Fluid Manipulations Using a Piezo Electric Transformer for Sensing and Spray Generation

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Fluid Manipulations Using a Piezo ElectricTransformer for Sensing and Spray GenerationApplicationsZeinab RamshaniWestern Michigan University, [email protected]

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FLUID MANIPULATIONS USING A PIEZO ELECTRIC TRANSFORMER FOR

SENSING AND SPRAY GENERATION APPLICATIONS

by

Zeinab Ramshani

A dissertation submitted to the Graduate College

in partial fulfillment of the requirements

for the degree of Doctor of Philosophy

Electrical and Computer Engineering

Western Michigan University

August 2017

Doctoral Committee:

Massood Z. Atashbar, Ph.D., Chair

David B. Go, Ph.D.

Bradley J. Bazuin, Ph.D.

Paul D. Fleming, Ph.D.

FLUID MANIPULATIONS USING A PIEZO ELECTRIC TRANSFORMER FOR

SENSING AND SPRAY GENERATION APPLICATIONS

Zeinab Ramshani, Ph.D.

Western Michigan University, 2017

A steady effort has been made on the development of fluid manipulation techniques for

sensing and actuating systems. Conventional techniques of fluid manipulation for sensing and

spray generation purposes, often need trained staff or high input power and they are

complicated, time consuming and expensive. Therefore, it is necessary to develop new systems

which can overcome these drawbacks.

In this dissertation, the author has developed piezoelectric-based systems for fluid

manipulation. The focus of this dissertation involves novel approaches of enhancing the

sensing and actuating systems using the piezoelectric devices. This includes designing a

system, which theoretically can overcome the limitations associated with conventional fluid

manipulation systems, fabricating the device and examining the functionality of the system to

prove the claim.

The purpose of the first project of this research work was to design and fabricate a

piezoelectric based system to enhance the sensitivity of the system towards the sensing of toxic

materials in liquid media. The functionality of the designed sensing system was investigated

towards several toxic heavy metals including lead, cadmium, nickel and mercury. The SH-

SAW sensor was fabricated on a 64° YX-LiNbO3 piezoelectric substrate using

photolithography techniques and placed in the sensor groove of an acrylic based material flow

cell. Then, varying concentrations of target analytes were injected into the flow cell using a

programmable syringe pump. A network analyzer was used to measure the phase response (S21)

of the SH-SAW sensor towards the test analytes. System control, data acquisition and post

processing of the network analyzer measurements was performed using a LabView™ based

application. The significance of this research was based on the contribution that this sensing

system could enhance the detection of the toxic heavy metal ions to pico molar concentration

levels while conventional methods often work in the micro molar concentration levels.

Further, a piezoelectric based system which can be utilized for spray generation from a

desired liquid was designed and fabricated. A linear 128 Y-cut lithium niobate (LiNbO3)

crystal was used and a 3D printed stand was designed to pin the piezoelectric transformer on

the second resonance standing wave nodes. The piezoelectric transformer (PT) was actuated

by a signal generator connected to a radio frequency (RF) amplifier. To generate the spray,

various aqueous solutions prepared using deionized (DI) water was filled in an adjacent

reservoir and a paper bridge was placed from the reservoir and in contact with the surface of

the PT. The generated piezoelectric driven spray resulted in a broad area, uniform, continues

spray appropriate for coating applications. The spray generation system also made it possible

to generate the spray out of the liquid by applying around 15 Vamp, AC input voltage amplitude,

while traditional techniques typically require around 100 Vamp for spray formation.

Finally, the PT driven spray generation technique was used for membrane coating

applications. Different polymers such as poly(allylamine hydrochloride) (PAH)/ poly(styrene

sulfonate) (PSS) and poly(diallyl-dimethylammonium chloride) (PDADMAC) were

sequentially sprayed on to a polycarbonate track-etched (PCTE) membrane. The polymer

coated membrane was tested towards water permeability and ion rejection ratio to investigate

the functionality of this novel spray generation system for membrane coating purposes.

ii

ACKNOWLEDGEMENTS

In the beginning, I would like to express my sincere appreciation to Prof. Massood

Zandi Atashbar for all his help, advice and guidance trough 5 years of my research program.

I am very grateful of Prof. David B. Go who gave me an opportunity to pursue my

research work in University of Notre Dame as well as Dr. Paul Rumbach and Dr. Michael

Johnson, who passed their knowledge to me. Without their support, the second phase of this

dissertation would not have been possible.

My thanks also go to the other committee members Dr. Bradly Bazuin and Dr. Paul D.

Fleming for their time and comments.

I would like to show my appreciation to Dr. Dean Johnson who helped me through my

teaching experience in Western Michigan University.

I would like to express my great thanks to my fellow lab members Dr. Ali Eshkeiti, Dr.

Sai Guruva Reddy Avuthu, Dr. Amer Chlaihawi, Sepehr Emamian and Dinesh Maddipatla for

their support and friendship and specially Dr. Binu Narakathu who was not only a perfect co-

worker in the lab but also a dedicated teacher for me.

Finally, I would like to dedicate this dissertation to my family. To my dad who may not

always agree with me but when the time comes, supports me the most. To my mom and her

unconditional love. To my brothers who have taken care of all my responsibilities at home,

while I was pursuing my dreams overseas.

Zeinab Ramshani

Copyright by

Zeinab Ramshani

2017

iii

TABLE OF CONTENTS

ACKNOWLEDGEMENTS ........................................................................................... ii

LIST OF TABLES ....................................................................................................... vii

LIST OF FIGURES ................................................................................................... viii

CHAPTER

I. INTRODUCTION .................................................................................................. 1

1.1 Motivation ........................................................................................................ 1

1.2 Author’s Contributions ..................................................................................... 2

1.3 Dissertation Organization ................................................................................. 4

1.4 References ........................................................................................................ 6

II. LITERATURE REVIEW ....................................................................................... 9

2.1 Introduction ...................................................................................................... 9

2.2 10Fluid Manipulation Techniques for Sensing Applications ......................... 10

2.2.1 Toxic Heavy Metal Deficiencies ............................................................... 10

2.2.2 Sensors ....................................................................................................... 11

2.3 Fluid Manipulation Techniques for Spray Generation ................................... 17

2.3.1 Pneumatic Sprays ...................................................................................... 17

2.3.2 Acoustic Based Sprays .............................................................................. 17

2.3.3 Electrically-Driven Sprays ........................................................................ 20

2.4 Summary ........................................................................................................ 23

2.5 References ...................................................................................................... 24

iv

Table of Contents-Continued

CHAPTER

III. INVESTIGATION OF A PIEZOELECTRIC BASED SENSOR FOR TOXIC

HEAVY METAL DETECTION .......................................................................... 38

3.1 Introduction .................................................................................................... 38

3.2 Theory ............................................................................................................ 39

3.3 Experimental .................................................................................................. 42

3.3.1 Chemicals, Materials and Sample Preparation .......................................... 42

3.3.2 Synthesis of the Chemical Sensing Layer ................................................. 42

3.3.3 Sensor Fabrication ..................................................................................... 43

3.3.4 Flow Cell Fabrication ................................................................................ 45

3.3.5 Experiment Setup ...................................................................................... 46

3.4 Results ............................................................................................................ 46

3.5 Summary ........................................................................................................ 52

3.6 References ...................................................................................................... 53

IV. PIEZOELECTRIC TRANSFORMER BASED SPRAY GENERATIONSYSTEM ............................................................................................................... 59

4.1 Introduction .................................................................................................... 59

4.2 Experiments and Results ................................................................................ 60

4.2.1 Piezoelectric Transformer and Stand Design ............................................ 60

4.2.2 Spray Generation ....................................................................................... 61

4.2.3. Spray Coating Profilometry ....................................................................... 63

4.2.4 Volumetric Flow Estimation ..................................................................... 64

4.2.5 Linear Flow Speed Visualization ................................................................ 65

v


CHAPTER

4.2.6 Bulk Motion of Droplets Directly on the PT Device .................................. 67

4.2.4 Measurement of Output Voltage ............................................................... 67

4.2.5 Effect of Liquid Conductivity on Spray Current ....................................... 70

4.2.7 Analysis of the Load Resistance and the Input Current .............................. 72

4.2.6 Effect of Liquid Conductivity and Surface Tension on the Onset

Voltage ................................................................................................................. 73

4.2.7 Chemical Solution Preparation .................................................................. 75

4.4 Summary ........................................................................................................ 75

4.5 References ...................................................................................................... 76

V. PIEZOELECTRIC TRANSFORMER BASED SPRAY GENERATION

FOR THIN FILM MEMBRANE COATING ...................................................... 79

5.1 Introduction .................................................................................................... 79

5.1.1 Spray Coating Techniques ........................................................................... 79

5.1.2. Membrane Coating Materials ..................................................................... 80

5.2 Experimental .................................................................................................. 81

5.2.1 Spray Generation Setup ............................................................................. 81

5.2.2 Material Preparation .................................................................................. 82

5.2.3 Moving Stage (Lazy Susan) ...................................................................... 82

5.2.4 Water Permeability and Salt Rejection Tests ............................................ 83

5.3. Results ............................................................................................................ 84

5.3.1. Bilayer coating of PAH and PSS ................................................................ 84

5.3.2. PDADMAC Spray Coating ....................................................................... 89

vi


CHAPTER

5.4 Summary ........................................................................................................ 90

5.5 References ..................................................................................................... 90

VI. CONCLUSION..................................................................................................... 93

6.1 Conclusion ...................................................................................................... 93

APPENDICES

A. List of publications............. ............................................................................ 97

A.1 Inventions ........................................................................................ 97

A.2 Journal Papers .................................................................................. 97

A.3 Conference Papers ........................................................................... 98

B. Matlab code for scale used to graph the spray volumetric flow rate

used in Chapter 4 ......................................................................................... 100

vii

LIST OF TABLES

5.1 Water permeability and ion rejection results for PAH/PSS PT spray coating . 86

5.2 Water permeability and ion rejection results for PDADMAC PT spray coating .............................................................................................................. 89

viii

LIST OF FIGURES

Figure 2.1: Schematic of a planar IDE design SAW device ............................................. 15

Figure 2.2: Curved IDE designed SAW device ................................................................ 15

Figure 2.3: Flexible SAW device fabricated on a nanocrystal film ................................. 16

Figure 2.4: (a) Overcoming the surface tension to break the droplet in SAW devices.

(b) SAW device configurations. ....................................................................................... 18

Figure 2.5: Schematic of a typical electrospray system. High voltage applied between

the metal nozzle electrode and substrate generates the

spray from the precursor liquid delivered by the syringe. ................................................. 21

Figure 2.6: Paper spray setup used for mass spectrometry. .............................................. 23

Figure 2.7: Taylor cone formation in paper spray. ........................................................... 23

Figure 3.1: Schematic of a delay line SAW device with input and output IDEs .............. 40

Figure 3.2: Structure of QDPPZ ....................................................................................... 43

Figure 3.3: (a) Schematic of the SH-SAW device and (b) fabricated SH-SAW

sensor. ........................................................................................................................ 44

Figure 3.4: Flow cell with the SH-SAW sensor in the sensor groove (Inset: PDMS

microfluidic flow channel. ................................................................................................ 45

Figure 3.5: Experimental setup. ........................................................................................ 46

Figure 3.6: Magnitude (insertion loss) and phase of the frequency response for SH-

SAW sensor. ..................................................................................................................... 47

Figure 3.7 (a) SH-SAW sensor phase response (S21) towards varying concentrations of

PbNO3 and (b) Percentage change in frequency shift of SH-SAW sensor response...... .. 48

Figure 3.8: SH-SAW sensor phase response (S21) towards varying concentrations of

CdNO3 ........................................................................................................................ 50

Figure 3.9: SH-SAW sensor phase response (S21) towards varying concentrations of

Ni(NO3)2 ........................................................................................................................ 50

ix

List of Figures-Continued

Figure 3.10: SH-SAW sensor phase response (S21) towards varying concentrations of Hg(NO3)2 ................................................................................................................................................................................ 51

Figure 4.1: 3D printed PT stand to pin the device in L/4 of the tips. Conductive tape was used to apply the input voltage to the razor blade in contact with top and bottom electrodes. ........................................................................................................................ 61

Figure 4.2: (a) PT-driven electrospray configuration with solution delivery from a paper wick. (b) Geometric configuration of the PT with resulting standing displacement and stress wave. (c) Photograph of PT-generated electrospray of 50 mM NaCl solution. ................................................................................................................... 62

Figure 4.3: 3D profilometry of a glass slide coated by 1 µm beads. The roughness was determined to be 0.05 μm over an area of 15 mm × 5 mm. (b) SEM image of as

deposited particle ........................................................................................................... 64

Figure 4.4: Generated spray volume for 5 mM NaCl solution in DI water versus time. The slope of the curve fit indicates a constant volumetric flow rate of 20 µl/min over 5 min of applied constant 19 Vamp AC input voltage. The dash lines represent the standard deviation in the spray volume from three repeated experiments ...................................... 65

Figure 4.5: Front tracking of the red dye during spray generation ................................... 66

Figure 4.6: Distance of the front of red dye through the paper wick as a function of time with applied constant 20 Vamp AC input voltage at 59.91 KHz. The dotted line is a linear curve fit indicating a constant speed. ................................................................................ 66

Figure 4.7: Time evolution of droplet motion under a 13 Vamp, 59.9 kHz input to the PT. The droplet nearest the corner eventually translated to the edge of the PT, wetting it, but no spray was formed. The more central droplet only exhibited vibrational motion, but no bulk translation ...................................................................................................... 67

Figure 4.8: (a) PT output voltage measurement configuration. (b) PT output voltage as a function of the applied input voltage at a frequency of 60 kHz. .................................... 69

Figure 4. 9: PT driven generated spray current measurement configuration. ................... 70

Figure 4.10: (a) Logarithmic plot of output spray current for different (a) concentrations and (b) conductivity of HCl and NaCl in DI water with an applied voltage of 18 Vamp. The dash line in (b) is a linear curve fit with a coefficient of determination equal to r2 =0.76. ............................................................................................................... 71

x

List of Figures-Continued

Figure 4.11: Input current for different concentrations of HCl and NaCl in DI water for

an input voltage of 18 Vamp and slightly different frequency around 60 KHz dependent

on the liquid. ..................................................................................................................... 72

Figure 4.12: (a) PT equivalent circuit. (b) Simulated input and output current (I in , I out)

as a function of load resistance (RL) ................................................................................. 73

Figure 4.13: a) Logarithmic plot of the onset voltage for various concentration of NaCl

in DI water. The dash line is the linear curve fit with a slope equal to -0.32. b) Onset

voltage as a function of surface tension for various concentration of glycerol in DI

water ........................................................................................................................ 74

Figure 5.1: Sequential PT driven Spray coating of membrane using PAH and PSS by

placing the membranes on a Lazy Susan .......................................................................... 83

Figure 5.2: SEM images of PCTE membranes sprayed with different numbers of

bilayers. Different numbers of PAH/PSS bilayers were sprayed onto a 50 nm PCTE

membrane. The sprayed membranes were taped onto a SEM grid, sputtered with

iridium (2 nm), and visualized in SEM ............................................................................. 85

Figure 5.3: Logarithmic plot of the normalized water permeability vs. number

of bilayers......................................................................................................................... 87

Figure 5.4: Salt rejection vs. number of bilayers. A different number of PAH/PSS

bilayers were sprayed alternatingly with 10 s duration on a 50 nm PCTE membrane.

