in vacuum

overnight.

Results and Discussions

Pretreatment of Silicon Substrates

The silicon substrate was modified prior to the polyelectrolyte adsorption. After

being cleaned in the 'piranha solution', the silicon substrates were reacted with an amine-

terminated silane reagent (ABDMS). A typical XPS survey spectrum of the

substrate reacted with ABDMS is shown in figure 4.6 (a). The atomic nitrogen

concentration was calculated to be 4.39 at 15° takeoff angle, and 1.65 at 75° takeoff

angle, as indicated in Table 4.1. The dynamic water contact angle was 0a / Or = 80° / 32".

100

starting with the adsorption of PSS, 10 alternating layers ofpolyelectrolytes were

then built. Figure 4.6 (b) shows a representative XPS survey speetrum of silicon

substrate with 1 0 polycleetrolyte layers composed of PSS and PAl 1, with PAl 1 being the

outermost layer. Nitrogemsulfur ratio, calculated from 75° takeoff angle XPS atomic

concentration data, was ~1 .9. The water contact angles were Oa / Or = 91" / 8". These

results were in good agreement with the previous work done by l lsieh^' in the McCarthy

group.

1000.0 mo 800.0 700.0 Goo.o mn •loo.o m.o ?oo o loo oBIMDINC INtKCV. tV

niNlllNC INIIiCV. cV

Figure 4.6. XPS survey of a silicon substrate at 15° takeoff angle (a) reacted with

ABDMS for 3 days in toluene; (b) with 10 polycleetrolyte layers, PAIl as the outermost

layer.

101

)•.^PS concentration data for a clean silieon snhslrale. an ABOMS-

15" Takeoff Angle 75" Takeoff Angle

Substrate %Si %C %0 %N %S %Si %C %0 %N %S

Si 22.95 47.29 29.76 52.25 16.10 31.65

Si/ABDMS 24.87 38.48 32.26 4.39

Si/ABDMS/PSS-PAH(10 layers)

73.47 16.10 7.45 2.98

45.50 13.91 38.95 1.65

69.75 18.35 7.85 4.05

Adsorption of Polystyrene Colloids onto Polyelectrolyte Multilayers

Hsieh reported that after building up -10 multilayers, each polyelectrolyte layer

became stratified rather than being interpenetrated.''^ In all our experiments, the

adsorption of polystyrene colloids was performed onto the multilayer assemblies

consisting of 10 polyelectrolyte layers. After treating the silicon wafer with ABDMS, 1

0

alternating layers of PSS and PAH were deposited. Then, the substrates were immersed

into the dilute latex solution with a pH of 8.8. Although the polystyrene colloids were

negatively charged, they were observed to form aggregates on the surface if only stirred

at room temperature. This was overcome by using an ultrasonicator so that every 10

minutes, the solution in the ice bath was sonicated for 10 seconds. The wcllabilily of

these rough surfaces was explored before and alter they were hydrophohi/cd by

chemisorption of 1 -dodecancthiol (1-DCT).

102

The surface topography was imaged by performing AFM in TappingMode™.

The height images captured at 40 ^m x 40 ^m scanned areas were used in section

analysis software to calculate the Wenzel's roughness ratios. When performing such an

analysis, a straight line was drawn randomly across the scanned area, and the program

gave a graph of surface topography such as the one shown in figure 4.7. It calculated the

surface distance from a peak to a valley and the horizontal distance of the line. 8 or more

lines were drawn across a scanned area and the ratios of (surface distance)^ / (horizontal

distance)' were calculated. The Wenzel's roughness ratio was taken as their mean value.

I I

10.0 20.0 30.0 40.0

Surface distance 69.642 mhHoriz distanceCLJ 44,688 mm

Figure 4.7. AFM section analysis was performed using AFM height images to calculate

the roughness ratios of the substrates. The software provided both the surface distance

(peak to valley distance) and the horizontal distance. The roughness ratio was calculated2 2

using the following formula: r = (surface distance) / (horizontal distance) .

