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Characterization of electrowetting systems for microfluidic applications

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Characterization of electrowetting systems for microfluidic applications
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Mishra, Pradeep K
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Contact Angle
Corrosion
Impedance
Reliability
CYTOP
Dissertations, Academic -- Mechanical Engineering -- Masters -- USF   ( lcsh )
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non-fiction   ( marcgt )

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Abstract:
ABSTRACT: Electrowetting is the change in apparent surface energy in the presence of an electric field. Recently, this phenomenon has been used to control the shape and location of individual droplets on a surface. However, many microfluidics researchers have acknowledged unexplained behaviors and performance degradation. In this work, electrowetting systems are characterized with different methods. The electrowetting response is measured by measuring contact angle for different applied voltages. A novel technique for direct measurement of Electrowetting Force (EWF) using nano indenter is proposed in this work. The EWF measurements show that, for aqueous solution the EWF is more as compared to DI water. Additionally, the electrowetting system is found to be more susceptible for degradation when aqueous solution is used. The performance degradation due to defective dielectric layer is also investigated by measuring the electrowetting force. Degradation of EWOD systems with environmental exposure over time is further studied experimentally by contact angle and electrochemical impedance spectroscopy (EIS) measurements. The time constant of 'contact angle decay' with environmental exposure is found to be similar to the time constant of electrolyte diffusion in dielectric layer.
Thesis:
Thesis (M.S.M.E.)--University of South Florida, 2009.
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Includes bibliographical references.
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by Pradeep K. Mishra.
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Characterization of Electrowetting Syst em s for Microfluidic Applications by Pradeep K. Mishra A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Mechanical Engineering Department of Mechanical Engineering College of Engineering University of South Florida Major Professor: Nathan B. Crane, Ph.D. Alex Volinsky, Ph.D. Craig Lusk, Ph.D. Date of Approval: July 1, 2009 Keywords: Contact Angle, Corrosio n, Impedance, Reliability, CYTOP Copyright 2009, Pradeep K. Mishra 1

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2 DEDICATION To my Parents

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i ACKNOWLEDGEMENTS The author wishes to acknowledge the gr acious support of the many people that contributed to this work directly or indirectly. First of all, I am grateful to Prof. Nathan B. Crane for giving me the opportunity to undertake this work. He has been an advisor to me in more ways than just academically. He has been a great source of inspirations during course of this research work. The time and effort of Dr. Criag Lusk and Dr. Alex Volinsky as committee members are greatly appreciated. The permissi on to use X-ray diffraction lab (Dr. Alex Volinsky) and Corrosion lab (Dr. Alberto Sa gues) for various experimental works is greatly appreciated. I am greatly thankful to all my coll eagues at research lab for their help and support. My special thanks to Vivek Ramadoss for his valuable advices in early days of my graduate studies. My thanks to Mike Ne llis, Jeffrey L. Murray, Jairo Chimento, James Tuckerman, Ajay Rajgadkar and Gary Ha ndrick for their help and great company. Thanks to Kartikay Singh for giving excellent company outside the lab. Without my parents early support and en couragement, I never would have made it this far. My sincere thanks to my brothe rs and their family for having faith in me. Most of all Im certainly indebted to my best frie nd, Shikha, for her constant support and motivation. She has been amazing in all aspects. Thank you, I owe a lot to you This work has been supported in part through NACE Seed Grant 2008-09.

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TABLE OF CONTENTS TABLE OF CONTENTS.....................................................................................................iLIST OF TABLES.............................................................................................................ivLIST OF FIGURES............................................................................................................vABSTRACT...................................................................................................................xiCHAPTER 1INTRODUCTION......................................................................................11.1Thesis Statement.........................................................................................11.2Motivation...................................................................................................11.3Scope of Work............................................................................................31.4Thesis Outline.............................................................................................4CHAPTER 2LITERATURE REVIEW...........................................................................52.1Wettability...................................................................................................52.2Electrowetting.............................................................................................62.3Contact Angle Variation.............................................................................82.4Contact Angle Hysteresis............................................................................92.5Material Properties......................................................................................9 i

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2.6Effects of Gravity and Evaporation..........................................................122.7Electrowetting Characterization................................................................13CHAPTER 3EXPERIMENTAL PROCEDURES.........................................................153.1Introduction...............................................................................................153.2Sample Preparation...................................................................................153.3Contact Angle Measurement.....................................................................183.3.1Method..........................................................................................183.4Electrowetting Force.................................................................................203.4.1Introduction...................................................................................203.4.2Principle of Electrowetting Force Measurement..........................203.4.3Experimental Setup for Electrowetting Force Measurement........243.4.4Experimental Procedure................................................................253.5Electrochemical Impedance Spectroscopy...............................................253.5.1Introduction...................................................................................253.5.2Principle of Electrochemical Impedance Spectroscopy................263.5.3Experimental Procedures..............................................................28CHAPTER 4CONTACT ANGLE AND ELECTROWETTING FORCE MEASUREMENTS..................................................................................304.1Introduction...............................................................................................304.2Contact Angle Measurements...................................................................304.2.1Different Liquids...........................................................................30 ii