The salt rejection test was carried out in a dead-end filter cell filled with either 10 mM

KCl or 10 mM MgCl2. ....................................................................................................... 87

1

CHAPTER I

INTRODUCTION

1.1 Motivation

A steady effort has been made on the development of fluid manipulation techniques for

sensing and actuating systems. Conventional fluid manipulation mechanisms by external

pressure using pumps and air pressure are simple, but not precise1. Ultrasonic standing waves

produced by piezoceramic and sound reflector, have been used for fluid manipulations in a

variety of applications as a more accurate solution2. Nilsson et al. 3 have used this method in

separation of plasma and erythrocytes in blood stream based on particle gathering in the nodes

of the propagating ultrasonic standing wave. In this dissertation, the author focuses on two

aspects of the fluid manipulation applications: sensing and spray generation systems.

Among research developments in sensing systems, heavy metal detection systems have

been receiving an increasing attention due to its adverse effects in aquatic and atmospheric

ecosystems, even at the micro- or nano-molar concentration levels4,5. In the case of aqueous

systems, this eco- and phyto toxicity on vital microorganisms is influenced by environmental

factors6. Traditional detection methods for toxic heavy metals, mostly found in their ionic form

in nature, include spectrometric sensing systems or biological / chemical sensors.7,8,9,10,11,12,13.

Spectrometric sensing systems work based on the interaction between applied electromagnetic

radiation and the heavy metal ions, while the chemical and biological sensors rely on the change

on a specific ligand’s functionality in the presence of the heavy metal ions. However, these

techniques often need trained staff and they are complicated as well as time consuming14.

Therefore, it is necessary to develop sensing systems that are easy-to-use, cost effective, highly

sensitive and employ rapid detection techniques.

2

Various mechanisms can be used to manipulate and atomize fluids, from simple

approaches like a pneumatic atomizer, in which there is no control on the droplet size, to more

precise ones such as an acoustic based atomizer. Surface acoustic wave devices are proven to

work well for atomizing the desired aqueous sample in chemical analysis approaches and mass

spectrometry. However, they are often complicated to fabricate and need around 100 Vamp input

voltage15. To get an electrically driven spray, approximately 1-5 KVamp has to be applied to the

liquid to form the droplet plume in electrospray systems16 . Among all the drawbacks in

conventional spray generation systems, the high amount of required applied voltage can be a

significant reason for illustrating the need for more research in fluid manipulation.

Piezoelectric devices produce mechanical strain under applied electrical field which

results in propagating mechanical vibrations17. Acoustic waves have been used to manipulate

the liquid medium in sensing technologies, due to its benefits such as small sample volume,

high sensitivity and bio compatibility 18 , 19 , 20 . The high sensitivity arises from the large

electromechanical coupling factor of the piezoelectric crystal21. Moreover, the aforementioned

features make piezoelectric devices an apt choice in actuating systems such as aerosolization.

The mechanical vibration along with the large polarization produced on the surface of the

piezoelectric crystal can be used to overcome the liquid film surface tension and break it into

the droplets.

1.2 Author’s Contributions

In the first project, described in chapter3, the author designed and fabricated a surface

acoustic wave device, contributed in synthesis of proper sensing layer and conducted some

experiments in order to investigate the functionality of the system towards lead, mercury,

cadmium and nickel ions in pico molar concentration in water. From this part of research,

author published a decent journal paper along with 5 conference papers.

3

In the second project, described in chapter 4, author designed a novel piezoelectric

transformer based spray generation system. Author was responsible for design and fabrication

of the proper piezoelectric transformer and stand fabrication for fluid manipulation and spray

generation from a liquid reservoir. The author then performed some experiment to measure the

various spray parameters such as current and voltage and to study the effect of liquid properties

on the generated spray characteristics. From this research work, author published an

intellectual property, a patent and a decent journal paper.

In the third project, described in chapter 5, author found a practical use for the PT driven

spray system. The capability of the system was examined towards the membrane coating. The

author conducted different experiments to collect adequate data which was proving the

reliability of this approach. Form this part of the research, author was published an intellectual

property, a patent and a paper.

Overall, the author’s research work has resulted in two intellectual properties

disclosures, two patent applications, three high quality peer reviewed journal and six

conference publications mentioned in Appendix A. The results of the projects have been

published in prestigious journals such as Sensors and Actuators: B Chemical; Applied Physics

Letter and Analytical Methods. The research outcomes have been shared with other scholars in

this field during the author’s presentation and proceedings in international conferences such as

IEEE Sensors Conference (2013, 2015, 2016), Electro/Information Technology (EIT) (2013),

International Meeting for Chemical Sensors (IMCS) (2014), Frequency Control Symposium &

the European Frequency and Time Forum (FCS) (2015) and Electrostatic Society of America

(ESA) (2016). The author also was awarded the All-University Graduate Research and

Creative Scholar Award for 2016-17 by Western Michigan University.

4

1.3 Dissertation Organization

This dissertation presents details of the research projects that was performed, including

developing piezoelectric based systems for fluid manipulation and investigating the capability

of the designed piezoelectric based systems for sensing applications and spray generation.

In Chapter 2, the literature review is provided. This chapter presents a brief introduction

to conventional fluid manipulation systems used for sensing and spray generation. The basic

definition of sensor followed by previous sensing systems that have been successfully used for

toxic heavy metal detection in liquid media are discussed. Traditional techniques of spray

generation are also reviewed in this chapter. The drawbacks of these systems which led the

author to develop a piezoelectric based fluid manipulation system in this dissertation, are also

included in the discussions.

In Chapter 3, a project on developing a piezoelectric sensing system is presented. An

efficient piezoelectric transformer was fabricated for the detection of heavy metal compounds.

An acrylic flow cell, which consists of inlet and outlet ports for the microfluidic chamber as

well as polydimethylsiloxane (PDMS) based microfluidic channels was used to provide a user

friendly, re-usable sensing system. A chemical sensing layer was utilized to selectively bind to

heavy metal ions. The capability of the system for detecting varying concentrations of heavy

metal ions, such as lead (Pb), cadmium (Cd), mercury (Hg) and nickel (Ni) in liquid

environments, through phase shifts in the frequency response of the piezoelectric sensor, was

studied. This work demonstrated the capability of the developed system to detect heavy metal

compounds at pico molar concentration levels.

In Chapter 4, a project for developing a novel spray generation system using a

piezoelectric transformer is presented. The author demonstrated the use of a piezoelectric

transducer to induce the generation of a broad area electrospray. A piezoelectric transducer was

designed and top and bottom electrodes was patterned to generate adequate output voltage from

5

an applied input voltage. A proper PT stand was designed and 3D printed to access the

piezocrystal second resonant frequency. A continues broad area spray was formed by providing

a persistent liquid film using a paper bridge connected to a liquid sample reservoir. The effect

of various liquid parameters on the generated spray was studied

In Chapter 5, the piezoelectric transformer based spray production system was used for

membrane coating. The capability of the system for spray formation out of some high demand

coating materials was investigated. Polymer coating of a track etched membrane was chosen

due to the recent high demand. Solutions of these polymers were sprayed using the PT_ driven

spray system. Coated membranes were tested for the water permeability and ion rejected ratio.

The outcomes will be presented to indicate the abilities and limitations of the system for this

purpose.

Finally, in Chapter 6, a conclusion of the research work and suggestions for the future

works are presented.

6

1.4 References

1 Teh, Shia-Yen, Robert Lin, Lung-Hsin Hung, and Abraham P. Lee. "Droplet

microfluidics." Lab on a Chip 8, no. 2 (2008): 198-220.

2 Bruus, Henrik, Jurg Dual, Jeremy Hawkes, Martyn Hill, Thomas Laurell, Johan

Nilsson, Stefan Radel, Satwindar Sadhal, and Martin Wiklund. "Forthcoming Lab on a Chip

tutorial series on acoustofluidics: Acoustofluidics—exploiting ultrasonic standing wave forces

and acoustic streaming in microfluidic systems for cell and particle manipulation." Lab on a

Chip 11, no. 21 (2011): 3579-3580.

3 Nilsson, Andreas, Filip Petersson, Hans W. Persson, Henrik Jönsson, and Thomas

Laurell. "Autologous blood recovery and wash in microfluidic channel arrays utilizing

ultrasonic standing waves." In Micro Total Analysis Systems 2002, pp. 625-626. Springer

Netherlands, 2002.

4 Rooney, Corinne P., Fang-Jie Zhao, and Steve P. McGrath. "Phytotoxicity of nickel

in a range of European soils: Influence of soil properties, Ni solubility and

speciation." Environmental pollution 145, no. 2 (2007): 596-605.

5 Eshkeiti, A., Binu Baby Narakathu, A. S. G. Reddy, A. Moorthi, M. Z. Atashbar, E.

Rebrosova, M. Rebros, and Margaret Joyce. "Detection of heavy metal compounds using a

novel inkjet printed surface enhanced Raman spectroscopy (SERS) substrate." Sensors and

Actuators B: Chemical 171 (2012): 705-711.

6 Lock, Koen, and Colin R. Janssen. "Ecotoxicity of nickel to Eisenia fetida,

Enchytraeus albidus and Folsomia candida." Chemosphere 46, no. 2 (2002): 197-200.

7 Abebe, Fasil A., Carla Sue Eribal, Guda Ramakrishna, and Ekkehard Sinn. "A ‘turn-

on’ fluorescent sensor for the selective detection of cobalt and nickel ions in aqueous

media." Tetrahedron Letters 52, no. 43 (2011): 5554-5558.

7

8 Hu, Bo, Lin-Lin Hu, Ming-Li Chen, and Jian-Hua Wang. "A FRET ratiometric

fluorescence sensing system for mercury detection and intracellular colorimetric imaging in

live Hela cells." Biosensors and Bioelectronics 49 (2013): 499-505.

9 Trieu, Khang, Emily C. Heider, Scott C. Brooks, Fernando Barbosa, and Andres D.

Campiglia. "Gold nanorods for surface plasmon resonance detection of mercury (II) in flow

injection analysis." Talanta 128 (2014): 196-202.

10 Quang, Duong Tuan, and Jong Seung Kim. "Fluoro-and chromogenic

chemodosimeters for heavy metal ion detection in solution and biospecimens." Chemical

reviews 110, no. 10 (2010): 6280-6301.

11 Knecht, Marc R., and Manish Sethi. "Bio-inspired colorimetric detection of Hg2+ and

Pb2+ heavy metal ions using Au nanoparticles." Analytical and bioanalytical chemistry 394,

no. 1 (2009): 33-46.

12 Gammoudi, Ibtissem, Hakim Tarbague, Ali Othmane, Daniel Moynet, Dominique

Rebière, Rafik Kalfat, and Corinne Dejous. "Love-wave bacteria-based sensor for the detection

of heavy metal toxicity in liquid medium." Biosensors and Bioelectronics 26, no. 4 (2010):

1723-1726.

13 Kozlova, Tatiana, Chris M. Wood, and James C. McGeer. "The effect of water

chemistry on the acute toxicity of nickel to the cladoceran Daphnia pulex and the development

of a biotic ligand model." Aquatic Toxicology 91, no. 3 (2009): 221-228.

14 Hossain, SM Zakir, and John D. Brennan. "β-Galactosidase-based colorimetric paper

sensor for determination of heavy metals." Analytical chemistry 83, no. 22 (2011): 8772-8778.

15 Rajapaksa, Anushi, Aisha Qi, Leslie Y. Yeo, Ross Coppel, and James R. Friend.

"Enabling practical surface acoustic wave nebulizer drug delivery via amplitude

modulation." Lab on a Chip 14, no. 11 (2014): 1858-1865.

8

16 Törnkvist, Anna, Stefan Nilsson, Ardeshir Amirkhani, Lena M. Nyholm, and Leif

Nyholm. "Interference of the electrospray voltage on chromatographic separations using

porous graphitic carbon columns." Journal of mass spectrometry 39, no. 2 (2004): 216-222.

17 Park, Gyuhae, Hoon Sohn, Charles R. Farrar, and Daniel J. Inman. "Overview of

piezoelectric impedance-based health monitoring and path forward." The Shock and Vibration

Digest 38, no. 2 (2003): 91.

18 Luo, J. K., Y. Q. Fu, Y. Li, X. Y. Du, A. J. Flewitt, A. J. Walton, and W. I. Milne.

"Moving-part-free microfluidic systems for lab-on-a-chip." Journal of Micromechanics and

Microengineering 19, no. 5 (2009): 054001.

19 Renaudin, Alan, Vincent Chabot, Etienne Grondin, Vincent Aimez, and Paul G.

Charette. "Integrated active mixing and biosensing using surface acoustic waves (SAW) and

surface plasmon resonance (SPR) on a common substrate." Lab on a Chip 10, no. 1 (2010):

111-115.

20 Wang, Zhuochen, and Jiang Zhe. "Recent advances in particle and droplet

manipulation for lab-on-a-chip devices based on surface acoustic waves." Lab on a Chip 11,

no. 7 (2011): 1280-1285.

21 Kondoh, Jun, Takao Muramatsu, Tetsuo Nakanishi, Yoshikazu Matsui, and Showko

Shiokawa. "Development of practical surface acoustic wave liquid sensing system and its

application for measurement of Japanese tea." Sensors and Actuators B: Chemical 92, no. 1

(2003): 191-198.

9

CHAPTER II

LITERATURE REVIEW

2.1 Introduction

Fluid manipulation at the microfluidic scale plays a significant role in developing lab-

on-a-chip functional technologies. These miniaturized structures, need small volumes of

reagents and result in a fast and accurate response due to their enhanced sensitivity that is

critical for sensing systems1. In this chapter, the author has reviewed conventional methods

used for fluid manipulation in sensing or spray generation systems. After a brief explanation

of sensor basics and the elevating need for improvements in toxic heavy metal sensing systems,

due to their environmental drawbacks, the author introduces some traditional methods of toxic

heavy metal sensing systems (spectrometric, chemical/biological and acoustic sensors). The

reasons behind the author’s choice of the sensing method further in this dissertation, is also

discussed.

The well proved techniques of spray generations are also reviewed in this chapter.

Despite all the methods described (pneumatic, acoustic and electrical spray), the author

explains the necessity for further improvement in spray formation technology for specific

applications.

10

2.2 Fluid Manipulation Techniques for Sensing Applications

This section provides a brief historical background about conventional fluid

manipulation techniques for sensing applications. Sensors mostly produce a mechanical or

electrical signal in response to the change in their physical properties. Sensors have been widely

used in various applications since their first discovery in the early 19th century2. Among the

different usage of sensors, the author is focusing on the sensing systems that target toxic heavy

metal compounds in aqueous media.

2.2.1 Toxic Heavy Metal Deficiencies

Heavy metals are the elements with atomic weight between 63 and 209 g/mol in the

periodic table of elements3. The toxic effect of heavy metals has been a growing concern in the

biomedical and environmental industries, and hence a major focus of research in the

development of different sensing systems. Although, heavy metals are typically formulated as

non-toxic compounds, it has been shown that they can cause serious health issues even at micro

molar concentration levels4.