103

ionKinetic Study ofthc Adsorption Reh.vinrnf r^ iuj.,^

pi,3t ^,p.,„

behavior ofcarboxylatcd polystyrene eolloids onto polyclectrolyle nu.ll.layers with the

outermost layer being PAH was explored. 0.35 and 1 ^im diameter polystyrene

colloids were used for this purpose. The polystyrene colloids with 1 ^m diameter were

adsorbed for 1/6, 1/2, 1, 2, 4, 6, 10 and 24 hours. The Al'M height images were used to

calculate the Wen/el's roughness ratio of these surfaces. When the roughness ratios were

plotted against the time of adsorption in figure 4.X, it clearly showetl that the roughness

ratio increased linearly with time of adsorption until ten hours when it reached a value of

~2. 14 and then, it stayed almost constant. The A1<M images in figure 4.9 indicated that

as the adsorption time increased, the surface coverage of colloids also increased.

I I 1' I I I I -I 1 I I T T—I—r—

r

2.2 I-

2.1

S 2.0

</) 1.9en

.X

0^ 1 nO 1.7

1.6

1.5

1.4

0 5 10 15 20 25

Adsorplion Time (hrs)

Figure 4.8. The change in roughness ratio, calculated from the AFM height images, with

adsorption time of polystyrene colloids (1 ^im) onto the polyelectrolyte multilayers.

104

1 0 iiiiinilcs

(a)

1 .8 MM

0.9 MH

0.0 MM

Data tuptZ rangt

K«l«htl.fiO MM

40. U MN 0

Uatd 1g|>e

2 ranva Rd.Q da

40.0 tm

30 iiiiiiutcs

(b)

Data tui^a

7. ranueHeightl.RO MM

Data tupa2 rantfa

4(1.0 MMI'lta^e

fiO.O (le

60 iniiuilcs

(c)

Data tu|)*

2 ranwa 1 Jill MM

Data tui>«

2 ranuif

I'ij^iire 4.9. AI'M height (left) and phase (right) images of the surfaces obtained when (he

time of adsorption of polystyrene colloids ( I fini diameter) onto polyelectrolyte

multilayers was for (a) 10, (b) .10, (c) 60 minutes; (d) 2, (e) 4, (1) 6, (g) S, (h) 10 and (i)

24 hours, (continued next page)

105

2 hours

(d)

1.8 MH

0.9

0.0 MH • • .

(e)

Data type2 range

4 hours

Data typeZ range

6 hours

Height1.80 MM

40.0 MM 0

Data type2 range

Phase50.0 de

40.0 UM

40.0 MM 0Height1.80 iiM

Data typeZ range

40.0 MMPhase

50.0 de

(f)

40.0 MM 0 40.0 MM

Data typeZ range

Height1.80 MM

Data type2 range

Phase50.0 de

re 4.9 (continued).

106

8 hours

(g)

i.8 PH

0.9 MM

0.0 PM

(h)

Data type2 range

0 hours

Data type2 range

24 hours

Height1.80 MM

40.0 MM 0

Data typeZ range

Phase50.0 de

40.0 MM

Height1.80 MM

Data type2 range

40.0 MNPhase

50.0 ae

(i)

UM UTlAtA tui>p

2 J'«lbi|fc' l.SO un 2 J cuiyir 50.0

Figure 4.9 (continued).

107

The adsorption of polystyrene colloids witli 0.35 urn diameter was carried out lor

1/2, 2, 4, 6 and 1 0 hours. In contrast to 1 ^m, the adsorption kinetics for colloids w.th

0.35 diameter showed a faster adsorption occurring. The roughness rat.o had a sharp

increase with time in the Hrst two hours, and then leveled offal 1.9, as mdicaled ni

ngure 4.10. The surface coverage increased in a shorter period of time than observed for

I ^m colloidal adsorption, as seen from the AFM images in figure 4. 1 1 . 1 lence, wc

observed that the larger the diameter of polystyrene colloids, the more time it needed to

achieve a high surface coverage. It was not very clear why the surfaces with I

diameter colloids had relatively big gaps between the colloids, even at 24 hour

adsorption. We believe that the adsorption behavior was different for 0.35 ^im and 1 [im

due to the fad that the adsorption of colloids from the solution to the substrate was

0 2 4 6 8 10

Adsoiplion Tunc (hi s)

Figure 4. 1 0. The change in roughness ratio, calculated from AFM height images, with

adsorption time of polystyrene colloids (0.35 [im) onto the polyelectrolytc multilayers.

108

difftision-controlled at the early stages. The smaller the diameter, the easier for the

negatively charged colloids to difftise from the solution and adsorb onto the positively

charged surface. Also, the smaller diameters might have higher surface charge density

that resulted in faster adsorption. This difference in the adsorption behavior was also

shown, quantitatively, in the analyses of the evolution of roughness ratios with time of

adsorption, in figures 4.8 and 4.10.