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4.2.2Reversibility of Electrowetting.....................................................334.2.3Polarity Dependence.....................................................................364.3Electrowetting Force (EWF) Measurement..............................................384.3.1Predicted EWF..............................................................................384.4Measured EWF.........................................................................................404.4.1Polarity Dependence of Electrowetting System...........................464.5Electrochemical Corrosion........................................................................49CHAPTER 5ENVIRONMENTAL EXPOSURE OF THE ELECTROWETTING SYSTEM..............................................................575.1Introduction...............................................................................................575.2Contact Angle Measurement with Environmental Exposure....................575.3Electrochemical Impedance Spectroscopy...............................................63CHAPTER 6CONCLUSIONS AND FUTURE WORK...............................................696.1Conclusions...............................................................................................696.2Future Work..............................................................................................70REFERENCES.................................................................................................................72 iii

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LIST OF TABLES Table 1 Sample Specifications ..........................................................................................64Table 2 Dielectric Constant and Water Uptake for Samples ............................................66 iv

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LIST OF F IGURES Figure 1 Possible Shapes of a Droplet on a Surface.......................................................5Figure 2 Young Equation for Thre e Phase Interfacial Tension. ......................................6Figure 3 Electrowetting of a Droplet Basic Principle Demonstration ...........................7Figure 4 Electrowetting of a Droplet. A Coplanar Electrodes Configuration...............8Figure 5 Top View of the Sample and Stacked View of the Sample Used for Contact Angle Measurement and Electrochemical Impedance Spectroscopy ...................................................................................................16Figure 6 Top view of the Sample and Stacked View of the Sample for Electrowetting Force Measurement. ...............................................................17Figure 7 Contact Angle Measurement Set-up. A Goniometer is Used for Measuring the Contact Angle Using Sessile Drop Method. ...........................19Figure 8 Electrowetting Configurations and Equivalent Electrical Circuit ..................21Figure 9 The Equilibrium Position a nd Offset Position of Drop on Coplanar Electrodes During Electrowetting Experiemnt ...............................................22 v

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Figure 10 Schematic of Electrowetti ng Force Measurement and Picture of Experimental Set-up ........................................................................................24Figure 11 Schematic of Experi mental Set-up for EIS Testing ........................................29Figure 12 Experimental Set-up for EIS Testing ..............................................................29Figure 13 The Variation of Contact A ngle for DI Water for DC Voltage. The Thickness of Dielectric Layer is 1.78 m. The YoungLipmann Equation is Used to Estimate the Contact Angle at Different Voltages. ........32Figure 14 Contact Angle Measurements for Different Liquid on a Substrate having Dielectric Thickness of 2.05 m .........................................................33Figure 15 Reversibility of Electrowetti ng. Contact Angle Measurements for DI Water on Sample having Di electric Thickness of 2.05 m ............................34Figure 16 Reversibility of Electrowetti ng. Contact Angle Measurements for 1M NaCl Solution Sample having Dielectric Thickness of 2.05 m....................35Figure 17 Reversibility of Electrowetting. Contact Angle Measurements for 1mM NaCl Solution Sample having Dielectric Thickness of 2.05 m ..........35Figure 18 Asymmetric Electrowetting: Polarity Dependenc e of Electrowetting for DI Water on a Sample having Dielectric Thickness of 1.78 m. .............37Figure 19 Asymmetric Electrowetting: Polarity Dependenc e of Electrowetting for 1M NaCl Solution on a Sample having Dielectric Thickness of 2.05 m ...........................................................................................................37 vi

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Figure 20 Asymmetric Electrowetting: Polarity Dependenc e of Electrowetting for 1mM NaCl Solution on a Sample having Dielectric Thickness of 2.05 m ...........................................................................................................38Figure 21 Electrowetting Force (EWF) Pr ediction for Floating Drop on Coplanar Electrode and Sorted or Electrode Coated with Defect on Dielectric Layer. ..............................................................................................................39Figure 22 Comparison Between Measured and Predicted EWF .....................................39Figure 23 Measured Electrowetting Force for 1mM NaCl Solution. 75 DC Voltage Applied during time 20 to 40 second. Experimental Parameters: Droplet size = 55L; the Offset Side of Electrode Grounded, Offset = 3mm (b) EWF during Voltage Applied Period. ..............40Figure 24 Electrowetting Force for 1 mM NaCl Solution. DC Voltage Ramped up. The Voltage Applied : 25V, 50V, 60V, 75V, 90V, 110V, 120V, 130V. ...............................................................................................................42Figure 25 Electrowetting Force for 1M NaCl Solution. DC Voltage Ramped up. The Voltage Applied : 25V, 50V, 60V, 75V, 90V, 110V, 120V, 130V.........43Figure 26 EWF for DI Water. DC Voltage Ramped up. The Voltage Applied : 25V, 50V, 60V, 75V, 90V, 110V, 120V, 130V. ............................................43 vii