The rising demand for heavy metals in applications such as plating, mining, solar cells,

pharmaceutical and chemical industries intensifies their role as environmental pollutants5,6.

Toxic heavy metals can be released in the environment and pollute the area for a long time

during their mining process 7 . Some toxic heavy metals are also used to mine the other

materials8. These toxic ions accumulate in human body since they are not biodegradable like

organic contaminants9. In plants, toxic heavy metals can affect the root growth as well as water

entry and exit and cause some deficiencies in their physiological functions 10 . The extra

aggregation of the toxic heavy metals in vegetables from contaminated water can aafect food

safety11. The detection of these heavy metal compounds is thus of utmost importance.

11

2.2.2 Sensors

An electronic sensor is defined as a device which responds to a stimulant by producing

an electrical signal12. The sensor converts a physical property to a readable signal which makes

it possible to record the data for future analysis. The output of the sensor can be used to trace

any changes in amplitude, phase or frequency of the voltage or the current of the electrical

signal. Sensors can be classified as active or passive. An active sensor requires additional

energy source other than the physical property which being sensed, however a passive sensor

does not need any external source of energy for sensing activities13.

For example, there are passive and active sensors designed for satellites14 . Active

sensors, such as radar, use the electromagnetic radiation as the external source and compares

the sent and received signals to measure the distances to objects. On the other hand, a satellite’s

passive sensors may utilize reflected sunlight or thermal radiation.

2.2.2.1 Spectrometric Sensors

Spectrometric sensing systems are categorized as optical sensors, where the sensor is

designed to use a light source to detect the changes in the measurand. These sensing systems

use electromagnetic radiation and record changes in the intensity as a function of the frequency

or wavelength15.

After the initial launch of the atomic absorption spectrometer in the 19th century, an

enhanced cold vapor atomic absorption spectroscopy (CVAAS) was developed as a technique

for heavy metal ion detection. Hatch and Ott transferred mercury ions to the spectrometer

optical measurement by neutralizing the ions with stannous chloride to generate the vapor form

of mercury16. A pure, dry gas such as air or argon is used to carry the mercury vapor. The light

12

absorption of mercury is directly proportional with the ion concentration. However, any other

ions in the inert gas, which can reduce the florescence emission, can affect the results.

A faster spectrometric approach for heavy metal detection, when compared to the

atomic adsorption technique, is inductively coupled plasma mass spectroscopy (ICP-MS).

Argon atoms are ionized by being placed inside an electromagnetic field generated by an RF

oscillation circuit producing a plasma discharge. This can ionize an aerosolized heavy metal

sample, which is introduced to the discharge and connected to the mass spectrometer for heavy

metal ion detection17. Micro molar level concentration of heavy metal has also been detected

using inductively coupled plasma atomic emission spectrometry (ICP-AES). Excited atoms

and ions produced by inductive coupled plasma emit electromagnetic radiation at wavelengths

that are related to the specific material18.

Even though these methods have reliable results, they are relatively expensive, time

consuming and need trained personnel.

2.2.2.2 Chemical and Biological Sensors

A chemo/bio sensor records a readable signal in response to a controlled binding event

by combining a chemical/biological sensing element with a transducer19,20. Prasad et al. 21

demonstrated that specific antigen antibody binding can be used to detect low concentrations

of environmental and physiological hazardous agents for neural studies. Metal ions can change

the enzyme function, suppressing them. The immobility of enzyme oxides or proteins have

been used to detect micro molar concentration levels of the toxic heavy metal compounds in

liquid media22,23. The need to develop alternative techniques of enzyme based sensors seems

to be necessary due to enzyme price, time consuming reactions and the required complex

enzyme reactions since the pollutants could not be recognized with a simple enzyme

reaction24,25. For this reason, Yamasaki et al.26 implemented a microbial based sensing system,

13

for copper detection in water. The presence of copper was found to suppress the bacterial

growth and a detection limit was reported to be in the milimolar concentration level. However,

it is worth mentioning that the use of microbes in this detector can be hazardous to the user.

2.2.2.3 Acoustic Sensors

In acoustic sensors, the change in the environment can be detected by the investigation

of an acoustic wave, which propagates throughout the media. These types of sensors are

typically based on piezoelectricity, which involves electrical to mechanical vibration

conversion and vice versa. Piezoelectric sensors have been used in various sensing applications

for measuring the changes in the propagating acoustic wave characteristics, such as magnitude

or phase shift in the frequency response27. An example of an acoustic sensor is a quartz crystal

microbalance (QCM) sensor, consisting of a quartz crystal and metal electrodes, that shows a

shift in resonant frequency with any change in the applied mass 28 . QCMs have been

successfully tested for heavy metal ion detection in liquid media at milimolar concentration

levels29.

A surface acoustic wave (SAW) device contains patterned electrodes on the top

piezoelectric substrate. Even though different SAW device configurations have been used,

since its discovery by Lord Rayleigh in 188530, the use of interdigital electrodes (IDEs) or

interdigital transducers (IDTs) and reflectors by White and Voltmer in 196531 and Staples et

al. in 197432, respectively have proven to minimize the power dissipation and hence obtain

optimized SAW generation and detection. Several advantageous features such as small size,

high resonant frequency, low power consumption and compatibility with CMOS technology,

makes them an appropriate choice when compared to conventional sensing systems33, 34.

SAW devices have been typically used in the Rayleigh, Lamb, shear horizontal (SH-

SAW) and Love propagation modes35. In the Rayleigh propagation mode, particle displacement

14

has two components: one is parallel to the SAW propagation direction and the other is normal

to the substrate, thus forming an elliptical trajectory for the particle. Most of the SAW energy

is confined to the surface of the substrate and is often diminished to zero at a substrate depth

of 4 to 5 times the wavelength36. Due to the normal component of the particle displacement

and interference with other media, above the substrate surface, this mode is not suitable for a

liquid environment.

The Lamb wave37 is similar to the Rayleigh wave and are typically generated using

very thin substrates that are only a few wavelengths thick.38 Like the Rayleigh waves, Lamb

waves have both normal and shear components and can be used for gas sensing, but they are

also well-suited for liquid sensing. Due to the low phase velocity of Lamb waves, there is no

energy penetration inside liquid media placed on the SAW surface, and hence no acoustic

streaming.

On the other hand, in the SH-SAW propagation mode, the particle displacement

happens only in parallel with the substrate surface, which prevents the vibration vector from

transferring into any secondary media and hence prohibits SAW energy attenuation when the

substrate comes into contact with a liquid39. Knodoh et al. proved SH-SAW liquid sensing

system’s capability by detecting various types of Japanese tea. The liquid properties were

evaluated by monitoring the frequency shift and amplitude change of the SH-SAW sensor

output40. SH-SAW sensor, has also been used for investigating oil contamination of ground

water by detecting hydrocarbons in micro molar concentration levels41. The use of the SH-

SAW device is thus a promising solution for heavy metal compound detection in liquid

environments.

Often, a thin-film called a guiding layer is applied to the surface of the device to activate

a Love wave mode42, which also inhibits radiation of acoustic pressure into the liquid making

Love wave operation the most common for sensing in liquids.43

15

Devices that can generate SH-SAWs, with much less bulk leakage, began to appear in

the 1990s, and often use a Love thin-film wave guide to further prevent leakage into a liquid

on the surface.44,45,46,47

The SAW device is operated at a designed resonant (or center) frequency (f) imposed

by the IDE design, wherein a typical IDE the electrode spacing is an integer multiple or 1/2,

1/3, or 1/4 of the wavelength = 2π/k, with k being the wavenumber and wavelengths typically

between 0.01 mm to 1.0 mm.

A variety of IDE designs have been implemented to affect both the resonance and the

propagation of the SAW. The most basic is a planar design, where the IDE consists of simple

linear electrode ‘fingers’ and the interdigitated spacing is consistent for each finger pair, as

shown in Figure 2.1.

Figure 2.1: Schematic of a planar IDE design SAW device48.

Figure 2.2: Curved IDE designed SAW device49.

16

However, other clever IDE designs are possible, such as curved or focused IDE (Figure

2.2) configurations to produce focused SAWs 50 or variable-spacing ‘chirped’ IDEs that

produce multiple resonant frequencies.51

A single IDE design is generally used to generate planar traveling waves that propagate

along the surface of the device, called a traveling wave, but sets of two or more opposing IDEs

can also be used to set up constructive interference and produce standing surface waves. To

inhibit reflections and scattering off the edges of the SAW substrate and minimize wave

reflection interference, acoustic absorbents such as gels (e.g. alpha gel) are often applied to the

edges of the piezoelectric substrate.52

Ultimately, the IDE design depends heavily on the nature of the SAW device and the

application in question. The progress in SAW device fabrication methods have focused on

developing novel flexible substrates instead of conventional, rigid single-crystal piezoelectric

substrates. Jin et al. developed flexible SAW devices which operate in Rayleigh and Lamb

wave propagation mode by depositing ZnO nanocrystals on bendable Kapton polyimide

films.53

Figure 2.3: Flexible SAW device fabricated on a nanocrystal film54.

17

2.3 Fluid Manipulation Techniques for Spray Generation

Spray technologies for the atomization of fluids play an important role across a vast

range of technologies ranging from drug delivery 55 and chemical analysis 56 to combustion

systems57 and material coatings58. In the biomedical field, aerosolized drug delivery for cystic

fibrosis 59, pneumonia60, influenza61, respiratory tract infections62 and pulmonary aspergillus63

have been under clinical trials. The results illustrate the controllable and concentrated drug

delivery in targeted regions which positively affects the dosing and duration of the therapeutic

procedures. Various mechanisms can be used to atomize fluids including pneumatic 64 ,

acoustic65 and electrically-driven sprays66.

2.3.1 Pneumatic Sprays

Pneumatic sprays work based on the interaction of two fluid phases, gas and liquid.

Compressed and high velocity gas is pressurized into a liquid jet to produce the spray. These

types of atomizers are able to produce the spray out of viscous materials at high throughput,

which makes them a subject of interest in painting, air brushes and fuel combustion systems67.

This atomizer structure can be designed to be simple and cost effective. However, there

is no precise control over the resultant aerosol size and hence the thickness and uniformity of

the covered area68. The characteristics of the spray are drastically affected by the size, geometry

and location of the liquid nozzle69,64,70, 71.

2.3.2 Acoustic Based Sprays

After Wood and Loomis made the first acoustic based spray system in 1927, Sollner

explained that the mechanical vibration of the surface causes the cavitation beneath the surface

of the liquid, which results in mist formation72. Lang showed that when an ultrasonic transducer

18

bridges between liquid and air, generated acoustic waves produce capillary waves that are able

to atomize the liquid73 . Since then, ultrasonic atomizers have been used for a variety of

applications such as plastics, coatings or in more advanced spray production system designs,

for organic film deposition74,75.

Piezoelectric crystals convert the electrical signal to mechanical vibrations; hence they

are apt choices for this approach. Surface acoustic wave (SAW) devices, explained in part

2.2.2.3, are well-known spray generation techniques. The delivered electrical signal to the

SAW device converts to particle displacement and vibrations due to the piezoelectric features

of the substrate. A sufficient amount of the input electrical signal can initiate particle

displacement in the axis normal to the surface; big enough to leak the energy into the droplet

or liquid film placed on the substrate, as shown in Figure 2.4(a).

Figure 2.4: (a) Overcoming the surface tension to break the droplet in SAW devices. (b) SAW

device configurations76.

a)

b)

19

The pressure difference and induced capillary wave can destabilize the liquid surface

and overcome the surface tension at a specific threshold to break the liquid into atomized

droplets. In a typical SAW device, this threshold can be achieved by delivering around 100 V

amplitude sinusoidal input voltage77.

SAW based spray generation has been used and available since 1990. 78 , 79 , 80 The

physical mechanism behind the spray generation and the designing optimization for SAW

atomizer have been well documented in the literatures.81,82,83

Friend, Yeo and collaborators published several papers about the use of microscale

aerosolized droplets generated by SAW devices in pharmaceutical84 and chemical85 industries.

It was also demonstrated that the resonant frequency of the SAW device and the geometrical

configuration of the interdigital electrodes (IDEs), shown in Figure 2.4(b), can affect the size

of the droplets, the uniformity of the mist and the onset voltage for atomization that is

achievable by SAW device design and fabrication86,.

The resonant frequency of the SAW device is a function of the width of the metalized

IDEs patterned on the piezoelectric substrate, calculated by87

𝑓 ∝1

4𝑤(2.1)

where w is the width of the IDE. The radius (R) of the generated aerosol using the SAW

atomizer, on the other hand, this is88

𝑓 ∝1

𝑅(2.2)

Therefore, a fabricated SAW device with high center frequencies (in the MHz range)

can produce relatively small and mono-dispersed atomized droplets. For atomization

applications, the shape of the IDEs can play a major role on the minimum required input

voltage, known as onset voltage. This is because patterning specific IDE configurations will

enable the acoustic power to be focused on a desired point of the substrate89. However, SAW

20

device fabrication means patterning micron size metalized fingers on the piezoelectric substrate

in a clean room, which can be complicated, time consuming and expensive.

2.3.3 Electrically-Driven Sprays

Electrically-driven sprays, where a high voltage is applied to a liquid flow or film, have

a long and distinguished history90,91. To this day, electrosprays are widely used for mass

spectrometry systems92 and emerging applications include micro-propulsion93, spray coating94,

materials synthesis95 and printing96.

In electrosprays, positive and negative ions get separated by the electrical field. In the

late 16th century, William Gilbert discovered the effect of the electrical field on water droplets,

and in the mid-20th century, Sir Geoffrey Ingram Taylor modeled the shape of the cone made

by applying the electrical field to the droplets97. A high voltage between 1 to 20 kV is applied

between a nozzle electrode and a flat grounded electrode (Figure 2.5). The liquid is provided

continually to the nozzle electrode and a substrate is placed on the grounded electrode for thin

film formation. When the input voltage, applied to the liquid in the nozzle electrode, crosses

the threshold, the liquid is drawn into a cone. The liquid reaching the nozzle electrode tip forms

a Taylor cone, which emits a liquid jet through its apex due to charge accumulation at the apex

of the cone which overcomes the surface tension of the liquid.

In fact, Taylor cones are the result of the electrified interface between the conductive

liquid and air, when the charge reaches a critical level. Initially, a 100 µm size thin liquid jet

and eventually a series of small and highly charged liquid droplets will be formed. The droplet

diameter is directly proportional to liquid mass density and indirectly proportional with liquid

conductivity and surface tension98. One limitation of most electrospray systems is the nature

of the droplet plume, which typically arises from the tip of a liquid cone (Taylor cone99 or cone-

21

jet100) and spreads conically to cover an area101. Thus, the projected area of the droplet plume

is circular, which is not ideal for applications such as uniform coating.

Figure 2.5: Schematic of a typical electrospray system102. High voltage applied between the metal

nozzle electrode and substrate generates the spray from the precursor liquid delivered by the syringe.

The size of this plume can be affected by the spray and solution conditions 103 .