1 12 hour

(a)

600.0 nM

300.0 HM

0.0 HM

Data type2 range

HeightBOO RM

40.0 MN 0

Data typeZ range

Phase60.0 de

40.0 MM

2 hours

(b)

40.0 MMData typeZ range

Phase60.0 de

Figure 4.11. AFM height (left) and phase (right) images of the surfaces obtained when

the time of adsorption of polystyrene colloids (0.35 |im diameter) onto polyelectrolyte

multilayers was for (a) 1/2, (b) 2, (c) 4, (d) 6 and (e) 10 hours, (continued next page)

109

4 hours

(c)

600.0 HH

300.0 nM

0.0 nM

Data typeZ range

Height600 HN

40.0 MM 0

Data typeZ range

Phase60.0 de

40.0 UN

6 hours

(d)

Data typeZ range

Height600 nN

1 0 hours

(e)

Data type2 rang*

40.0 MM 0 40.0 MMHeight600 nN

Data typeZ range

Phase60.0 de

^ure 4.1 1 (continued).

110

After colloidal adsorption, the substrates were dried overnight and then coated

with ~1 10 A gold. These were then immersed into an ethanol solution of 1-DCT that

resulted in the formation of self-assembled monolayers of alkanethiols on gold. The

substrate with multilayer assemblies was also hydrophobized via this method for -

comparison and the XPS atomic concentration data is shown in Table 4.2.

Table 4.2. XPS atomic concentration data for an ABDMS-modified silicon substrate andafter building 1 0 polyelectrolyte layers composed of PSS and PAH, PAH as theoutermost layer which were then hydrophobized with self-assembled monolavers of 1-

DCT.^

1 5° Takeoff Angle 75° Takeoff Angle

Substrate %Au %C %0 %S %Au %C %0 %S

Si/ABDMS 10.54 79.88 7.63 1.95 43.68 52.86 - 3.46

Si/ABDMS/PSS-PAH 13.55 68.81 14.71 2.94 42.75 54.14 - 3.11

(10 layers)

Figure 4.12 shows the dynamic water contact angle versus time of adsorption of 1

|Lim and 0.35 |im colloids after the substrates were hydrophobized. For both surfaces, the

advancing contact angle increased while the receding contact angle decreased with

adsorption time. However, there was a more gradual increase (decrease) in advancing

(receding) water contact angles for the surfaces with 1 |im colloids than for the ones with

0.35 |im. In other words, the hysteresis rose linearly with time in the case of 1 \\.m and

reached a maximum value of -122° after 24 hours of adsorption, whereas for 0.35 |im

colloids, the hysteresis increased substantially from 77° to 125° in the first two hours, and

111

stayed almost constant afterwards. What this meant physically was that the water

droplets pimied on these surfaces due to the high hysteresis. They did not roll off the

surfaces even at -80° tilt angle. Hence, these surfaces were not promising candidates as

ultrahydrophobic surfaces although they had very high advancing water contact angles.

(a)80

60

140 h

120

o 100

2 80

60

40

0

I I

I

5 10 15 20

Adsorption Time (hrs)

nJ 1 1 I u

25

(b)

o-4—

»

oU

0 2 4 6 8

Adsorption Time (hrs)

Figure 4.12. The advancing () and the receding () water contact angle with time of

adsorption of (a) 1 |Lim, (b) 0.35 |im colloids after the substrates were hydrophobized with

self-assembled monolayers of 1-DCT.

112

The dynamic contact angles of the substrates with 1 ^im and 0.35 mii colloids were

measured using water and hexadecane as probe fluids, and they are given in Table 4.3

and 4.4.

Table 4.3. Dynamic contact angles for surfaces with 1 |am colloids, before and afterhydrophobizing with self-assembled monolayers of 1-DCT, using water and hexadecane(HD) as probe fluids, (time = 0 is for polyelectrolyte multilayers with PAH as theoutermost layer)

Au/1-DCT Au/1-DCTTime

(hours) water

Or

water

Oa

water

Or

water

Oa

HDOr

HD0 91° 8° 118° 92° 49° 17°

1 92° 8° 118° 90° 49° 17°

2 91° 10° 128° 82° 46° 9°

4 99° 9° 138° 69° 45° 10°

6 106° 10° 146° 59° 40° 9°

10 107° 9° 155° 57° 37° 9°

24 109° 7° 163° 42° 26° 8°

Table 4.4. Dynamic contact angles for surfaces with 0.35 |im, before and after

hydrophobizing with self-assembled monolayers of 1-DCT, using water and hexadecane

(HD) as probe fluids.