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Figure 27 Electrowetting Force for 1m M Na2So4 Solution. DC Voltage Rampup. The Voltage Applied : 25V 50V, 60V, 75V, 90v, 110V, 120V, 130V. ...............................................................................................................44Figure 28 EWF for 1mMNaCl. 75V DC Pu lse was Applied. The Electrode with Offset Droplet Position was Negativ e at the Beginning of the Test. During Second DC Pulse the Pola rity was Reversed Making the Electrode with Offset Dropl et as Positive and so on. .....................................47Figure 29 EWF for DI Water. 75V DC Pulse was Applied. The Electrode with Offset Droplet Position was Negativ e at the Beginning of the Test. During Second DC Pulse the Pola rity was Reversed Making the Electrode with Offset Dropl et as Positive and so on. .....................................48Figure 30 EWF for 1mM Na2SO4. 75V DC Pulse was Applied. The Electrode with Offset Droplet Position was Negative at the Beginning of the Test. During Second DC Pulse the Polarity was Reversed Making the Electrode with Offset Dropl et as Positive and so on ......................................49Figure 31 Electrochemical Corrosion: EWF Measurement at 75V DC Polarity Change for 1mM NaCl Droplet. Cha nge in Sign of the Force and Spikes in EWF Spectrum Due to Electrochemical Corrosion. .......................52Figure 32 The Damaged Surface on Substr ate due to Electrochemical Corrosion for EWF Measurement Corresponding to Data from Figure 31 .....................52 viii

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Figure 33 Electrochemical Corrosion: EWF Measurement at 75V DC Polarity Change for 1mM NaCl Droplet. Ch ange in Sign of the Force and Spikes in EWF Spectrum Due to Electrochemical Corrosion. .......................53Figure 34 The Damaged Surface on Substr ate due to Electrochemical Corrosion. The Damaged Surface on Substrate due to Electrochemical Corrosion for EWF Measurement Corresponding to Data from Figure 33 .....................53Figure 35 SEM Image of Damaged Test Spot Due to Electrochemical Corrosion. Magnification: x200 .......................................................................................54Figure 36 SEM Image of Damaged Test S pot due to Electrochemical Corrosion. Magnification: x2200 .....................................................................................54Figure 37 EDS Analysis of Damaged Test Spot. ............................................................55Figure 38 Electrochemical Corrosion: EWF Measurement at 75V DC Polarity Change for DI Water. Change in Sign of the Force and Spikes in EWF Spectrum due to Electrochemical Corrosion ..................................................55Figure 39 The Corrosion of Al Film and Delimitation of CYTOP .............................56Figure 40 Change in Contact Angle for 1mM NaCl Droplet over Time ........................59Figure 41 Change in Contact Angle for 1mM Na2SO4 with Immersion Time ..............60Figure 42 Change in Contact Angle for DI Water with Immersion Time ......................61 ix

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Figure 43 Time Constant of Steady D ecay in Electrowetting Response due to Liquid Exposure ..............................................................................................62Figure 44 The Change in Contact Angl e Modulation for Different Fluids with Immersion Time. .............................................................................................63Figure 45 Percentage Increase in Capacitance with Exposure Time ..............................64Figure 46 Time Constant of Water Mo lecule Diffusion in Dielectric Layer .................65Figure 47 Comparison of Different Sample s with Respect to Capacitance after the First Data Collected of th e Experiment (t=155 seconds); Percentage Increase in Capacitance at 4hrs and Dielectric Constant Calculated at t=155 seconds...........................................................................67 x

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Characterization of Electrowetting Syst em s for Microfluidic Applications Pradeep K. Mishra ABSTRACT Electrowetting is the change in apparent surface energy in the presence of an electric field. Recently, this phenomenon has been used to control the shape and location of individual droplets on a surface. However, many microfluidics researchers have acknowledged unexplained behaviors and perf ormance degradation. In this work, electrowetting systems are ch aracterized with different methods. The electrowetting response is measured by measuring contact an gle for different app lied voltages. A novel technique for direct measurement of Electro wetting Force (EWF) us ing nano indenter is proposed in this work. The EWF measurements show that, for aqueous solution the EWF is more as compared to DI water. Additiona lly, the electrowetting system is found to be more susceptible for degradation when a queous solution is used. The performance degradation due to defective dielectric layer is also investigated by measuring the electrowetting force. Degrad ation of EWOD systems with environmental exposure over time is further studied experimentally by co ntact angle and electrochemical impedance spectroscopy (EIS) measurements. The time constant of contact angle decay with environmental exposure is found to be similar to the time constant of electrolyte diffusion in dielectric layer. xi