Alternative strategies, such as using alternating current (AC) fields, can also be used to generate

linear jets of droplets rather than plumes104. Yet, developing ways to generate a uniform, broad

area droplet plume could be important for applications such as spray coating.

As another type of electrically-driven spray formation, paper spray is an ambient

ionization method that is well known as a reliable, time efficient, easy way for mass

spectrometry with no sample preparation105. Similar to electrospray ionization, it appears to

produce charged droplets via Taylor cones. Typically, 3-5 kV input voltage is required and

applied to a piece of paper which contains the desired liquid, via copper clips to make the spray

out of the liquid as shown in Figure 2.6106. Delivering a sufficient electrical field will produce

charged droplets by liquid breakage due to Columbic forces according to107

𝐹 =𝑉

𝑟 (2.3)

22

where V is the applied voltage and r is the radius of the cut paper knowing that the

minimum necessary field strength for field ionization which is around 107 Vcm-1 107.

For this purpose, various types of papers, ranging from chromatographic paper to fiber

glass papers, have been cut to have a sharp point with a tip angle between 60° to 150° and used

for paper spray108. Research has shown that fiber glass paper shows poor performance when

compared to chromatographic papers109. The sharp tip of the paper can provide a higher electric

field as well as help in microfluidic transportation110 . High resolution images and phase

Doppler analysis have demonstrated the mechanism behind paper spray ionization by Espy et

al.107. High speed photographic images suggested this phenomenon occurs in two separate

modes depending on the solvent flow rate. Taylor cones are formed and numbered in the

(Figure 2.7). The first mode happens for high solvent flow rates by applying 3.5 to 4.5 kV and

0.1 to 0.2 µA, where Taylor cones result in a wide range of droplet size formations. After a

sufficient amount of solvent is depleted, mode two will occur. In this mode, the solvent flowrate

is smaller and the current is higher when compared with the first mode; also, corona discharge

generates smaller and monodispersed droplets. The short spitting mode in paper spray makes

it a proper method for mass spectrometry but not practical in procedures that needs a longer

endurance such as coating applications.

23

Figure 2.6: Paper spray setup used for mass spectrometry 111.

Figure 2.7: Taylor cone formation in paper spray107. Cones are shown with numbers.

2.4 Summary

This chapter provided a brief background and introduction to fluid manipulation in

sensing and spray generation systems. The conventional systems for sensing toxic heavy metal

ions in liquid media were introduced. The conventional sensing systems are expensive, time

consuming and need complicated procedures with trained staff which suggests introducing a

new procedure that overcomes these drawbacks. Spray generation technologies were also

24

summarized in this chapter. Based on the high applied voltage, small area coverage and non-

continuity of the spray generated by the conventional systems, this field of study still needs an

innovative idea that overcomes these deficiencies. The author will discuss how piezoelectric

devices can be a proper choice to design new systems for sensing and spray generation

techniques that conquer the obstacles of the traditional approaches.

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38

CHAPTER III

INVESTIGATION OF A PIEZOELECTRIC BASED SENSOR FOR TOXIC

HEAVY METAL DETECTION

3.1 Introduction

Some of the most common heavy metal pollutants are lead (Pb) and cadmium (Cd)1.

Pb has been categorized as a neurotoxic element due to its severe impact on the nervous system,

causing neurotoxicity and nephrotoxicity in the gastrointestinal and renal systems2,3,4,5. Cd,

which has been increasingly used in industrial product lines, is known to cause kidney failure,

hypercalciuria and osteoporosis6,7,8. Mercury (Hg) is the second most toxic heavy metal in the

planet. Hg is a neurotoxicant, which targets the central nervous system as well as liver and

heart muscles in the human body9. Nickel (Ni) has been seeing a rising demand in the surface

coating and jewelry industries. Ni is a carcinogenic contaminant, which causes pulmonary

fibrosis, asthma, lung and nasal cancer in case of long term exposures10. All these elements are

often released from natural and manmade sources and have the capability to accumulate in the

vital organs and cause fatalities11,12.

Heavy metal detection methods including spectrometric techniques such as cold vapor

atomic absorption spectroscopy, inductively coupled plasma mass spectroscopy,

electrochemical impedance spectroscopy, UV-visible absorbance spectroscopy, x-ray, laser

and colorimetric analysis as well as biosensing detection techniques based on enzyme

activation or bacteria have been reported13,14,15,16,17,18,19,20. However, these techniques are often

expensive, time consuming, laborious, hazardous and require complex labeling methods.

39

Therefore, it is necessary to develop sensing systems with easy-to-use, cost effective, highly

sensitive and rapid detection techniques.

In this work, the author has designed and fabricated an efficient piezoelectric device,

consisting of gold (Au) interdigital electrodes (IDEs) on a 64° YX-LiNbO3 based piezoelectric

substrate known as a shear horizontal surface acoustic wave (SH-SAW) sensor, for the

detection of heavy metal compounds. The main focus of the author was to design a system

which has a small limit of the detection to be able to show a recordable signal for low

concentrations of the toxic heavy metals in liquid media. The range of the detected heavy

metals by this system was desired to be as wide as possible. Proper chemical sensing layer,

Phenol or naphtho[2,3-a]dipyrido[3,2-h:2',3'-f]phenazine-5,18-dione (QDPPZ) are used to

selectively bind to the heavy metal ions. An acrylic based flow cell was used to hold the sensor

and PDMS microfluidic channel. The capability of the system for detecting different

concentrations of heavy metal compounds, such as lead nitrate (PbNO3), cadmium nitrate

(CdNO3), mercury (II) nitrate, (Hg(NO3)2) and nickel (II) nitrate (Ni(NO3)2) in liquid

environments, through phase shifts in the frequency based response (S21) of the SH-SAW

sensor are investigated.

3.2 Theory

For sensing applications, the SAW devices typically work at higher frequencies (~100-

500 MHz) whereas most microfluidic devices operate at lower frequencies (~10-100 MHz).

The most common piezoelectric substrates are single crystal lithium niobate (LiNbO3), lithium

tantalate (LiTaO3), and quartz (SiO2) with common cuts including 128 y-x cut for LiNbO3 and

ST and AT cuts for quartz substrates.

The piezoelectric based SH-SAW devices, generally consists of two sets of metalized

input and output IDEs mounted on a piezoelectric substrate. The surface acoustic wave is

40

generated by an impulse or sinusoidal input electrical signal applied at the input IDEs. It then

propagates on the piezoelectric substrate through the delay line, which is the distance between

the input and output IDEs and is converted back to an electrical signal, as shown in Figure 3.11.

Figure 3.1: Schematic of a delay line SAW device with input and output IDEs.

The SH-SAW device output signal can finally be obtained from the output IDEs. The

SAW resonant frequency can be mathematically calculated by21

𝑓0 =𝑣𝑠

𝜆 (3.1)

𝜆 = 4×𝑤 (3.2)

where, f0 is the resonant frequency of the SAW, vs is the SAW propagation velocity on

the piezoelectric substrate, λ is the SAW wavelength and w is the width of the IDEs. The

resonant frequency of the SH-SAW sensor is dependent on the velocity of the acoustic wave

passing through the piezoelectric substrate. The velocity of the wave is known to vary due to

mass, stiffness, conductivity, dielectric coefficient, temperature and pressure changes22. The

absorbance of the different concentrations of toxic heavy metals by the sensitive layers, which

are coated on top of the piezoelectric substrate, causes a mass loading effect to take place. mass

loading also changes the SAW propagation velocity and thus the resultant center frequency,

which can be used to quantify the concentration levels23.

41

The principle behind SAW-based sensing devices is that the target chemicals that

adsorb on a functionalized surface on the SAW substrate fundamentally change the frequency

of the SAW, and measurement of the frequency shift (∆f) can be correlated to the concentration

of the target species. Based on this principle, SAW sensors were initially developed more than

three decades ago24,25, and are currently employed as fundamental components in a variety of

microsensing systems. 26 The main advantage associated with SAW sensors is their high

sensitivity and capability for the trace detection of a wide variety of chemical materials

including bio/chemicals27,28, organic and inorganic vapors29,30, and explosives.31

Chemical SAW sensing systems utilize a sensing layer on the SAW substrate that is

designed to react or bind with the analytical target. The sensing layer interacts with the target

to cause detectable changes in the features of the acoustic wave, such as the velocity or

amplitude, which are then manifested as a frequency shift or insertion loss.

According to the literature, increasing the resonant frequency can enhance the sensor

performance by increasing the amount of the frequency shift response to the change in media

and subsequent sensitivity. However, at frequencies in the GHz range, the noise factor will

have more effect on the results. The sensitivity of the sensor is defined as32

𝑆 =∆𝑓

𝑓0⁄

𝑢(3.3)

where the Δf is the frequency shift of a sensor with resonant frequency equal to fo in

response to the unit measurand of u.

42

3.3 Experimental

3.3.1 Chemicals, Materials and Sample Preparation

PbNO3, CdNO3, Hg(NO3)2 and Ni(NO3)2 in crystalline form, were purchased from the

Sigma Aldrich Chemical Company. Various solutions of deionized water and PbNO3, CdNO3,

Hg(NO3)2 and Ni(NO3)2 in concentrations of 1 pM ,100 pM, 1 nM, 100 nM, 1 μM, 50 μM,

100 μM and 750 μM were prepared. PbNO3, CdNO3, Hg(NO3)2 and Ni(NO3)2 are all soluble

in the DI water and the process will not be affected by NO3- anions33. All test analytes were

stored at −20 ºC in 10 ml aliquots before use. Tubing (Inner diameter - 0.01"; Outer diameter

– 0.0625") and tube connection accessories for sample transfer were purchased from Upchurch

Scientific.

3.3.2 Synthesis of the Chemical Sensing Layer

The sensitive layer for this experiment, which has been synthesized for sensing

applications, was acquired from the Chemistry Department at Western Michigan University.

The chemical sensing layer QDPPZ was synthesized using a two-step process34,35,36,37. In the

first step, concentrated sulfuric acid (H2SO4, 20 mL) and concentrated nitric acid (HNO3,

10 mL) were added dropwise to a mixture of 1,10-phenanthroline (1.00 g, 5.56 mmol) in the

presence of potassium bromide (KBr) (5.95 g, 50 mmol) at 0°C. The solution was refluxed for

2 hours then cooled to room temperature, yielding a black, oily product. The contents of the

flask were diluted with 400 ml deionized water and neutralized with sodium bicarbonate

(NaHCO3), yielding a clear yellow solution. The product was extracted with methylene

chloride and dried over anhydrous magnesium sulfate (MgSO4). The solvents were removed

using a rotary evaporator, resulting in a yellow solid. The product was purified by

43

recrystallization from methanol. The average yield of the product, 1,10-phenanthroline-5,6-

dione was (1.11 g, 5.31 mmol) which was calculated to be 95%. The resulting material was

achieved to be formulated as 1H NMR (400 MHz, CdCl3, 25 oC) δ: 9.12-9.10 (t, 2H, J = 2.95

Hz), 8.51-8.48 (d, 2H, J = 1.83 Hz), 7.60-7.55 (m, 2H, J = 4.71 Hz).

In the second step, 1,10-phenanthroline-5,6-dione (0.50 g, 2.38 mmol) was refluxed in

ethanol for 15 min. 9,10-diaminoanthroquinone (0.981 g, 2.38 mmol) was then added, resulting

in a purple solution, and the solution was refluxed for 4 hours. The dark purple product was

collected using vacuum filtration, washed with methanol and concentrated in vacuum. The

reaction yield was 80%. Resulting material was achieved to be formulated as 1H NMR (400

MHz, CdCl3, 25 oC): δ 9.83 (d, 1H), 9.64 (d, 1H), 9.28 (d, 2H), 8.64 (dd, 2H), 8.27 (q, 2H),

7.82 (m, 4H). The final structure is shown in Figure 3..

Figure 3.2: Structure of QDPPZ.

3.3.3 Sensor Fabrication

Figure 3.3(a) illustrates the schematic of the SH-SAW device. The SH-SAW sensor

was fabricated on a 64° YX-LiNbO3 piezoelectric substrate using photolithography techniques.

Eight pairs of input and output IDEs, 0.1 μm thick Au, were patterned by metal sputtering

44

technique on the piezoelectric substrate. The electrode aperture is 760 µm, while electrode

width and gap are 10 μm thereby resulting in a 40 µm acoustic wave length (λ). The SH-SAW

sensor also consists of 120 reflectors fabricated on the outer side of the input and output IDEs

with similar dimensions and 20 reflectors in between the IDEs to reduce the scattering losses

of surface acoustic waves38. Since the SAW propagation velocity on the 64° YX-LiNbO3

piezoelectric substrate is 4474 m/s and the designed wavelength is 40 µm, the resultant

resonant frequency is calculated to be 111.8 MHz39. The photograph of the fabricated SH-SAW

sensor, with overall device dimensions of 11 × 12 mm2 is shown in Figure 3.3(b). Two sets of

input IDE, output IDE, reflectors and electrical pads were patterned on one substrate for cost

efficiency purposes. Author just used one set for this experiment.

Figure 3.3:(a) Schematic of the SH-SAW device and (b) fabricated SH-SAW sensor.

Input IDEs Output IDEs

Reflectors ReflectorsReflectors

45

3.3.4 Flow Cell Fabrication

The flow cell for this experiment was provided by the Sensor Technology Laboratory

(STL) in the Electrical and Computer Engineering Department at Western Michigan

University. The flow cell, with overall device dimension of 70 × 50 × 52 mm (w/l/h), was

designed in AutoCAD™ and CNC machined using acrylic material (Figure 3.4). A

microfluidic flow channel, with dimensions of 710 × 6800 × 710 µm (w/l/h) and a total

channel volume of approximately 3.4 µl, was also fabricated with PDMS. The flow of the test

analyte through the PDMS microfluidic channel was obtained by integrating two sets of inlet

and outlet ports in the flow cell.

Figure 3.4: Flow cell with the SH-SAW sensor in the sensor groove (Inset: PDMS microfluidic

flow channel.

The effective closing of the flow cell, which results in the tight sealing of the PDMS

microfluidic flow channel around the sensing area of the SH-SAW sensor, was made possible

by the use of an axially magnetized set of neodymium magnets (Diameter - 0.25"; Thickness -

0.375"; Magnetic strength – 13,200 Gauss) purchased from K&J Magnets, Inc.

SH-SAW Sensor in Sensor Groove

Gold Tipped Electrical Spring

Probes

Neodymium Magnets for proper

flow cell closing PDMS Microfluidic Channel

(Volume ~ 3 µl)

46

3.3.5 Experiment Setup

The experiment setup is shown in Figure 3.5.The SH-SAW sensor was placed in the

sensor groove of the flow cell. Calibration for the wires and probes was done before the

measurements. Before use and at the end of each experiment, the sensor was cleaned with

acetone, and then blow dried with pressurized air. The measurements were performed at

constant room temperature (25 °C), using a heater occupied with thermocouple, since any

changes in the temperature would affect the SAW velocity and attenuation40.

Initially, a reference signal for deionized (DI) water was obtained. Then varying

concentrations of analytes were injected into the flow cell using a KD Scientific (KDS210P)

programmable syringe pump, at a flow rate of 50 μL/min. An Agilent 4395A network analyzer

was used to measure the frequency response (S21) of the SH-SAW sensor towards the test

analytes. System control, data acquisition and post processing of the network analyzer

measurements was performed using a LabView™ based application.

