Contact Angle Measurements and Surface Energy

Ashley TraceyGraduate StudentDepartment of Materials Science and EngineeringUniversity of Washington

θ

Outline

Introduction Techniques and methodology Contact angle and surface energy

theory Application to research

Surface Energy

The energy that is associated with unsatisfied bonds at a surface› Surfaces are usually only a few molecular

layers thick Surfaces are at a higher free energy

than bulk phases Contact angles measured to calculate

surface energy

Contact Angles

The contact angle, θ, is the angle that a small drop of liquid makes as it meets the surface or interface of another phase, usually a solid› The contact angle is always measured

through the liquid phase

liquid

vapor

θ solid

Some Interesting Observations about Contact Angles

The contact angle of water on skin is about 90 degrees› If it were 0 degrees, external water could

penetrate the pores The contact angle of water on Teflon® is

about 110 degrees The contact angle of water on a bird’s

feather is as high as 150 degrees› The feather is a rough surface with air

trapped within and between the individual fibrils, which have hydrophobic surfaces

Lotus Leaf Effect

Lotus leaf effect› Due to trapped air

bubbles in the microroughness features on the leaf’s hydrophobic surface

Contact Angles

Why does the drop

spread on some

surfaces?

• Non-wetting surface – a sphere has minimum surface to volume ratio

• Drop volume is constant

• Thus, drop surface area must increase

Same Liquid

Solid Surface energy

increasing

Same Solid

Liquid surface tension

decreasing

Techniques to Measure CA

Instru

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Gon

iom

ete

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urfa

ce A

naly

st

Bench top device (VCA Optima video goniometer)› Multiple fluids

Handheld device (Brighton Surface Analyst)› Water

Goniometer Methodology Contact angle of a 1μL drop of

fluid is measured – side view› Determine average contact angle

from 10 drops For surface energy calculations

using Owens-Wendt model, need average contact angle for at least 2 fluids

Complete wetting when θ approaches zero

Hydrophobic surface

Hydrophilic surface

Concerns and Considerations in Contact Angle Measurements

Liquid penetrating the surface Liquid dissolving the surface Contamination of the liquid Size of drop and effect of gravity Evaporation Usually limited to ambient temperature Surface roughness Surface heterogeneity Surface group mobility and kinetics Advancing vs. receding contact angles Limited to relatively low energy solids (polymers) Individual eyeball “bias”

Surface Energy Calculated from Contact Angle

Three parameters influence the shape a drop takes on a solid surface› γsl : solid-liquid surface energy

› γsv : solid-vapor surface energy

› γlv : liquid-vapor surface energy

γsl = γsv -γlv cosθ(1)

These three values and contact angle are related by Young’s equation:

γsl γsv

γlv

solid

liquid

vapor

θ

Young’s Equation

Liquid-vapor surface energy and contact angle can be determined empirically› Need to develop model to determine other

values

γsl = γsv -γlv cosθ(1)

Calculating Surface Energy cont.

-

Many models developed to calculate solid-liquid and solid-vapor surface energies› Neuman Method (1 component)› Fox-Zisman Method (1 component)› Fowkes Method (2 components)› Owens-Wendt Method (2 components)› Van-Oss-Chaudhury-Good Method (3

components)

Calculating Surface Energy

Berthelot assumed (late 1800s):

› Wsl is the interfacial work of adhesion

› Wss is the work of cohesion of a solid

› Wll is the work of cohesion of a liquid

The work of cohesion: Wcoh = 2γ, thus (2) becomes:

Wsl = √(WssWll)

(2)

Wsl = 2√(γssγll)(3)

Calculating Surface Energy cont.

The Dupre equation for the work of adhesion for two dissimilar materials:

Equating (3) and (4):

Equation (5) is the starting point for deriving equations to determine γsv

Wsl = γsv + γlv – γsl

(4)

γsl = γsv + γlv – 2√(γsvγlv)(5)

Owens-Wendt Model

Takes the surface energy to be composed of two parts› Dispersive component (d)

Van der Waals interactions› Polar component (p)

polar interactions, hydrogen bonding, acid-base interactions

γ = γp + γd

(6) Using equation (6), equation (5) becomes:

γsl = γsv + γlv – 2√(γsvp γlv

p) – 2√(γsvd γlv

d) (7)

Owens-Wendt Model cont.

Equating Young’s equation (1) and Owens-Wendt’s equation (7) and some algebra yields:

Equation (8) takes the form y = mx + b, where:› y = γlv(cosθ + 1)/2√γlv

d

› x = √(γlvp/γlv

d)

› m = √γsvp POLAR COMPONENT

› b = √γsvd DISPERSIVE COMPONENT

(8)

Owens-Wendt Model cont.

Equation (8) has two unknowns (γsvp and γsv

d)› We need the contact angle of at least two fluids, so

we can plot x vs. y (Klaeble plot) and extrapolate m and b to calculate γsv

p and γsvd

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.600.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00

f(x) = 1.4942357943749 x + 6.77028180906058R² = 0.928733807113008

Kaelble Plot

Owens-Wendt Spontaneous Wetting

Substituting θ= 0° for spontaneous wetting and γlv = γlv

p + γlv

d into equation (8) and some algebra:

Rearranging (10):

Equation (11) takes form of ax2 + bx + c = 0, can solve for roots using quadratic formula:

γlvp + γlv

d = √(γsvp γlv

p) + √(γsvd γlv

d)(10)