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CHAPTE R 1 INTRODUCTION 1.1 Thesis Statement The thes is will address th e characterization methods of electrowetting systems response. A novel technique of measuring capill ary forces in electrowetting system and investigate the reliability of electrowetting systems. The electrowetting system response for different parameters such as liquid t ype, voltage type and voltage polarity is characterized by contact angle and electrow etting force (EWF). The liquid exposure effects on electrowetting system response are ex amined by measuring the contact angle. The electrochemical impedance spectroscopy is used to investigate the electrochemical corrosion in electrowetting systems. This chapter will review the motivation and scope of this research work. 1.2 Motivation In last two decades microfluidic devices have been investigated and numbers of products are already available on the market. The manipulation of small liquid droplets has been used in most of the Microfluidic devices. The application of microfluidics includes continuous flow of liquid, individua l droplet movement, DNA chips, molecular biology, tunable micro lenses, acoustic dr oplet ejection, fuel cells and more. Electrowetting is a powerfu l actuation mechanism in which the surface energy of a fluid is being changed by applying electrical energy. Electrowetting has been 1

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used in a number of applicati ons from digital microfluidics to displays. The transport of liquid droplets on an array of electrodes has be en demonstrated by Sung et al [1, 2]. The mixing of droplets and inducing a chemical reaction based on electrowetting has been demonstrated by Pail et al. and Fowler et al. [3, 4]. Electrowetting based optical switches have been demonstrated by Yang et al [5] .T hermal management of electronic devices by liquid cooling have been investigated in recen t years [6-10]. Mugele et al. [11] reported transport and mixing of droplets for printing. Chen et al. [12] used electrowetting for suction of liquids in microtubes. Electrowetting based optical devices ar e commercially available. The difference in the refractive index of various liquids has been used as the basic aspect for electrowetting based tunable mi cro lenses. In 2000, Berge et al. [13] reported the tunable micro lenses based on electrowetting principl e. The authors used non-polar oil droplet and salt solution in a closed cell and used the difference in refractive index along with contact angle difference for c ontrolling the focal length of le nses. Later Krupenkin et al. [14] reported similar tunable micro lenses with a lateral positioning option of lenses. In 2004, Kuiper et al. [15] reported the tunable lenses with integrated CCD cameras for a cell phone display. Hayes et al. [16] reported the use of electrowetting in display technology by using oil droplet having colored die and slat solution on predefined and patterned electrodes. Later, Heikenfeld et al [17, 18] and Roques et al. [19] also reported electrowetting based display. Li quavista Inc. commercially launched the electrowetting based display in 2006. 2

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1.3 Scope of Work The electro wetting based systems have been proven to have a great importance in commercially available products. The design a nd characterization of such electrowetting actuated devices will need to focus on e xploiting device scaling while optimizing for reliability and lifetime. However, many mi crofluidics researchers have acknowledged unexplained behavior and performance degr adation. These include spurious droplet motion, oscillations, voltage polarity dependence, and sensitivity to the ambient medium [20-29]. The characterization of electrowetti ng system is often carried out by contact angle measurements due to simple equipm ent configurations, direct wetting angle measurements and large body of comparative measurements. However, the accuracy and response time of system of contact angle measurement is limited. Verheijen et al [30] experimentally measured the capacitance of electrowetting systems. The measurements were repeatable but the response time of the measurements was similar to contact angle measurements. In this work a novel technique for di rect measurement of capillary forces in electrowetting has been invest igated. The electrowetting force (EWF) has been measured by a modified Hysitron Triboindent er. The effects of different parameters of interest such as voltage type, voltage polarity and liquid type in electrowetting system have been investigated. Additionally, due to the functional nature of electrowetting in liquid environment with different i ons, dielectric materials and strong electric field make the system prone for electrochemical corrosion a nd could further degrad e the performance of 3

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the sys tem. However, due to compact nature of electrowetting system the electrochemical corrosion has been never investigated befo re. In this work, the Electrochemical Impedance Spectroscopy technique has been us ed to study corrosion in the electrowetting system. 1.4 Thesis Outline This thesis proceeds as follows: Chapter 2 reviews the previous work done on electrowetting processes and summarizes their outcomes. Chapter 3 details the experimental technique used in this work for characterizatio n of electrowetting. Chapter 4 discusses the experimental re sults of contact angle measurement and electrowetting force measurement and compares the variations due liquid type and voltage polarity. In chapter 5, the affects of environmental exposure on el ectrowetting system is reported. Chapter 6 concludes the work done and recommends future aspects for improvement and optimization of the electrowetting processes. 4

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CHAPTE R 2 LITERATURE REVIEW 2.1 Wettability The behavior of any liquid dr oplet over a surface is know n as the W ettability of the liquid and surface. When a liquid droplet is kept over a surface, how well it sticks or spreads depends on the wetting characteristic s of the droplet and the surface. The interaction between a liquid dr oplet and surface droplets is often characterized by a contact angle. Consider the case of a liquid droplet on a surface, the possible shape of the droplet on a surface is shown in Figure 1 Figure 1 Possible Shapes of a Droplet on a Surface 5