Figure 3.5: Experimental setup.

3.4 Results

To investigate the practicality of the system, results were first obtained towards

different concentrations of PbNO3. Figure 3.6 shows the frequency response (S21) of the SH-

SAW measured using network analyzer which contains both magnitude or insertion loss and

Agilent 4395ANetwork Analyzer

Flow Cell with SH-SAW Sensor

KD Scientific Programmable Syringe

Pump (KDS210P)

LabViewTM

application on PC

Withdrawal

Infusion Connected

Via SMA Cable

Via GPIB

Connected

47

phase. The author decided to record the phase aspect of the frequency response since it results

in an easier to detect shift.

Figure 3.6: Magnitude (insertion loss) and phase of the frequency response for SH-SAW sensor.

Figure 3.7 shows the frequency response (S21) of the SH-SAW towards different

concentrations of PbNO3. It was observed that the resonant frequency of the reference signal

established by DI water shifted from 108.564 MHz to 108.551 MHz, 108.543 MHz,

108.529 MHz, 108.508 MHz, 108.503 MHz, 108.499 MHz, 108.494 MHz and 108.479 MHz

for the 1 pM, 100 pM, 1 nM, 100 nM, 1 μM, 50 μM, 100 μM and 750 μM concentrations of

PbNO3 solution, respectively (Figure 3.7 (a)). This results in frequency shifts of 13 kHz,

21 kHz, 35 kHz, 56 kHz, 61 kHz, 65 kHz, 70 kHz and 85 kHz along with percentage changes

of 0.011 %, 0.019 %, 0.032 %, 0.051 %, 0.056 %, 0.059 %, 0.064 % and 0.078 % for the 1 pM,

100 pM, 1 nM, 100 nM, 1 μM, 50 μM, 100 μM and 750 μM concentrations of PbNO3 solution,

respectively when compared with DI water (Figure 3.7 (b)). The experiment was repeated

48

seven times using two different fabricated sensors and standard deviations of 3.5 kHz, 1.7 kHz,

2.1 kHz, 6.8 kHz, 3.5 kHz, 4.6 kHz, 2.1 kHz and 5.6 kHz from the average value of 15 kHz,

22 kHz, 33 kHz, 48 kHz, 57 kHz, 64 kHz, 69 kHz and 80 kHz were achieved for the 1 pM,

100 pM, 1 nM, 100 nM, 1 μM, 50 μM, 100 μM and 750 μM concentrations of PbNO3 solution,

respectively .

Figure 3.7: (a) SH-SAW sensor frequency response (S21) based on phase variation towards

varying concentrations of PbNO3 and (b) Changes in frequency shift of SH-SAW sensor response.

Then, varying concentrations of CdNO3 were injected onto the SH-SAW sensor. It was

observed that the resonant frequency of the reference signal established by DI water shifted

from 109.290 MHz to 109.100 MHz, 109.076 MHz, 108.989 MHz, 108.979 MHz,

(

a)

b)

49

108.963 MHz, 108.948 MHz, 108.944 MHz, 108.935 MHz and 108.933 MHz for the 1 pM,

100 pM, 1 nM, 100 nM, 1 μM, 50 μM 100 μM, 250 μM and 750 μM concentrations of CdNO3

solution, respectively. The frequency response (S21) of the SH-SAW sensor demonstrated

frequency shifts of 190 kHz, 214 kHz, 301 kHz, 311 kHz, 327 kHz, 342 kHz, 346 kHz,

355 kHz and 357 kHz along with percentage changes of 0.173 %, 0.195 %, 0.275 %, 0.284 %,

0.299 %, 0.312 %, 0.316 %, 0.324 % and 0.326 % for the 1 pM, 100 pM, 1 nM, 100 nM, 1 μM,

50 μM 100 μM, 250 μM and 750 μM concentrations of CdNO3 solution, respectively when

compared with DI water (Figure 3.8). The experiment was repeated eight times using two

different fabricated sensors and standard deviations of 4.2 kHz, 1.1 kHz, 1.1313 kHz, 1.6 kHz,

1.7 kHz, 2.5 kHz, 1.6 kHz, 1.5 kHz and 1.3 kHz were obtained from the average value of

109 kHz, 129 kHz, 175 kHz, 192 kHz, 203 kHz, 217 kHz, 221 kHz, 235 kHz and 271 kHz for

the 1 pM, 100 pM, 1 nM, 100 nM, 1 μM, 50 μM 100 μM, 250 μM and 750 μM concentrations

of CdNO3 solution, respectively.

Also, Figure 3.9 shows the SH-SAW sensor resonant frequency shift for the varying

concentrations of Ni(NO3)2 solution, when compared with DI water. Frequency shifts of

273.3±60 kHz, 441.7±50 kHz, 566.7±49 kHz, 673.3±43 kHz and 806.7±57 kHz was observed

for the 1 pM, 100 pM, 1 nM, 100 nM and 1 μM concentrations of Ni(NO3)2 solution,

respectively.

50

Figure 3.8: SH-SAW sensor frequency shift towards varying concentrations of CdNO3 .

Figure 3.9: SH-SAW sensor frequency shift towards varying concentrations of Ni(NO3)2.

The resonant frequency shifts can be attributed to the change in SAW propagation

velocity, in the delay line, caused by the varying concentrations of the test analytes. The

development of a selective and sensitive chemical sensing layer relied on the deposition of the

organic ligand, QDPPZ, which is a 1,10-phenanthroline derivative that consists of nitrogen and

oxygen groups that will coordinate to attach the heavy metal ions. Initial work has

demonstrated that the quinolone unit was essential in inducing selectivity towards the heavy

metal ions, which was not observed if 1,10-phenanthroline alone was used. The binding of

51

QDPPZ to the heavy metal ions causes an increase in mass between the electrodes, thus

affecting the surface acoustic wave function. As the ions flow through the sample channels,

QDPPZ molecules coordinate with the metal ions, forming a complex and increasing the mass

of the material between the electrodes.

However, QDPPZ was not an apt choice for mercury ion detection. Performing the

same experiment using mercury nitrate did not show any frequency shift in our SAW sensing

system. This suggested that unlike lead, cadmium and nickel, QDPPZ is not an adsorbent for

mercury ions. In the second attempt for mercury nitrate detection using our system, the author

used Phenol as the sensing layer on the SAW sensor, based on the approved binding event

between phenolic group and metals such as copper41, and the ability of the system towards the

mercury nitrate detection was tested by introducing different concentration of Hg(NO3)2

solution.

Figure 3.10 shows the SH-SAW sensor resonant frequency shift for the varying

concentrations of Hg(NO3)2 solution, when compared with DI water. Frequency shifts of

184.8±23 kHz, 378.3±62 kHz, 458.3±80 kHz, 600.0±96 kHz and 748.3±116 kHz were

observed for the 1 pM, 100 pM, 1 nM, 100 nM and 1 μM concentrations of Hg(NO3)2 solution,

respectively.

Figure 3.10: SH-SAW sensor frequency shift towards varying concentrations of Hg(NO3)2.

52

These sets of frequency responses also displayed detection levels as low as picomolar

concentrations and the ability of this piezoelectric sensor to distinguish among a wide range

(micro, nano and pico level) of sample concentrations. The results satisfied the main goal of

this chapter which was introducing a sensing system with a small limit of detection.

The results obtained demonstrated that phenol and QDPPZ can be employed as

sensitive layers for Hg(NO3)2 and PbNO3, CdNO3 and Ni(NO3)2, respectively. It is worth

noting that the approved toxicity level of Pb and Cd according to the United States Food and

Drug Administration (USFDA) is 1.25 mM and 207 µM, respectively 42 .The United States

Environmental Protection Agency (EPA) reported maximum allowable level for Hg(NO3)2 and

Ni(NO3)2 in drinking water to be 10 nM and 0.3 µM, respectively43, 44. Considering these

toxicity limits, this system can follow any trace of these toxic heavy metals before the toxicity

reaches to the dangerous level to avoid any health issues.

3.5 Summary

In this chapter, the author provided a brief introduction about the conventional methods

of toxic heavy metal detection in aqueous solution as well as the needs of improving these

methods. The author then described theoretical explanations of the piezoelectric sensing based

system. This was followed by a detailed report of the performed experiments including the

chemicals, materials and sample preparation; synthesis of the chemical sensing layer; sensor

fabrication; flow cell fabrication and experiment setup. Final results of the experiments are also

represented.

To summarize, a SH-SAW sensor was successfully fabricated on a 64° YX-LiNbO3

piezoelectric substrate. An efficient flow cell was also designed and fabricated using acrylic

material. The flow cell consisted of inlet and outlet ports for the microfluidic chamber and

PDMS based microfluidic channels. The feasibility of using the SH-SAW sensor for detecting

53

heavy metal compounds was demonstrated through the quantitative detection of PbNO3,

CdNO3, Ni(NO3)2 and Hg(NO3)2. A proper chemical adsorbent was used as the sensing layer.

The measured resonant frequency response shift of the SH-SAW sensor demonstrated

a 13 kHz, 190 kHz, 273 kHz and 184 kHz frequency shift for the 1 pM concentrations of

PbNO3, CdNO3, Ni(NO3)2 and Hg(NO3)2 respectively, when compared with the DI water. The

results show the capability of the SH-SAW sensor to detect pico molar concentrations of toxic

heavy metals in water, which is several orders of magnitude lower than the maximum allowable

level for toxicity in drinking water.

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26 Inoue, Yasunobu, Yoshihiro Kato, and Kazunori Sato. "Surface acoustic wave

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28 Sankaranarayanan, Subramanian KRS, Reetu Singh, and Venkat R. Bhethanabotla.

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59

CHAPTER IV

PIEZOELECTRIC TRANSFORMER BASED SPRAY

GENERATION SYSTEM

4.1 Introduction

The drawbacks of the conventional spray generation systems, were discussed in detail

in the literature review (Section 2.2), including the lack of thickness and uniformity control in

pneumatic sprays, the time and money consuming fabrication process of surface acoustic

devices in acoustic spray systems, and applied high applied voltage and the plum shaped spray

area in electro sprays. These challenges indicate the necessity of further research and system

design in this area of this technology. In this chapter, the author has been investigating the

ability of piezoelectric crystals, in a simple and reliable design, significantly improve spray

generation .

In this study, the author utilized a piezoelectric transformer for inducing the generation

of a broad area electrospray. A piezoelectric transformer (PT) is a piezoelectric device that

directly amplifies a low input alternating current (AC) voltage (~10 V) to values that are several

orders of magnitude larger (~102-103 V) through the electromechanical coupling effect of the

piezoelectric substrate.

The physics underlying the generation of this spray are complex, and the exact

mechanism is unclear. It is well known that the PT generates both mechanical displacement

(vibration) and high surface voltages (with accompanying electric field) at the end of the crystal

where it is in contact with a paper wick. It has been previously demonstrated that ultrasonic

60

piezoelectric actuation can be used to vibrationally break a liquid film and atomize liquid on

the surface1. Furthermore, on 128°YX LiNbO3 crystals, both surface acoustic Rayleigh and

Lamb waves have been shown to atomize fluid on the surface2,3, although at radio frequencies

(~MHz) much higher than those used here.

In this chapter, the author introduces a novel approach of broad spray formation using

a piezoelectric transformer (PT). A 100 mm × 15 mm × 0.5 mm (l/w/t) 128°YX LiNbO3

piezoelectric crystal PT device was used at the second resonant frequency. Using a rectangular

micro fiber glass paper, liquid film was delivered to the crystal surface. Applying the AC

voltage at the resonant frequency led to the liquid film break down and spray formation along

the entire width of the PT device. A comprehensive study was performed by the author to

investigate the mechanism and the effect of parameters such as liquid properties or power

delivery on the spray formation.

4.2 Experiments and Results

4.2.1 Piezoelectric Transformer and Stand Design

A 100 mm × 15 mm 5 mm (l/w/t) cut of a piezo electric crystal was used as the

piezoelectric (PT) device. Although different geometric configurations can be used4, the author

utilizes one that is based on a linear 128 Y-cut lithium niobate (LiNbO3) crystal. Bottom and

top electrodes were formed by painting silver electrodes half the length of the PT on both sides.

A proper stand was designed using AutoCAD software and 3D printed to hold the PT in a

location that support the second resonance standing wave nodes (Figure 4.1).

61

Figure 4.1: 3D printed PT stand to pin the device in L/4 of the tips. Conductive tape was used to

apply the input voltage to the razor blade in contact with top and bottom electrodes.

The PT was actuated by a signal generator (Agilent Model 33220A) connected to an

RF amplifier (Powertrone Model 500A). Sinusoidal waveforms were used for the studies. An

oscilloscope (Tektronix DPO 2024B) was used to measure the applied voltage and current via

a resistor-capacitor-inductor (RLC) notch filter set at the driving frequency. A PT-driven

electrospray was generated by applying an AC input voltage, at the second harmonic resonant

frequency of the 128°YX LiNbO3 crystal, to two electrodes on the top and bottom of the

piezoelectric surface. The resonant frequency is a strong function of the surrounding

capacitance of the system, and for these studies, the second resonance was typically ~60 kHz.

4.2.2 Spray Generation

To generate the spray, various deionized (DI) water based aqueous solutions were filled

in an adjacent reservoir and a paper wick was placed in the reservoir and in contact with the

surface of the PT, as shown in Figure 4.2(a). The paper bridge serves to continuously wick the

solution to the PT surface , allowing the generation of a continuous electrospray, similar to

methods used for surface acoustic wave atomization5. Various types of paper were examined

62

as the wick, the paper_ PT contact and spray consistency were each observed. Ultimately,

fiberglass paper (Ahlstrom Company) produced the most consistent spray.

As shown in Figure 4.2(b), inputting an AC voltage at one end of the crystal generates

a standing mechanical wave, and due to electromechanical coupling, a large polarization is

generated along the surface of the crystal. By mounting the crystal, so that the nodal resonance

of the standing wave is pinned, substantial surface voltages are reached at the crystal tip.

Figure 4.2: (a) PT-driven electrospray configuration with solution delivery from a paper wick.

(b) Geometric configuration of the PT with resulting standing displacement and stress wave. (c)

Photograph of PT-generated electrospray of 50 mM NaCl solution.

Notably, it has been shown that these surface potentials are sufficient to breakdown gas

and form a plasma6,7,8. Here, the author utilizes this surface voltage to generate a continuous

electrospray by supplying an aqueous solution to the crystal surface.

Figure 4.2(c) shows an image of a PT-generated electrospray of a sodium chloride

solution (50 mM, NaCl) using an input AC voltage amplitude of 15 Vamp at 59.78 kHz. The

author found that the spray forms directly from the paper wick, and only when the wick is in

good contact with the PT. Unlike conventional electrospray, there is no secondary or ‘counter’

electrode used here, nor is the liquid actively pumped through a capillary by an external source

such as a syringe pump. Instead, liquid flow is driven through a combination of capillary action,

63

to saturate the paper, and an electrokinetic flow as mass is removed by the spray. Thus, the

spray is inherently self-limited by the amount of electrokinetic flow that can be generated.