1(√γlvd)2

- √(γsvd) √(γlv

d) + [γlvp - √(γsv

p) √( γlvp) = 0 (11)

x = [-b ± √(b2 – 4ac)]/(2a)

ax2 bx c = 0+ +

Owens-Wendt Wettability Envelopes

Spontaneous Wetting

Non Wetting

Contact Angle Measurements in

ResearchPotential Surface Preparation Inspection

Method for Composite Bonding

Motivation

Adhesive bonds are highly desired in the aerospace industry› Eliminate local stress concentrations present at the

sites of fasteners› Reduce part count and weight increase efficiency

However, adhesive bonds are VERY sensitive to surface preparation› Want to be able to inspect a surface prior to

bonding to ensure proper surface preparation CONTACT ANGLE MEASUREMENTS

Voids

Contaminants

Surface Preparation

Protect surface prior to bonding Remove/prevent a weak boundary layer Maximize adherend/adhesive intimate

molecular contact Ensure intrinsic adhesion forces across the

interface yield required joint strength/service life

Generate a specific adherend surface topography

Assist in the adhesive hardening Surface preparations for composites:

› Peel ply (as tooled)› Abrasion (sanding or grit blasting)

Potential Surface Preparation Inspection Method for Composite Bonding

Motivation › Most important step for bonding is SURFACE

PREPARATION!!› Inspect the surface prior to bonding to ensure proper

surface preparation Objective

› Develop quality assurance (QA) technique for surface preparation

Approach› Investigate different surface preparations and process

variations using benchtop and handheld devices CFRP prepared with peel ply for secondary bonding Contamination

Composite Bonding and Surface Preparation Peel ply is a desirable surface preparation

because it produces repeatable and consistent surfaces

› Composite bonds are materials system specific (prepreg/surface prep/adhesive) potential problems that could degrade bond quality: Incorrect peel ply? Contamination?

Peel ply

Composite

Peel Ply Used for Structural Composite Bonds

Precision Fabrics 60001PolyesterPrecision Fabrics 60001Polyester

PF 52006NylonPF 52006Nylon

PF 60001 SRBSiloxane Coated PolyesterPF 60001 SRBSiloxane Coated Polyester

Last layer on part before curing Protects surface during handling Removed just prior to bonding Clean, chemically active surface

Materials

Toray 3900/T800 unidirectional laminates Precision Fabric Group 60001 polyester peel ply Precision Fabric Group 52006 nylon peel ply Precision Fabric Group SRB release ply Autoclave cure of composites Fluids used for contact angle analysis:

› De-ionized water (DI water)› Ethylene Glycol (EG)› Glycerol (Gly)› Diiodomethane (DIM)› Formamide (Form)

Goniometer Methodology

Using a goniometer, the contact angle of a 1μL drop of fluid is measured – side view› Peel ply removed and contact

angles measured within 1 hour› Four fluids, 10 drops per fluid

were evaluated on each surface

› Average contact angle and standard deviation were calculated to determine surface energies

Side-view of drop as viewed from goniometer

camera

Drop application: dispense drop, raise

surface

θ

Surface Analyst™ Methodology

Using the Brighton Surface Analyst, the contact angle of a 1.38μL drop (20 69nL drops) of water is measured – top view› Contact angle is calculated by

fitting the circumference to the volume of the drop

› Average contact angle and standard deviation were calculated for comparison to water contact angles measured with the goniometer

http://www.btgnow.com/SEP.html

d3/V = (24sin3θ)/(π(2-3cosθ+cos3θ))

Top-view of drop as viewed from Surface Analyst camera

Note: rectangle in image is a reflection of light from camera

Drop application: dynamically apply 20

small drops to form 1 drop

Experimental Overview

Compare bench top instrument (goniometer) with field device (Surface Analyst™)

Can contact angle detect different peel or release ply materials? › Nylon, polyester, SRB

What level of siloxane detectable with contact angle?› Various levels of contamination

Assess potential QA methods ability to identify less than desirable process conditions

31

Contact Angle Sensitive to Surface Preparation

Contact angle can identify different surface preparation materials What level of surface contamination is detectable?

Polyester SRB Nylon0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Goniometer

Surface Analyst

Surface Preparation Material

Norm

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d A

vera

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ate

r C

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An

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(N

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d t

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Poly

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Pre

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d C

A)

Effect of Peel Ply Contamination

Surface contamination can be detrimental to bonding› Siloxane: worrisome contaminant degrades

bond quality1

1 VanVoast, P.J., P.H. Shelley, R.L. Blakely, C.B. Smith, M.P. Jones, A.C. Tracey, B.D. Flinn, G. Dillingham, B. Oakley. “Effect of Varying Levels of Peel Ply Contamination on Adhesion Threshold.” SAMPE 2010 – Seattle, WA May 17-20, 2010.

Amount Siloxane

Contamination

Total Surface Energy

GIC (adhesive: AF555M)1

(mN/m) (J/m2)0% 48 > 750

0.01% 46 ---0.1% 45 ---0.4% 42 7091% 43 6032% 42 382

Goniometer Results: Wettability Envelopes

Contact angle sensitive to < 0.1% siloxane contamination

Need to characterize with Surface Analyst™

0 2 4 6 8 10 12 140

5

10

15

20

25

30

35

40

45

50 0% (control)0.01%0.10%0.40%1%2%

Polar surface energy (mN/m)

Dis

pe

rsiv

e s

urf

ac

e e

ne

rgy

(m

N/m

)

Inc. S

iloxa

ne

Conclusions

Contact angle measurements can be used to detect differences between surfaces› Hydrophobic vs hydrophilic

Contact angle measurements can be used to calculate surface energies

Research application: composite bonding› Surface preparations› Peel ply contamination

Acknowledgements

FAA: JAMS, AMTAS Boeing Company

› Adhesive Bonding Group Precision Fabric Group Richmond Aircraft Products Airtech International Prof. Mark Tuttle (UW) Prof. Allan Hoffman (UW)