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The funda mental phenomena of contact angl e and wettability ca n be described by the Young equation. Figure 2 shows the three phase contac t line and the Young equation can be expressed by following equation, cosLV SL SV Equation 1. Young's equation for interfacial tensions where stands for interfacial energy and S,L and V stands for solid, liquid and vapor respectively and for contact angle. Figure 2 Young Equation for Three Phase Interfacial Tension. 2.2 Electrowetting Miniaturization increases surface to volum e ratios bringing more challenges in control of surface and surface energies [23]. In 1875, Gabriel Lippmann demonstrated a relationship between electrical and surface tension phenomena. This relationship allows efficient control of the shape and motion of a liquid meniscus by applying a voltage. The liquid changes shape when a voltage is applie d in order to minimize the total energy of the system (sum of surface tens ion energy and electrical energy). Today, this effect, known as electrowetting, has shown potenti al importance in many applications. 6

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The idea behind electrowetting is to make the surface highly wettable by an applied electric field. In electrowetting on a di electric, the change of contact angle as a function of the applied voltage can be related to the dielectric thickness () and dielectric strength (R) as proposed by the YoungLippmann equation [23]. lv ro oV 2 coscos2 1 Equation 2 Young-Lipmann equation It employs control of voltage that changes the interfacial energy of the liquid-solid interface [23]. By alternatively applying voltage across an electrode, the fluid can be efficiently moved as desired, by changing contact angle of the liquid on the surface. This is proved by many works done so far by appl ying large voltage on dielectric called Electrowetting on Dielectric (EWOD) that provided large changes in the contact angle. Figure 3 Electrowetting of a Droplet. Basic Principle Demonstration Figure 4 shows most common configurations em ployed in the electrowetting phenomenon. In the grounded droplet method, th e liquid is grounded while in the floating droplet, the droplet floats across two electrodes with a pplied voltage across them. 7

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Figure 4 Electrowetting of a Droplet. A Coplanar Electrodes Configuration. 2.3 Contact Angle Variation There is a considerable amount of lite rature on contact angles and wetting phenomena [21, 23, 24, 26, 31-33]. The voltage applied between the droplet and the counter-electrode generates el ectric charges on both conduc ting surfaces. The mechanism of charge generation, and its distribution, can be rather complicated for real electrolytes, but the assumption of an ideally conducting fluid with a surface charge density is usually sufficiently accurate. The Young-Lippmann equation predicts a near parabolic curve for a sessile drop on a single dielectric plate, rela ting contact angle to the capacitive voltage across the plate. One of the Electrowetting objectives is to maximize the accessible contact angle range. Contact angle decreases to a mini mum value upon applying the voltage. Minimum contact angle is known as Saturated Contact Angle and corresponding voltage is known as Saturation Voltage. A reasonable amount of research was done to find out the contact angle sa turation phenomena [24, 31]. Verheijen and Prins [26] found indications that the insu lator surfaces were ch arged after driving a droplet to contact angle satura tion. They suggested that charge carriers are injected into the insulators. These immobilized charge carriers then partially screen the applied electric field. It seems clear that divergi ng electric fields at the co ntact line can induce several distinct non-linear effects and each of them may independently cause saturation. Which 8

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effect dom inates depends on the specific conditions of each experiment and identifying these conditions requires more work in the future. Quinn et al. [24] studied contact angle saturation in electrowetting. The saturation of contact angle was related to the magnitu de of the capillary force variation. Also contact angle saturation phenomena are related to the materials properties. Above the contact angle threshold the electrowetting phenomena is considered to be the non equilibrium process. 2.4 Contact Angle Hysteresis Contact ang le hysteresis is another piece of the boundary physics needed to complete the model of droplet motion using EWOD forces. Hysteresis refers to the difference in contact angles between the a dvancing and receding ends of sessile drops [34]. It is a direct consequence of contact line pinning, which acts as a force that resists any sliding motion, and it can be seen when wa ter droplets stick to the side of a solid surface. For a sessile drop on a single plate, it can be seen that the advancing and receding contact angles are greater and smaller, respectively, than the nominal contact angle. The contact angle of the droplet and c ontact angle hysteresis is strongly influenced by the surface morphology. Bhadur et al. [35] studied the electr owetting on the rough surface. Analysis was done based on the mi nimization of energy. Dynamic electrowetting based on the surface design was proposed. 2.5 Material Properties In bas ic Electrowetting theory the liquid droplet is considered as perfect conductor. The requirements regarding the concen tration and nature of charge carries are 9