Unlike conventional electrospray, the droplets do not appear to be formed from Taylor cones;

rather the behavior is more like a spitting mode9. Also, of note is that, because there is no

counter electrode and the field is inherently AC, the droplets leaving the paper are not directed

by an electric field as in conventional DC electrospray. Instead, they essentially follow ballistic

projectile trajectories under the influence of gravity, falling to the surface beneath the PT. The

spray is generated uniformly along the width of the LiNbO3 crystal, which is 15 mm for these

experiments, such that the spray covers a much wider area than the circular area covered by a

typical capillary electrospray. This could be especially useful for spray applications to cover a

wide area.

4.2.3. Spray Coating Profilometry

To demonstrate this spray generation system’s potential for wide area coating purposes,

the author sprayed fluorescent microspheres onto a glass slide placed beneath the PT. Figure

4.3(a) shows the 3D profilometry of a glass substrate spray-coated by 1 μm red fluorescing

polymer microspheres (Duke Scientific Corp.) diluted in an aqueous solution of 50 mM NaCl.

The spray was operated at 20 Vamp AC input voltage for 70 seconds. The surface roughness of

was measured to be 0.05 μm over an area of 15 mm × 5 mm using a profilometer (Bruker from

Lafayette Instrument Co.). This roughness is negligible when compared with the 1 µm bead

diameter, and indicates uniform coating. Figure 4.3(b) provides a scanning electron microscope

(SEM) image of the deposited beads, showing fairly uniform coverage.

64

Figure 4.3: 3D profilometry of a glass slide coated by 1 µm beads. The roughness was determined

to be 0.05 μm over an area of 15 mm × 5 mm. (b) SEM image of as deposited particles.

4.2.4 Volumetric Flow Estimation

The author measured the weight (and hence volume) of the collected spray using a

digital lab scale as a function of time to estimate the volumetric flow rate. The PT was mounted

above the sensitive plate of a precise weight balance (Ohaus Adventurer Pro Digital Balance)

connected to a PC and a Matlab-based program for data acquisition (Appendix A). The

generated spray was directly collected in a container on the balance, and the weight was

averaged over 30 s at a rate of 10 samples/s.

(b)

50 µm

10 µm

65

Figure 4.4: Generated spray volume for 5 mM NaCl solution in DI water versus time. The slope of the

curve fit indicates a constant volumetric flow rate of 20 µl/min over 5 min of applied constant 19 Vamp AC

input voltage. The dash lines represent the standard deviation in the spray volume from three repeated

experiments.

The entire experiment was conducted in a glass box to limit the influence of random

drafts in the laboratory and limit the influence of evaporation. Figure4.4 shows the measured

volume as a function of time for a duration of 5 minutes, illustrating the overall stability of the

spray.

The author estimated that the volumetric flow rate is ~12-30 µL/min, depending on the

exact experimental conditions (input voltage, spray solution). For comparison, most mass

spectrometry applications use rates between 0.1-10 L/min, and most spray coating

applications have rates of 10-600 L/min10,11.

4.2.5 Linear Flow Speed Visualization

To confirm that the flow was not simply due to capillary action, the author also tracked

the solution front through the paper wick using red dye. The pumping effect of the PT-driven

spray system was investigated by tracking the flow front of red dye through the paper (Fig.

4.5).

66

Figure 4.5: Front tracking of the red dye during spray generation.

Using videography, the flow front was visualized and a frame-by-frame analysis was

used to analyze the flow.

Figure 4.6 shows the front distance as a function of time, and the linear relationship

indicates a constant flow speed of ~0.12 mm/s. In contrast, the distance-time relationship for

strict capillary flow is non-linear (d ~ t1/2), and the author measured that wicking alone over 10

min caused the front to progress approximately 3 mm . The solution front progresses linearly

with time, indicating a constant speed through the paper wick. This evidence indicates that the

PT is actively pumping the fluid during the spray process faster than capillary action alone.

Figure 4.6: Distance of the front of red dye through the paper wick as a function of time with

applied constant 20 Vamp AC input voltage at 59.91 KHz. The dotted line is a linear curve fit indicating a

constant speed.

67

4.2.6 Bulk Motion of Droplets Directly on the PT Device

To assess whether this phenomenon was similar to other conventional PT based spray

formation mechanisms, two 10 ml droplets of DI water were placed on the surface, one near

the center and the other at the corner of the PT device as shown in Figure 4.7. Upon application

of a 13 Vamp, 59.9 kHz input, no bulk translation was observed for the droplet that was placed

at the center, but the one closer to the corner was translated to the edge of the PT. As shown in

Figure 4.7, the droplet subsequently wetted the edge of the PT crystal, however no spray was

formed. Further, for droplet volumes as small as 200 nL, no spray was generated and for

droplets greater than 15 mL, no bulk motion of the droplet or atomization was observed. This

suggests that the mechanism is not vibrational, but rather electrical.

Figure 4.7: Time evolution of droplet motion under a 13 Vamp, 59.9 kHz input to the PT. The

droplet nearest the corner eventually translated to the edge of the PT, wetting it, but no spray was

formed. The more central droplet only exhibited vibrational motion, but no bulk translation.

4.2.4 Measurement of Output Voltage

Prior to behavioral studies of this novel approach of spray generation, the author first

confirmed that the output voltage was of sufficient magnitude to generate an electrospray.

The output voltage of the transformer is proportional to the applied input voltage,

crystal properties, and device geometry. It can be determined using12

t

68

𝑉𝑜𝑢𝑡

𝑉𝑖𝑛∝ 𝑘23𝑘33𝑄

𝐿

𝑇(4.1)

where Vin is the input voltage, k23 and k33 are the transverse and longitudinal vibration

coupling coefficients, Q is the crystal loss factor, and L and T are the length and thickness of

the crystal, respectively. For a 128° Y-cut LiNbO3 crystal, Q and the product k23 k33 are

reported to be approximately 104 and 0.3, respectively12. In these studies, the crystal length is

100 mm and the crystal thickness is 0.5 mm, allowing the voltage gain to be estimated as

1.5×105.

To attempt to confirm this gain, the piezoelectric transformer output voltage was

measured via a piece of wire mounted on the tip of the PT and connected to an oscilloscope, as

shown in Figure4.8(a). As shown in the plot (Figure 4.8(b)), the output voltage increases

linearly with input voltage, and a curve fit shows a voltage gain of about 124. This is

significantly lower than that predicted by Eq. (4.1). However, this can be attributed to the

inherent challenge of measuring the output voltage. Discharges tend to form at triple points and

the output wire connected to the PT forms such triple points. These discharges, which were

observed visually, generated a significant voltage drop, reducing the value measured from the

wire itself. Also, the output wire and oscilloscope were changing the load resistance, which

was suggesting the replacement of a super conductive wire could increase the measured gain.

This experiment, however, does confirm that the gain is at least 102 for this configuration. Thus,

even for an input of 10 Vamp, an output voltage of about 1240 V is expected, which is more

than sufficient to generate an electrospray.

69

Figure 4.8: (a) PT output voltage measurement configuration. (b) PT output voltage as a function

of the applied input voltage at a frequency of 60 kHz.

(a)

70

4.2.5 Effect of Liquid Conductivity on Spray Current

To explore whether the spray behaves similar to a conventional electrospray, a series

of studies were conducted to investigate the effect of the electrical conductivity and surface

tension of the solution on spray production. Based on electrospray theory, the author expects

to see that the output spray current ispray increases non-linearly with the liquid conductivity and

surface tension following

𝑖𝑠𝑝𝑟𝑎𝑦 ∝ (𝛾𝜆𝑚𝑜 𝐶)𝑛 (4.2)

where 𝛾 is the surface tension, 𝜆𝑚𝑜 is the limiting molar conductivity, C is the

concentration of the electrolyte, and n is an empirical constant between 0.2 13 and 0.5 14.

To study the effect of the electrolyte conductivity, aqueous solutions of sodium chloride

(NaCl) and hydrochloric acid (HCl) were used for testing due to the considerable difference in

their limiting molar conductivity. The limiting molar conductivity of (Na+) in water is

50 Ω-1cm2mol-1, whereas for H+ it is 350 Ω-1cm2mol-1, due to the high mobility of the proton13.

To study the effect of the conductivity of the liquid on the generated spray current, the

output current was measured by placing a grounded collecting electrode beneath the PT

connected to a picoammeter and a PC containing a LabVIEW base program for data acquisition

and further analysis (Figure 4.9).

Figure 4.9: PT driven generated spray current measurement configuration.

Ground

http://pubchem.ncbi.nlm.nih.gov/compound/hydrochloric_acid

71

Solutions of 5-20 mM NaCl and HCl in DI water were tested, corresponding to

conductivities of 0.48-2.3 mS/cm and 0.77-5.2 mS/cm for NaCl and HCl, respectively. Sprays

were generated with a constant 18 Vamp AC input voltage.

As expected, the output current increased monotonically with solution conductivity as

shown in Figure4.10(a). When the limiting molar conductivity is accounted for, by multiplying

with the solution concentration, all the data is plotted on a logarithmic scale and all the data for

the most part collapse, as shown Figure 4.10(b). A value of n = 0.35 can be extracted from a

linear curve fit, consistent with the expectations according to Eq. (4.2). Curve fits of the

individual solution data yield values of n = 0.45 and 0.37 for NaCl and HCl, respectively, which

are also consistent with Eq. (4.2).

Figure 4.10: (a) Logarithmic plot of output spray current for different (a) concentrations and (b)

conductivity of HCl and NaCl in DI water with an applied voltage of 18 Vamp. The dash line in (b) is a

linear curve fit with a coefficient of determination equal to r2 =0.76.

72

4.2.7 Analysis of the Load Resistance and the Input Current

One notable by-product of increasing the solution conductivity is that the input current

to the PT also increased. The input current applied to the PT was measured using an

oscilloscope (Tektronix DPO 2024B) for spray of HCl and NaCl solutions, each using an 18

Vamp input voltage. Figure 4.1 shows the input current as a function of the solution

concentration, so that, the input current increases monotonically with solution concentration

and thus conductivity. The author attributed this to the fact that increasing the conductivity

effectively decreases the output load resistance.

Figure 4.11: Input current for different concentrations of HCl and NaCl in DI water for an input

voltage of 18 Vamp and slightly different frequency around 60 KHz dependent on the liquid.

The author simulated equivalent circuit for the piezoelectric transformer15 using LT

Spice software (Figure 4.12(a)), and the effect of the change in the load resistance was

investigated. Results showed that for smaller values of load resistance, both the input and

output current of the piezoelectric transformer were increased, as shown in Figure 4.12(b).

According to these results, decreasing the output load resistance increases the input current and

total power consumed by the PT, which is consistent with increasing the conductivity of the

solution (reducing the load resistance) as discussed before.

73

Figure 4.12: (a) PT equivalent circuit. (b) Simulated input and output current (I in , I out) as a

function of load resistance (RL).

4.2.6 Effect of Liquid Conductivity and Surface Tension on the Onset

Voltage

The increase in the input current (and thus power) with conductivity for a fixed input

voltage of 18 Vamp potentially suggested that the minimum input voltage to onset the spray,

Vin,onset, is also a strong function of the conductivity. The onset voltage is set by the Maxwell

pressure induced by the electric field at the end of the PT, which must overcome the surface

tension, 𝛾, of the liquid film to induce a spray16. If it is assumed that the output voltage at the

edge of the PT is linearly proportional15 to the input voltage Vin, as suggested by the

measurements of the output voltage, it is anticipated that

𝑉𝑖𝑛,𝑜𝑛𝑠𝑒𝑡 ∝ 𝛾0.5 ( 𝜆𝑚𝑜 𝐶)−0.5 (4.3)

(a)

74

Figure 4.13(a) confirms that the onset voltage does decrease non-linearly with ionic

concentration. It is important to note that, for all of these studies, the surface tension was

measured to be 73 dyn/cm, when the conductivity was varied, which precludes the possibility

that surface tension was causing the variations in current and voltage that were measured. The

surface tension of solutions was measured using an FTA 100 Angstroms instrument. A 4 mL

syringe was used to inject the solution through the device, and results were obtained by

calculating the average of three different sequential measurements.

Interestingly, the exponent factor was measured to be -0.32, which is slightly lower

than the expected relationship from a simplified balance of Maxwell pressure and surface

tension or electrostatic and capillary pressure17. While this could be attributed to the effect of

mechanical vibration on the atomization, further study is required to resolve this completely.

Nevertheless, the general trend is consistent with the argument that the electric field induced

by the PT is primarily responsible for inducing the spray.

Figure 4.13: a) Logarithmic plot of the onset voltage for various concentration of NaCl in DI

water. The dash line is the linear curve fit with a slope equal to -0.32. b) Onset voltage as a function of

surface tension for various concentration of glycerol in DI water.

To explore the effect of surface tension, the polar solvent glycerol was added to aqueous

solutions of NaCl (0.53 mS/m) to reduce the surface tension from γ = 73 to 69 dyn/cm

(corresponding to glycerol concentrations from 0.01-200 mM). As expected, the onset voltage

75

increases as γ increases (Figure 4.13(b)). However, unexpectedly, both the spray output current

and PT input current were unaffected as the surface tension was varied for a constant input

voltage. For the same input voltage (21 Vamp), the input current (irms) for all the samples was

essentially 84 mA, and the spray current (ispray) was 20 nA. As the surface tension does not

impact the output load resistance, it is not surprising that the PT input current was not affected.

But Eq. (4.2) shows that the spray current should be proportional to a positive power of the

surface tension. The author attributed this anomaly to the fact that the resolution to detect the

expected minute changes in current (~0.0001%) was not sufficient as predicted by Eq. (4.2) for

the small range of surface tension values explored here. But it is possible that there may be

more than conventional electrospray mechanisms at play, altering the dependence of these

parameters on surface tension.

4.2.7 Chemical Solution Preparation

Sodium chloride (NaCl), glycerol and Hydrochloric acid (HCl) were purchased from

Sigma Aldrich. Aqueous solutions at different concentrations were made by diluting using

deionized (DI) water (18 MΩ). Solutions of 5-20 mM NaCl and HCl in DI water were prepared,

which corresponds to conductivities of 0.48-2.3 mS/cm and 0.77-5.2 mS/cm for NaCl and HCl,

respectively.

4.4 Summary

This chapter provided a novel approach to create a spray generation system using a

piezoelectric device. A detailed report of this work including the piezoelectric transformer and

stand design, spray generation, spray coating profilometry, volumetric flow estimation, linear

flow speed visualization, bulk motion of the droplet directly on a PT device, measurement of

output voltage, effect of solution conductivity on the output current, analysis of the load

76

resistance and the input current, effect of liquid conductivity and surface tension on the onset

voltage and chemical solution preparation has been presented.

To summarize, the author successfully generated a continuous spray by pinning a

128°YX LiNbO3 piezoelectric crystal in the standing wave mode and providing the desired

liquid film using a paper wick. The paper covers the PT width which enabled a final spray that

was as wide as the PT device. Profilometry of the coated area by the generated spray

demonstrated a uniform coating. Volumetric flow rate of the produced spray was measured to

be 12-30 µL/min, depending on the exact experimental conditions (input voltage, spray

solution) and the front dye tracking proved the pumping effect of the PT device. To verify

whether electrical or mechanical feature of the piezo electric crystal is the dominant reason for

the spray generation, bulk motion of the droplet was recorded.