. Au/l-DCT Au/l-DCT

Time Ba Or Oa 9a 9r

(hours) water water water water HD HD1 81° 9° 139° 62° 35° 7°

2 99° 6° 159° 34° 18° 6°

4 102° 8° 163° 38° 25° 7°

6 105° 8° 161° 35° 26° 7°

10 100° 7° 161° 37° 23° 7°

113

Summary ol the Rouah Surfaces Prepared

Our aim was to vary the roughness of the substrates by simply using eollo.ds of

different diameters and adsorbing them onto the polyeleetrolyte multilayers. The ehange

in surface roughness would ultimately affect the wettability of surfaces. The colloids

with 0.1, 0.35, 0.5, 1, 2, 3.3 and 4.5 ^im diameters were adsorbed onto the multilayer

assembly for 6 hours. The AFM height and phase images are shown in figure 4. 1 3. The

surfaces consisting of 0.1 and 0.35 ^m colloids had a full surfiice coverage. 0. 1 ^m

colloids showed 3-dimensional aggregation on the surface. As the diameter increased to

0.5 [im and higher, the surfiice coverage decreased. A possible explanation for the

decrease in surface coverage with increasing diameter is that the adsorption process is

diffusion controlled and the smaller colloids diffused faster from the solution and adsorb

onto the PAH surface layer faster than the larger ones. Hence, it would take longer than

6 hours for the larger colloids to have a full coverage on the surface, as observed for the

case with 1 )iim colloids.

These rough surfaces were further coated with gold and reacted with l-DCT to

impart hydrophobicity. Table 4.5 lists the dynamic water contact angles for the surfaces

prepared by using polystyrene colloids with sub-micron and micron diameters. There

was a clear difference in the wettability behavior between the surfaces that had less than

1 ^m colloids and the ones with greater than 1 ^im. When the diameter of the colloids

were between 0.1 pm and 1 pm, the hydrophobi/cd surfaces had very high advancing

water contact angles (-160"), except for 0.5 pm where a good surface coverage was not

1 14

achieved. These surfaces exhihiled remarkahle hysteresis with h.gh advancing hut low

receding water contact angles. When these surfaces were tdted, even at SO", the droplets

stayed pinned. There was great resistance to rolling across the surface due to the high

hysteresis. When the diameter was greater than 1 ^m, the surface coverage of the colloids

decreased remarkably as can be seen from the AhM images. They did not alTcct the

wettability of the surfaces. Hence, the dynamic contact angles of these surfaces after

hydrophobi/ation were very similar to that of a smooth surface consisting of only

polyelectrolyte multilayers which exhibited Oa / Or ~ 118"/ 92".

Table 4.5. Dynamic contact angles for surfaces prepared by adsorbing polystyrenecolloids with the indicated diameters, before and after hydrophobi/ing with self-

assembled monolayers of dodecanethiol (Au/l-Dd ), using water and hexadecane (ill))

as probe (luids. r is the roughness ratio calculal(ul iVom the A1<'M height image with 40|j,m X 40 |im scanned area.

Diameter

(^m)r

Oa

water

Or

water

Au/1

Ba

water

-Dcr

Or

water

Au/1

Oa

IID

-DCl

0R

111)

0 1 91° 8° 118° 92° 49° 17°

0.1 1.81 75" 90163" 95" 45" 90

0.35 1 .93 105° 8° 161° 35° 26° 70

0.5 1 .36 72" 6° 135" 85" 46" 11"

1 1.85 106° 10° 146° 59° 40° 90

2 1.61 70" 5° 124" 82" 47" 11"

3.3 1.9 78" 5° 117" 95" 46" 23"

4.5 1.47 81" 6" 118" 96" 45" 17"

115

0.1 |„im

250.0 HM

125.0 HH

0.0 HM

(a)

600.0 RH(b)

300.0 HM

0.0 nH

0.35 |im

•hi

.0 LIH

60.0 ae

0.5 |im

850.0 HM

425.0 RM

0.0 HM

(C)

PhaseBU.O dw

1 urn

1 .8 MM

0.9 MM

0.0 iJM

(d)

Figure 4. 1 3. AFM height (left) and phase (right) images of the surfaces obtained using

(a) 0.1, (b) 0.35, (c) 0.5, (d) 1, (e) 2, (f) 3.3 and (g) 4.5 [im diameter polystyrene colloids

that were adsorbed onto the polyelectrolyte multilayers for 6 hours, (continued next

page)

116

2 f.im

(e)

3.5 MM

1.8 UN

0.0 MM

40.0 Aw

3.3 |Lim

(05.0 MM

2.5 UM

0.0 MM

(g)5.0 MM

2.5 MM

0.0 MM

4.5 |im

•10.0 MM

h.OO MMData tu|i<

7 r-4(iiiti HO.O (!•

Figure 4.13 (conlinucd).