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not strict. Most authors report no af fect of concentration of salts in the liquid on the electrowetting. On the other hand the prope rties of insulating layer are much more critical. Substantial work has been done to optimize the insulating layer thickness and properties of the insulating layer for low voltage electrowetting. The two criteria for insulating material optimization can be obtained from the Young-Lipmann equation. First one can obtain maximum c ontact angle at zero voltage a nd second criteria should be the possible minimum insulating layer thickne ss. The minimum thickness of dielectric layer will lead to a high cap acitance value and accordingly the contact angle modulation will be high. The first choice can be met by hydrophobic insulating polymer. Thin layers of amorphous fluoropolymer (Teflon or CYTOP or Paralyne ) are often used. These organic compounds can be easily deposited wi th thickness ranging from few nanometers to micrometers range by spin coating or dip coating methods. The commonly used inorganic compounds for dielect ric layers are silicon dioxide and silicon nitride. In general, thin film deposition tech nique is being used for inorganic dielectric layer. To make surface hydrophobic, a thin layer of hydrophobic compound is coated over the inorganic substrate. Seyrat et al. [34] in 2001 reported on the different thickness of amorphous fluoropolymer used and their critical effect on the reversible electrowetting. Coatings dried at room temperature are typically observed to have reduced electrowetting modulation (i.e. difference betw een the contact angle at zero volts and saturated contact angle) and fewer adherences to the electrowetting theory. Peykov et al. [32] studied the electrowetti ng with parylene and Teflon dielectric layers with thin gold film as electrode. A double layer effect was studied in depth and 10

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theoretical model was developed for the cont act angle saturation. The model proposed in this work predicts that for an electrowetti ng device in which an aqueous droplet can be forced to completely wet a hydrophobic surface a surface with the same surface energy as the liquid is required. Indeed the work presented a more detailed consideration of electrowetting taking into account th e structure of the double layer. Moon et al. [36] in 2002 reported low voltage electrowetting. Three different types of dielectric layers namely, Teflon, silicon dioxide and Parylene with varying thickness was studied. A cont act angle modulation of 40 was reported for the actuation voltage of 15V. Cahill et al. [37] studied the platinum and copper electrodes for EWOD experiment. The leakage current and resist ance were measured experimentally to characterize the dielectric materials. Br eakdown voltage tests were performed for Parylene C and SiN. Raj et al. [38] studied the composite dielectric materials for low voltage electrowetting. The composite diel ectric was made of aluminum oxide and silicon nitride. CYTOP was spin coated over the composite dielectric to make surface hydrophobic. The aluminum oxide, thickness of 100 nm, wa s deposited by atomic layer deposition. Later, silicon nitride (thickness of 150 nm) was deposited over the aluminum oxide using plasma enhanced chemical vapor technique. To reduce the contact angle hysteresis, the electrowetting experiment was carried in dodeca ne oil environment. The authors reported the optimal thickness of the CYTOP to be 50 nm for nominal electrowetting actuated devices. The CYTOP thickness greater or lower than 50 nm may cause charging in dielectric layer. 11

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Banpurkar et al. [39] reported the electrowetting of aqueous gelatin m aterial in the form of a small droplet over Teflon coated subs trate. The authors e xplored the possibility of the electrowetting of soft matter materials for inner sight of the rheological properties of such materials in liquid phase. 2.6 Effects of Gravity and Evaporation Another consideration when using a singleplate design is the e ffect of gravity on the motion of large drops. The ratio of gravitational to surface tension forces is characterized by the Bond number. Although it is not clear at what Bond number gravity affects electrowetting translation, a Bond number of unity corresponds to a droplet volume of 82 l for water [21]. Thus, for maximum EWOD applications droplet volumes correspond to Bond numbers mu ch less than unity and ar e therefore typically small enough that surface tension domi nates gravitational forces. With the droplet exposed to the ambi ent air, one must consider evaporative losses of the droplet. Chen et.al. [21] repor ted that a 5 l droplet requires 33 min. to evaporate from a Teflon surface under a lami nar flow hood. It is obs erved that droplets on hydrophobic surfaces such as Teflon have a longer evaporat ion time than droplets on hydrophilic surfaces. When the droplet is prot ected from air flow, the evaporation time can be substantially increased. In the same article, they claimed that in a zero humidity environment, a 250 l droplet takes 7 minut es for evaporation. For many microfluidic applications and testing done in this work, this time span is sufficient. 12

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2.7 Electrowetting Characterization Baviere et al. [40] reported dynam ics of droplet actuated by electrowetting in air. The transportation of small volume dropl ets were investigated by stroboscopic observations. The effect of viscosity on the dr oplet velocity with the varying voltage was studied. The electrowetting actua ted droplet velocities were found highly sensitive with the viscosity of fluid. A high velocity of 114 mm/sec with the actuation voltage of 100V was reported. Baird et al. [33] proposed a method to examine electrostatic force on microdroplets transported via Electrow etting on Dielectric (EWOD). The force distributions on advancing and receding fluid faces are detailed in each case. Dependence of the force distribution and its integral on sy stem geometry, droplet location and material properties are described. A co mparison of scaling propertie s and force distribution for both cases are given. The effect of the divergent charge density on possible explanations for contact angle saturation such as char ge trapping, local diel ectric breakdown, and corona discharge were studied. Both analytic al results for integrated total forces and numerical results for the force distribution were compared and proven to be in agreement. Walker et al. [41] discussed the m odeling and simulation of a parallel electrowetting on dielectric device that studied droplet movements through surface defects. The simulations were compared to th at of the experiments for a splitting droplet various factors affecting elec trowetting effects were. The g overning fluid equations and boundary conditions along with contact hys teresis were developed. A numerical simulation was described which uses a level set method for tracking the droplet boundary. 13