The output voltage gain of the PT was measured to be 124 and compared to the

calculated value. Different concentrations of HCl and NaCl in DI water were used for spray

generation and results showed that the liquid conductivity and the spray current are

monolithically related as it is predicted in a conventional electrospray system. The simulations

done by the author also indicated that the input and output current increased when the liquid

conductivity was elevated. The minimum voltage for liquid film breakage was shown to

decrease as the liquid surface tension reduced.

4.5 References

1 Dobre, M., and L. Bolle. "Practical design of ultrasonic spray devices: experimental

testing of several atomizer geometries." Experimental Thermal and Fluid Science 26, no. 2

(2002): 205-211.

77

2 Rezk, Amgad R., Leslie Y. Yeo, and James R. Friend. "Poloidal flow and toroidal

particle ring formation in a sessile drop driven by megahertz order vibration." Langmuir 30,

no. 37 (2014): 11243-11247.

3 Yeo, Leslie Y., and James R. Friend. "Surface acoustic wave microfluidics." Annual

Review of Fluid Mechanics 46 (2014): 379-406.

4 VanGordon, J. A., B. B. Gall, S. D. Kovaleski, E. A. Baxter, R. Almeida, and J. W.

Kwon. "High voltage production from shaped piezoelectric transformers and piezoelectric

transformer based circuits." In 2010 IEEE International Power Modulator and High Voltage

Conference, pp. 334-337. IEEE, 2010.

5 Ho, Jenny, Ming K. Tan, David B. Go, Leslie Y. Yeo, James R. Friend, and Hsueh-

Chia Chang. "Paper-based microfluidic surface acoustic wave sample delivery and ionization

source for rapid and sensitive ambient mass spectrometry." Analytical chemistry 83, no. 9

(2011): 3260-3266.

6 Kim, Hyun, Albrecht Brockhaus, and Jürgen Engemann. "Atmospheric pressure

argon plasma jet using a cylindrical piezoelectric transformer." Applied Physics Letters 95, no.

21 (2009): 211501.

7 Johnson, Michael J., and David B. Go. "Ferroelectric crystals for the low-voltage

operation of surface dielectric barrier discharges." Applied Physics Letters 105, no. 26 (2014):

264102.

8 Itoh, Haruo, K. Teranishi, and S. Suzuki. "Discharge plasmas generated by

piezoelectric transformers and their applications." Plasma Sources Science and Technology 15,

no. 2 (2006): S51.

9 Sleighter, Rachel L., and Patrick G. Hatcher. "The application of electrospray

ionization coupled to ultrahigh resolution mass spectrometry for the molecular characterization

of natural organic matter." Journal of Mass Spectrometry 42, no. 5 (2007): 559-574.

78

10 Tait, Jeffrey G., Barry P. Rand, and Paul Heremans. "Concurrently pumped

ultrasonic spray coating for donor: acceptor and thickness optimization of organic solar cells."

Organic Electronics 14, no. 3 (2013): 1002-1008.

11 Mahmood, Khalid, Bhabani Sankar Swain, and Hyun Suk Jung. "Controlling the

surface nanostructure of ZnO and Al-doped ZnO thin films using electrostatic spraying for

their application in 12% efficient perovskite solar cells." Nanoscale 6, no. 15 (2014): 9127-

9138..

12 Nakamura, Kiyoshi, and Yoshinori Adachi. "Piezoelectric transformers using

LiNbO3 single crystals." Electronics and Communications in Japan (Part III: Fundamental

Electronic Science) 81, no. 7 (1998): 1-6.

13 Tang, Liang, and Paul Kebarle. "Effect of the conductivity of the electrosprayed

solution on the electrospray current. Factors determining analyte sensitivity in electrospray

mass spectrometry." Analytical Chemistry 63, no. 23 (1991): 2709-2715.

14 Ganan-Calvo, A. M., J. Davila, and A. Barrero. "Current and droplet size in the

electrospraying of liquids. Scaling laws." Journal of Aerosol Science 28, no. 2 (1997): 249-

275.

15 Alonso, J. Marcos, Carlos Ordiz, Marco A. Dalla Costa, Javier Ribas, and JesÚs

Cardesin. "High-voltage power supply for ozone generation based on piezoelectric

transformer." IEEE Transactions on Industry applications 45, no. 4 (2009): 1513-1523.

16 Guerrero, I., R. Bocanegra, F. J. Higuera, and J. Fernández De La Mora. "Ion

evaporation from Taylor cones of propylene carbonate mixed with ionic liquids." Journal of

Fluid Mechanics 591 (2007): 437-459.

17 Fernández de La Mora, Juan. "The fluid dynamics of Taylor cones." Annu. Rev. Fluid

Mech. 39 (2007): 217-243.

79

CHAPTER V

PIEZOELECTRIC TRANSFORMER BASED SPRAY GENERATION FOR

THIN FILM MEMBRANE COATING

5.1 Introduction

Layer-by-layer (LbL) deposition of multilayer films has been widely used in the

fabrication of various sensors including gas, humidity, and bio sensors1. The LbL deposition

method entails alternating deposition of oppositely charged ions to form stable multilayer films

via electrostatic interactions or hydrogen bonding2 . There are many approaches of building

multilayer films such as dip coating3, inkjet printing4, spin-assisted assembly and electrophoretic

deposition 5 . However, scalability and thin-film uniformity are still challenging for

commercialization of these techniques.

5.1.1 Spray Coating Techniques

Spray technologies have been investigated for their potential capabilities of material

coating. There are various methods of breaking the liquid droplet down to the spray. One of the

simplest and widely used approach is a pneumatic spray which does not contain a precise or

uniform droplet size and hence is not a proper choice for use in membrane coating. Among the

vibrational approach of spray generation, surface acoustic wave atomizers have been utilized

for material coating. Although this technique provides a uniform coating, the complicated

procedure to fabricate this device along with the 100 V range of required input voltage can

undermine their efficiency6.

80

Electrosprays are well known to form a reliable continuous spray from a liquid sample.

However, the need of high input voltages (kV), along with costly equipment, does not make it

an apt choice in large scale sensor fabrication processes. Moreover, the principal of the

electrospray phenomenon is based on the Taylor cone formation and droplet plumes produce a

circular injected area which is not ideal for uniform material coating7.

5.1.2. Membrane Coating Materials

Among the other coating methods, polymer coating techniques have been witnessing a

rising demand due to the vast range of applications for polymer coated membranes in fuel cells8,

solar cells9, isopropanol dehydration10,surface treatment of metallic coronary stents11 and the

next generation of drug delivery systems12.

The key factor in membrane coating is to find a balance between flux and separation. In

other words, the liquid flow rate through the membrane needs to stay significant, while the target

ions are blocked by the coated membrane. It has been proved that alternating coating of a

membrane with opposite charge polymers can produce a thin film13. Layer by layer deposition

of anionic and cationic polyelectrolytes such as poly (allylamine hydrochloride) (PAH) and

poly(styrenesulfonate) (PSS) on a solid membrane was shown to be successful in the separation

of ethanol and water mixtures14.

Ouyang et al. 15 reported that adding a layer of poly(diallyl-dimethylammonium

chloride) (PDADMAC) to the ion rejection membrane, coated with other polymers, enhances

the liquid flux trough the membrane by increasing the swelling of films contacting the

PDADMAC.

Here, the author demonstrated an innovative procedure for LbL thin film deposition

using a piezoelectric driven spray generator. As a proof-of-concept, some of the previously

81

used polymers for membrane coating such as poly (allylamine hydrochloride) (PAH),

poly(styrenesulfonate) (PSS), poly(diallyl-dimethylammonium chloride) and poly(diallyl-

dimethylammonium chloride) (PDADMAC) were sprayed using the novel PT-driven spray

formation technology introduced by the author in chapter 4. A polycarbonate track-etched

(PCTE) membrane was spray coated by the polymer mixtures and water permeability and ion

rejection ratio of the final PT spray coated membrane were measured. As a proper solvent for

polymer based mixtures, dimethyl sulfoxide (DMSO) has been widely used due to its

nontoxicity, polymeric and water solubility capabilities, hence the author included this chemical

to the spray formation list to examine the ability of PT spray formation technology for different

probable polymer mixtures16. This method has several advantages due to its cost efficiency,

quick response and low material consumption when compared with the traditional thin film

deposition methods.

5.2 Experimental

5.2.1 Spray Generation Setup

Spray formation was achieved via the same PT driven setup mentioned in previous

chapter. A 15 mm (or 30 mm) × 100 mm × 1.5 mm (w/l/t) 128°YX lithium niobate (LiNbO3)

crystal was purchased, and bottom and top electrodes were painted on the PT surface using

silver paint (conductive silver paint from SPI company). A PT stand was designed and

fabricated using a 3D printer to mount and pin the PT at two locations corresponding to nodes

in the displacement wave related to second resonant frequency, which was around 60 KHz and

63 KHz for 15 mm and 30 mm width PT devices, respectively. A signal generator and a radio

frequency amplifier provide the required input current at the desired frequency. A liquid sample

82

was delivered to the surface of the PT by a paper wick placed between a liquid reservoir for

coating material and the PT surface.

5.2.2 Material Preparation

PAH (15 kDa), PSS (70 kDa) and Poly(diallyldimethylammonium chloride)

(PDADMAC) ( 20 wt. % in H2O) in liquid form were purchased from Sigma-Aldrich. Aqueous

solutions of the polyelectrolytes at 20 mM (based on repeat unit molecular weight) were

prepared. The pH of the PAH and PSS solution were unadjusted. A PCTE membrane (pore

diameter: 50 nm, membrane thickness: 10 μm, porosity: 3×108 pores/cm2, Whatman) was used

as a template.

5.2.3 Moving Stage (Lazy Susan)

To provide a moving stage for high throughput membrane coating, a circular, slow

spinning stage (lazy susan) was designed and made by the author (Figure 5.1).

A DC gear box reduction motor was used to produce 5 RPM vertical spinning motion

by applying 12 V input voltage delivered by a DC power supply. To change the motion to a

horizontal direction, a vertical shaft worm gear box was connected to the motor. This shaft also

reduced the speed of spinning 30 times due to the size difference. A gearbox ratio of 30:1

means that the output side is about 30 times faster than the input side. This gear box works

using a 30:1 ratio worm drive, which is able to hold the position when the power is not applied

to prevent any interference in the experiment result when any electrical interruptions occurs.

A 10 inch diameter plexiglass sheet was used as the moving stage connected to the

motor shaft. A simple variable power supply driver motor was also used to change the power

delivered to the motor by a resistor based voltage divider. The author used this configuration

83

to control the motor speed, which leads to the variable plexiglass speed of spinning. This

enabled the exposure of the membrane with the coating spray for different time durations. The

whole structure was mounted on an aluminum sheet for support and stability.

Figure 5.1: Sequential PT driven Spray coating of membrane using PAH and PSS by placing the

membranes on a Lazy Susan.

5.2.4 Water Permeability and Salt Rejection Tests

The PT- driven spray coated membrane was placed in a water filled cell (Amicon model

8003) and stirred. A pressure of 4 bar was applied to drive water through the membrane, which

was collected in a small beaker. The collected mass was measured using a scale, which was

connected to a PC with a LabVIEW™ based program for data acquisition and recording the

change in mass over time. The hydraulic permeability of the membrane was calculated using

the slope of this graph.

In salt rejection tests, 10 mM solutions of single salts (i.e., KCl, MgCl2) were used as

the feed solutions and a pressure of 4 bar was applied to drive the flow. The solution was

collected in a glass reservoir. Ion chromatography (Dionex ICS-5000) was used to analyze the

84

concentration of ions in the feed (cf) and permeate (cp) solutions. The percent rejection, R, was

calculated according to

𝑅(%) = (1 −𝑐𝑃

𝑐𝐹) ×100 (5.1)

5.3. Results

5.3.1. Bilayer coating of PAH and PSS

A paper was placed between a reservoir which contained PAH or PSS solution and PT

surface. Autor found it useful to soak the paper in the reservoir in the beginning of the

experiment to provide a better contact between the paper and the PT edge which can enhance

the uniformity of the generated spray. The PCTE template membrane was placed 3 cm beneath

the PT on the moving stage. By introducing around 15 Vamp AC input voltage in 59.8 KHz

frequency, the spray was generated. The spray was formed along the width of the PT, which is

15 mm in this experiment, and the spray droplets are of sufficient size that fall to the surface

beneath in the PT (in a manner analogous to a shower). As such, the spray deposits over an

area of 15 mm by 0.5 mm. Membrane was exposed to PAH spray for 30 seconds and after

drying up, which took almost 5 minutes, PSS was sprayed over the membrane for 30 second to

from the first bilayer coating of the membrane.

The process was repeated to form multiple bilayers and to coat the PCTE membrane

completely. Repeated spray coating of the membrane using PAH and then PSS resulted in the

fabrication of multilayer thin films on the membrane surface. As shown in Figure 5.2, scanning

electron microscope (SEM) images revealed that the native 50 nm diameter pores of the bare

PCTE membrane were coated by the sprayed polymers until total coverage was achieved.

85

Figure 5.2: SEM images of PCTE membranes sprayed with different numbers of bilayers.

Different numbers of PAH/PSS bilayers were sprayed onto a 50 nm PCTE membrane. The sprayed

membranes were taped onto a SEM grid, sputtered with iridium (2 nm), and visualized in SEM.

Table 5.1 shows the results for water permeability performed for bare, 1 bilayer, 2

bilayers, and 3 bilayers coated membranes using 30 s spray durations for each layer. The water

permeability decreased rapidly with increasing number of bilayers, suggesting that a successful

LbL thin film was sprayed on the PCTE membrane. The sprayed LbL thin film also showed

higher salt rejection at a higher number of sprayed bilayers. The water permeability of the

membrane sprayed with three bilayers was not measurable, indicating the pores of the

membrane were completely blocked by the sprayed films.

86

Table 5.1: Water permeability and ion rejection results for PAH/PSS PT_ spray coating.

Number of

bilayer

Water

permeability

L/m2/hours/bar

Ion rejection

ratio

Bare 425

1 bilayer 41±5

2 bilayers 3.5 25% for 10 mM NaCl

32% for 10 mM MgCl2

3 bilayers Too slow

In some cases, it is desirable to prepare thin films with a high number of bilayers while

maintaining membrane permeability. This condition can be achieved by shortening the PT

spray durations. A duration of 10 s was used instead of 30 s to alternatingly spray PAH/PSS

bilayers. Figure 5.3 shows a logarithmic plot of how the water permeability changes with

different number of bilayers. The water permeability decreased from 425 L m-2 hr-1 bar-1 for

the bare PCTE membrane to 0.9 L m-2 hr-1 bar-1 with 5 bilayers. The water permeability was

unmeasurable for the membrane with 6 bilayers. The results show that by shortening the

spraying duration, a thinner film is deposited for each bilayer. The small values of the water

permeability of the coated membrane suggest the absence of big defects in the LbL films.