117

Solution Casting Polystyrene Colloids onto PolvelectrolytP Mnltil.yers

A full coverage could not be achieved with the surfaces prepared by adsorbing

carboxylated polystyrene colloids onto polyelectrolyte multilayers. Hence, solution

casting technique was used to have a full coverage of colloids on the surfaces. 0.35, 0.5,

1, 2 and 3.3 ^im colloidal suspensions were spread onto multilayer assemblies forming

0.5 |im

o«t« tup*2 rangr*

1 |im 2 \im

0 40.0 UM 0 40.0 UM 0 UH 0 40.0 UH

2 r«n«* 2.00 UM Z r*r>«« £0,0 im ^ rang* 2. SO iim 2 r«ng« 60.0 d»

3.3 |im

Figure 4. 14. AFM height (left) and phase (right) images of the surfaces obtained by

using 0.35, 0.5, 1, 2 and 3.3 urn diameter polystyrene colloids that were solution-casted

onto polyelectrolyte multilayers, and dried for 3 days.

118

surfaces as shown in figure 4.14. These surfaces were further hydrophobized to explore

their wettability. Table 4.6 gives the dynamic contact angles (for water and hexadecane)

for these surfaces after hydrophobizing them with SAMs of thiols. The water contact

angle hysteresis of these surfaces were less, but still was not low enough to impart

ultrahydrophobicity.

Table 4.6. Dynamic contact angles for surfaces prepared by solution casting polystyrenecolloids and hydrophobizing with self-assembled monolayers of 1-dodecanethiol (Au/1-DCT), using water and hexadecane (HD) as probe fluids, r is the roughness ratiocalculated from the AFM height image with 40 \im x 40 |im scanned area.

Diameterr

Ga Gr Ga Gr(Mm) water water HD HD

0.35 1.83 159° 115° 26° 7°

0.5 1.34 124" 72" 37" 8"

1 2.1 161° 116° 24° 40

2 2.31 150° 118° 29° 8°

3.3 1.94 161° 119° 24° 11°

119

Conclusions

Submicron and micron scale rough surfaces were prepared by adsorbing

carboxylated polystyrene colloids onto polyelectrolyte multilayer assembly consisting of

10 alternating layers of anionic polyelectrolyte, PSS, and cationic polyelectrolyte, PAH.

The negatively charged colloidal particles were efficiently adsorbed onto the outermost

cationic polyelectrolyte surface, showing no aggregation as indicated in the AFM images.

Using various diameters of colloids enabled us to develop a better understanding of their

adsorption behavior onto the polyelectrolyte multilayers. A kinetic study of adsorption

was performed using 0.35 |Lim and 1 |^m diameter colloids. AFM analyses showed that

the smaller the diameter, the faster the adsorption. An almost full surface coverage was

achieved using colloids with submicron diameter. Also, the flexibility of altering the

roughness of the surface was gained by varying the diameter of the colloids. The

wettability was explored before and after the rough surfaces were hydrophobized by

chemisorption of an alkanethiol. The advancing water contact angle increased and the

receding contact angle decreased as the surface coverage (and the roughness) increased

for both 0.35 |im and 1 \im colloidal surfaces. The advancing water contact angle was as

high as -163° for the surface with 1 ^im colloids after 24 hours of adsorption. However,

these surfaces exhibited a remarkable hysteresis of -122" which resulted in the pinning of

the water droplet on the surface. For instance, the water droplets were not rolling off the

surface even at high tilt angles. This was an observation along the lines of our discussion

about wettability and hysteresis in the first chapter, such that even though the advancing

contact angle was very high, the hysteresis was the real determining factor for

120

hydrophobicity rather than the absokite values of water contact angles. From this study,

we concluded that hydrophobic surfaces could not be achieved by making rough surfaces

by colloidal adsorption.

121

Notes and References

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