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Berthier et al. [42] pro posed a technique to find to investigate m inimum and maximum actuation voltages in electrowetti ng. He formulated maximum voltage as a threshold beyond which there is no more gain in the capillary e ffect due to th e saturation. Calculation was done to determine the electrowetting force on a EWOD system considering the contact angle hy steresis and an analytical relation was obtained to derive the minimum actuation potential. Verhejen et al. in 1999 [26] reported the effect of trapped charges on the EWOD. The experiments were conducted on silicon su bstrate with aluminum (100 nm thickness) as electrode, an insulating layer of Paryle ne C (10m thickness) and a thin hydrophobic layer AF 1600. The EWOD experiments were co nducted inside the silicone oil to reduce the contact angle hysteresis. The 10 l si zed aqueous solution potassium chloride and potassium sulphate was used for EOWD testing. Verhejen et al. [26] reported that up to a threshold voltage, the charge remains in liq uid and is not trapped. However, once the threshold voltage is reached, th en interaction of ions inside the liquid has more attraction towards the solid as compared to li quid. A modified Young Lipmanns equation accounting for the trapped charges was propos ed. Later, contact angle was estimated from the capacitance values of EWOD sy stem using the modified Young Lipmann equation. 14

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CHAPTE R 3 EXPERIMENTAL PROCEDURES 3.1 Introduction The characterization of the electrowetting on dielectric (EWOD) in this work has been done by multiple experimental techniques. In this chapter, the basic principle, experimental set up and procedur es are discussed in detail. 3.2 Sample Preparation All exper iments in this work are carried out on silicon wafers. All fabrication steps were done at Nanomate rials and Nanomanufacturing Research Center (NNRC) facility at USF. First of all, a silicon dioxide layer of ~500nm is thermally grown over Si wafer. Thereafter a ~ 300nm thick Al metal thin film is deposited by sputteri ng process. Further fabrication process depends on the experiment of interest. Fo r the contact angle measurement and EIS experiment, a thin amorphous fluorocarbon polymer (CYTOP, Asahi Glass Co., Ltd) is spin coated whic h works as a dielectric and hydrophobic layer. In this work, all the samples are coated wi th two layers of CYTO P. Two layers of CYTOP have fewer defects as compared to a single layer. Two layers of CYTOP is spin coated on almost 3/4th of the Si wafer surface except fo r ~1.5 cm near the flat edge of the wafer to make electrical connection with the aluminum electrode as shown in Figure 5 After the first coating of CYTOP wafers were baked at 90C for 30 minutes 15

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in a conventional oven. Then a second layer of CYTOP is spin coated and final baking is done at 150C for 1 hour. Figure 5 Top View of the Sample and Stacked View of the Sample Used for Contact Angle Measurement and Electrochemical Impedance Spectroscopy For electrowetting force measurement, copl anar electrodes were patterned. After the aluminum film deposition, Shipley 1813 phot oresist was spin coated and baked on a hot plate at 90C for 90 seconds. Thereafter, using th e mask aligner, photoresist coated wafers were exposed to UV light for desire d pattern. Then the sample was developed using MF319 developer to develop the pattern exposed by UV light. Wet etching technique was used to etch the aluminum an d make coplanar electrodes. The aluminum etchant was heated to 60C in a beaker and developed samples were immersed in etchant 16

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for 30 seconds. Then wafers were rinsed in D I water and dried. Later, solvent (ethanol and methanol) were used to strip off the photoresist with a result shown in Figure 6 Then two layers of CYTOP were spin coated and baked. Figure 6 Top view of the Sample and Stacke d View of the Sample for Electrowetting Force Measurement. The thickness and roughness of CYTOP film were measured by the step profilometer and used in all analysis. The electrical connections across the electrode 17

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during the experim ent are made by putting c onductive copper tape on the aluminum film near the edge. 3.3 Contact Angle Measurement Contact an gle measurement (CA) is a simple technique for measuring the wettability of liquid and surface. In general surface energy is measured by two different methods, namely, goniometry and tensiometry. In goniometry, a goniometer is used for the direct measurement of contact angle. The analysis of contact angle is based on the shape of the droplet in goniometry. The cont act angle is measured by drawing a tangent at the edge of droplet and surface interface. In tensiometry the inte rfacial energy is found by measuring the attraction force between so lid and liquid droplet provided geometry of solid and surface tension of liquid are known. The contact angle analysis of fibers is often done by tensiometry. The change in contact angle with voltage in electrowetting can be captured using goniometer due to small-sized droplets. In this work, a Ra mehart Inc. (model number: 100-00-115) goniometer is used for the contact angle measurements. 3.3.1 Method The contact angle is m easured by sessile droplet method. In the measurement the following assumptions are made: first the drop is symmetric about vertical axis and secondly, droplet is in not moving. The droplet size is kept in the range of 5-10 micro liters, to reduce the effect of gravity. In this way, the measurement of contact angle leads the measurement of interfacial energy between droplet and surface. The goniometer available in the lab has movement in all three axes. Before any experiment, the panel is aligned to be flat a nd calibrated accordingly. The dispensing of a 18