87

Figure 5.3: Logarithmic plot of the normalized water permeability vs. number of bilayers. A 50

nm PCTE membrane was used as the substrate membrane with a different number of sprayed PAH/PSS

bilayers with 10 s durations. The water permeability test was measured in a dead-end filter cell. The error

bars meant three measurements with the same membrane and were barely seen since they were smaller

than the symbols.

Figure 5.4: Salt rejection vs. number of bilayers. A different number of PAH/PSS bilayers were

sprayed alternatingly with 10 s duration on a 50 nm PCTE membrane. The salt rejection test was carried

out in a dead-end filter cell filled with either 10 mM KCl or 10 mM MgCl2.

Figure 5.4 shows how the rejection of 10 mM KCl and MgCl2 changes with the number

of bilayers sprayed on the PCTE membrane. The results illustrate that by increasing the number

of bilayers, a higher salt rejection is achieved for the sprayed membrane.

Since there is no waste of coating material, unlike dip coating approach, the amount of

the required polymer solutions for membrane coating is considerably decreasing. The

membrane diameter is 2.5 cm while the moving stage diameter is 25cm, which makes it

possible to mount 30 membranes on the moving stage simultaneously. If the speed of spinning

88

in moving stage is set to expose each membrane with the PT spray for 20 second, bilayer

coating of 30 membranes is possible within 26 minutes. This can prove the capability of this

PT driven spray generation approach as a high throughput membrane coating system.

89

5.3.2. PDADMAC Spray Coating

To prove the ability of this novel spray formation technology for polymer coating of

the membrane, PDADMAC was tried.

The membrane was placed on the Lazy susan, at a distance of 3 cm below the PT device.

A DC motor was reconfigured to provide a 0.1 cm/s spinning speed for moving the stage. The

author exposed the membrane with PT driven spray of PDADMAC for 25 s. A mixture of 0.1

Molar of PDADMAC and 1% poly vinyl alcohol (PVA) was diluted 33% with DI water to

provide the coating material for the membrane. Table 5.2 shows the results for this experiment.

Table 5.2: Water permeability and ion rejection results for PDADMAC PT_spray coating.

Number of

layer

Water

permeability

lit/m2/hours/bar

Ion rejection ratio

1 layer 300

3 layers 300

6 layers 300

20 layers 50 66% ± 2 for 10 mM KCl

The results represent the capability of the system for spray production. Applying just

12 Vamp to PT was sufficient to generate a uniform continues spray from the PDADMAC

solution. The water permeability was almost as high as a bare membrane with no coating layer

in the first 5 layers coating. The author attributed this to the dilution factor used for the coating

mixture. The results suggest that the percentage of the polymer per sprayed droplet was very

low in the first stages of the coating.

90

5.4 Summary

A piezoelectric transformer was used to generate a uniform spray by applying 15 to 25

Vamp to the crystal at its second resonant frequency. A continuous supply of solution from a

paper wick was delivered to the surface of the PT, without the need for an external pump. Layer-

by-layer spraying of multilayer thin films with PAH/PSS and PDADMAC were used as a model

system to demonstrate the PT-driven spray technique as a material-deposition method. Results

show that the spray duration and number of bilayers have a significant impact on the water

permeability and salt rejection of the sprayed membrane.

5.5 References

1 R. Article, D. Rawtani, and Y. K. Agrawal, “Emerging Strategies and Applications of

layer-by-layer self-assembly,” Nanobiomedicine, pp. 1–15, 2014.

2 S. Rajesh, Y. Yan, H. Chang, H. Gao, and W. A. Phillip, “Mixed mosaic membranes

prepared by layer-by-layer assembly for ionic separations,” ACS Nano, vol. 8, pp. 12338–

12345, 2014.

3 B. W. Stanton, J. J. Harris, M. D. Miller, and M. L. Bruening, “Ultrathin, multilayered

polyelectrolyte films as nanofiltration membranes,” Langmuir, vol. 19, pp. 7038–7042, 2003.

4 P. Gao, A. Hunter, S. Benavides, M. J. Summe, F. Gao, and W. A. Phillip, “Template

synthesis of nanostructured polymeric membranes by inkjet printing,” ACS Appl. Mater.

Interfaces, vol. 8, pp. 3386–3395, 2016

5 J. L. Lutkenhaus and P. T. Hammond, “Electrochemically enabled polyelectrolyte

multilayer devices: from fuel cells to sensors,” Soft Matter, vol. 3, p. 804, 2007.

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7 A. Ganan-Calvo, J. Davila, and A. Barrero, “Current and droplet size in the

electrospraying of liquids. Scaling laws,” Journal of Aerosol Science vol. 28, pp. 249-275,

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"Transport Properties of Plasma Polymerized Anion Exchange Membrane for Direct Methanol

Alkaline Fuel Cells." ECS Transactions 50, no. 2 (2013): 2109-2118.

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polymerized polyamide composite membranes to isopropanol dehydration: Effect of

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93

CHAPTER VI

CONCLUSION

6.1 Conclusion

In this dissertation, the author has successfully developed a piezoelectric based system

for fluid manipulation. Piezoelectric devices produce mechanical strain under applied electrical

field, which results in propagating mechanical vibration. This acoustic wave has been used to

manipulate the liquid medium in sensing technologies due to their benefits such as small

volume of required sample, high sensitivity, bio-compatibility and non-invasive procedures.

The high sensitivity arises from the large electromechanical coupling factor of the piezoelectric

crystal.

Moreover, the aforementioned features make piezoelectric devices also apt choices in

actuating systems such as spray formation. The mechanical vibration along with the large

polarization produced on the surface of the piezoelectric crystal can be used to overcome the

liquid film surface tension and break it into the droplets.

In this work, a piezoelectric based sensor, which employs photolithographically

patterned electrodes was used for sensing applications. In addition, the author explored the

possibility of using piezoelectric transformers to generate a continuous uniform spray by

applying low input voltages when compared to the conventional spray production techniques.

The dissertation was organized and pursued in three projects in order to achieve the

research outcomes.

94

In the first project, an efficient piezoelectric based detection sensing system for the

detection of heavy metal compounds was developed. A photolithographically fabricated

piezoelectric sensor, which incorporates Au IDEs was used for the detection of various heavy

metal compounds. Proper sensitive layers were chosen for the selective detection of the heavy

metal compounds including lead, cadmium, nickel and mercury nitrate. Based on the results

from the piezoelectric sensor, the author concluded the capability of the developed piezo

electric based system to detect heavy metal compounds at pico molar concentration levels using

the frequency shift.

The author was responsible for design and fabrication of the SAW device and finding

proper sensing layer. Experiment setup and controlling the environmental factors in order to

preserve the experiment repeatability was also done by author.

In the second project, the author designed and fabricated a novel piezoelectric

transformer for fluid manipulation and spray generation. A micro fiber glass paper was used to

wick the liquid film out of the contained reservoir. Based on the imaging results, a large area

uniform spray was generated along the width of the piezoelectric transformer. The author

demonstrated that bringing a thin-film of liquid to the surface of a PT via a wick generated a

spray of droplets. The author’s studies strongly suggest that the spray mechanism is

electrospray in nature, and thus parameters such as solution conductivity and surface tension

can be used to manipulate the spray behavior. By spraying directly from the wick/PT interface,

the nebulization in many ways resembles a free-surface electrospray rather than capillary

electrospray. In the author’s opinion, the broad area nature of the spray makes it very appealing

for emerging applications in materials synthesis and coating, were uniform deposition may be

possible over large substrates simply by scaling the size of the PT crystal.

95

The author was responsible for design and fabrication of the device and stand. Proper

experiments were designed and conducted by author to study the key factors of this

phenomenon.

In the third project, the author tried to use the PT spray generation system for coating

purposes. Some previously proven polymeric materials for membrane coating were chosen:

poly(allylamine hydrochloride) (PAH), poly(styrene sulfonate) (PSS) and (PDADMAC) were

sprayed on to a polycarbonate track-etched (PCTE) membrane. SEM results illustrated the

polymer thin film formation on the membrane. Water permeability and ion rejection ratio of

the coated membrane were measured. The author concluded that the number of layers has an

optimum number for each desired polymer coating material and it occurs where the polymer

film is uniformly coated on the membrane surface to result in a good ion rejection ratio, while

the polymer film is still not too thick to block the liquid flow through the coated membrane.

There are several possibilities to improve the current projects which are covered in this

dissertation. The author has some suggestions for further studies:

• The ability to integrate the SH-SAW sensor into a hand-held and

portable detection system.

• Improve the selectivity of the SH-SAW sensor to exposed

chemicals by studying target to sensing layer binding conditions.

• Enhance the experiment repeatability by exploring the other

methods of the sensing layer deposition which results in a uniform film.

• The effect of the piezoelectric transformer geometry on the

discharge points of the edge of the PT device can be investigated using software

simulations such as COMSOL.

• The resultant aerosols size and dispersity can be measured using

video graphic techniques.

96

• More suitable approach of liquid delivery to the PT surface needs

to be found to increase the produced spray uniformity along the width of the PT.

• The dependency of the spray formation towards the liquid

conductivity and surface tension can be the focus of the further studies to

investigate the capability of the system for spray generation from the

nonconductive solutions.

• The influence of the spray duration, temperature and PT to

membrane distance on the ion rejection ration can be investigated to find the

optimized condition.

• Since the positive affect of washing the membrane proven in the

conventional LBL membrane coating on the membrane function has already

been proven, a washing step can be added in between the two spray stations

above the moving stage.

• PT-driven spray technique can be extended to the layer-by-layer

deposition of a large variety of polymer materials for the rapid, scalable synthesis

of ion-exchange and ion-rejection membrane sensors.

97

Appendix A

List of publications

A.1 Inventions

A.1.1 Intellectual Property

[1] D. B. Go, Z. Ramshani, W. A. Phillip, P. Gao, M. Z. Atashbar “Multi-

Functional Thin Film Coating by Piezoelectric Driven Spray,” University of Notre Dame,

Intellectual Property Disclosure, UND CASE# 17-001.

[2] D. B. Go, Z. Ramshani, M. J. Johnson, M. Z. Atashbar “Piezoelectric Transfer

Driven Transformer Device,” University of Notre Dame, Intellectual Property Disclosure,

UND CASE# 16-048.

A.1.2 Patent

[1] D. B. Go, Z. Ramshani, W. A. Phillip, P. Gao, M. Z. Atashbar “Apparatus and

Method for Atomization of Fluid, Thin Film Synthesis and Coating,” U.S. Provisional Patent

62/414,317, filed 10/26/2016.

[2] D. B. Go, Z. Ramshani, M. J. Johnson, M. Z. Atashbar “Piezoelectric

Transformer-Driven Electrospray Device,” U.S. Provisional Patent 62/319,775, filed

04/07/2016.

A.2 Journal Papers

[1] Ramshani, Zeinab, Michael J. Johnson, Massood Z. Atashbar, and David B.

Go. "A broad area electrospray generated by a piezoelectric transformer." Applied Physics

Letters 109, no. 4 (2016): 044103.

98

[2] Ramshani, Zeinab, Avuthu SG Reddy, Binu B. Narakathu, Jared T. Wabeke,

Sherine O. Obare, and Massood Z. Atashbar. "SH-SAW sensor based microfluidic system for

the detection of heavy metal compounds in liquid environments." Sensors and Actuators B:

Chemical 217 (2015): 72-77.

[3] Go, David, Massood Atashbar, Zeinab Ramshani, Huse Chia Chang. “Surface

acoustic wave devices for chemical sensing and microfluidics: A review and perspective”.

Analytical Methods, 2017, (in print)

A.3 Conference Papers

[1] Ramshani, Zeinab, Binu B. Narakathu, Sepehr Emamian, Avuthu SG Reddy,

and Massood Z. Atashbar. "Investigation of SH-SAW sensors for toxic heavy metal detection."

SENSORS, 2013 IEEE, pp. 1-4. IEEE, 2013.

[2] Ramshani, Zeinab, Binu B. Narakathu, Avuthu SG Reddy, Massood Z.

Atashbar, Jared T. Wabeke, and Sherine O. Obare. "SH-SAW-based sensor for heavy metal

ion detection." Frequency Control Symposium & the European Frequency and Time Forum

(FCS), 2015 Joint Conference of the IEEE International, pp. 536-540. IEEE, 2015.

[4] Eshkeiti, A., Z. Ramshani, S. Emamian, B. B. Narakathu, S. G. R. Avuthu, M.

M. Ali, A. Chlaihawi, M. K. Joyce, and M. Z. Atashbar. "A stretchable and wearable printed

sensor for human body motion monitoring." SENSORS, 2015 IEEE, pp. 1-4. IEEE, 2015.

[5] Ramshani, Zeinab., B. Narakathu, Massood Z. Atashbar, Kapseong Ro, J-C.

Song, Daewoo Lee, and C. H. Lee. "Design of a novel 3D localization and identification system

using surface acoustic (SAW) devices." Electro/Information Technology (EIT), 2013 IEEE

International Conference on, pp. 1-4. IEEE, 2013.

[6] Ramshani, Zeinab, Binu B. Narakathu, Avuthu SG Reddy, Massood Z.

Atashbar. "Heavy Metal Detection using Shear Horizontal Surface Acoustic Wave (SH-SAW)

Sensors. International Meeting on Chemical Sensors (IMCS), pp. 88, 2014.

99

[7] Ramshani, Zeinab, Gao Peng, William A. Phillip, Massood Z. Atashbar, and

David B. Go. "Piezoelectric Transformer-Driven Spray Coating for Membrane Sensor

Fabrication." SENSORS, 2016 IEEE International Conference on, pp. 142-144. IEEE, 2016.

100

Appendix B

Matlab code for scale used to graph the spray volumetric flow rate used in Chapter 4

“

function curpre

obj1(1)=serial('COM5','baudrate',2400,'databits',7,'parity','none','flowcontrol','none','te

rminator','CR/LF'); %from balance user manual

for i = 1:length(obj1)

fopen(obj1(i));

fprintf(obj1(1),'P');

fscanf(obj1(1));

end

t1=timer;

t1.startdelay=1;

t1.startfcn = @starttimer;

t1.TimerFcn = {@collectdata,obj1};

t1.StopFcn = {@stopdata,obj1};

%--------------------------

t1.Period = 0.1; %time between readings in seconds

t1.TasksToExecute = 4000; %number of readings

%-----------------------------

t1.ExecutionMode = 'fixedSpacing';

101

start(t1)

function starttimer(t1,event)

t0=event.Data.time;

data=[0 0];

assignin('base','data',data);

assignin('base','t0',t0);

function collectdata(t1,event,obj1)

fprintf(obj1(1),'P')

ot=fscanf(obj1(1))

out=sscanf(ot,'%f %*s');

tn_ = event.Data.time;

t0_=evalin('base','t0');

assignin('base','data',vertcat(evalin('base','data'),[etime(tn_,t0_) out]))

function stopdata(t1,event,obj1)

for i=1:length(obj1);

fclose(obj1(i));

delete(obj1(i))

clear obj1(i)

end

delete(t1)

clear t1

display('Timer ended');

Documents

Fluid Manipulations Using a Piezo Electric Transformer for Sensing and Spray Generation