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droplet on substrate is done precisely by a mi crometer syringe having micro liter resolution. The syringe is attached to the clamp, and the droplet is released along with the upwards movement of panel (attached to goniometer) containing substrate, until the droplet touches the substrate. The manual dispensing of droplet on substrate needs to be done carefully to avoid the scratch in the substrate. Then by moving the panel along X and Y axis, droplet is focused for accurate measurement. Thereafter, by moving the panel in X direction, the edge of droplet is located and contact angle is measured directly along the tangent at the edge of droplet and surface shown in Figure 7. Figure 7 Contact Angle Measu rement Set-up. A Goniometer is Used for Measuring the Contact Angle Using Sessile Drop Method. For electrowetting experiments, a wire electrode is dipped half way inside the liquid. The copper tape is attached to the aluminum film. Then alligator clips were attached to wire electrode and copper tape connected to the power source. The polarity of electrodes can be changed simply by changing the position of alligat or clips attached to the power source. The 19

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voltage output of the power source can be changed by a knob attached to the machine. During the electrowetting experiment, first the contact angle is measured without any voltage applied. Then, voltage was applied small steps (5-10V) and contact angle were measured. After measurement, the voltage output was kept off and contact angle was measured again to investigate the hysteresis of electrowetting. The contact angles measured in lab have tolerance of +/-2 as the measurements can be performed only manually. 3.4 Electrowetting Force 3.4.1 Introduction Electrowetting is an effective met hod to m anipulate droplets in Digital Microfluidics. The forces induced due to the principle of electrowetting is the main source of control of these droplets by causing an apparent change in the surface energy due to the voltage applied across the substrate on which the droplet is located. These forces can greatly affect the performance of electrowetting device s. Therefore, force measurement and optimization are critical to process improvements. A novel method has been developed to measure thes e forces in two-dimensions. 3.4.2 Principle of Electrowetting Force Measurement In the case of interest, the drop is positioned over two electrodes with a voltage difference across the electrodes. The electrode s are covered with a dielectric material. The droplet can be considered conductive so that the system is modeled as two capacitors in series. The electrowetting configurations are shown in Figure 8 20

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Figure 8 Electrowetting Configurations and Equivalent Electrical Circuit The voltage between each pad and the dr op will vary as the position of the droplet changes due to changes in the capa citor area and thus their capacitance. Neglecting the droplet resistan ce, the arrangement can be modeled as simple series capacitor circuit composed of two parallel plate capacitors. The voltages across the left and right capacitors ( VL, VR) are given as: tot RL R LV AA A V Equation 3. Voltage across left capacitor tot RL L RV AA A V Equation 4 Voltage across right capacitor 21

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The change in contact angle is driven by the red uction in eff ective surface surface energy by the energy stored in the capacitors Using these relationships, the total capacitive energy is given by 2 2 2 1 RRLlVCVCE Equation 5. The total energy stored in capacitor The area of the droplet over each electrode (AL, AR) is a function of the droplet position and the form of this function depends on droplet shape. In general, this shape will vary with the offset from the equilibri um position. If the droplet is kept middle between the coplanar electrodes, then the syst em remains in equilibrium. However, if the droplet is offset from the equilibrium positi on, the droplet tend to mo ve other side (not towards the offset side Figure 9 ). Figure 9 The Equilibrium Position and Offset Position of Drop on Coplanar Electrodes During Electrowetting Experiemnt 22

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In the EW F experiment, the droplet is sandwiched between the substrate and an electrically insulating coverplate that is we t by the fluid, the drop let contact area and shape on the substrate will approach the area of the coverplate as the droplet thickness decreases relative to the coverplate dimensions With this simplification, the area values of the droplet beneath a square coverplate with edge length s can be related to the displacement (x) of the coverplate from th e equilibrium position. x s sAL2 x s sAL2 22 s x s The energy and force as a f unction of position is then xV dx dE F xs V Etot R x Rtot2 0 22 0 24 8 Equation 6. The energy of the system and predicted model of electrowetting force where the force as a function of cover plate position is f ound by differentiating the energy E with respect to the displacement x. The static equilibrium positio n is at the center of the two drops with equal area on each electrode. In cases of limited viscous and electrical energy loss, the drop could oscillate around the equilibrium when disturbed. Another behavior that can be observed during electrowetting is induced by the presence of a hole in the dielectric layer. A hole would short the capacitor on one side so that no voltage will be applied across the regi on of the drop over the shorted electrode. The full voltage drop will occur across the othe r side. In this case, the electrowetting force will be constant with displacement. It is given by: 23

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2 02tot R xsV F Equation 7. The electrowetting force for a sorted capacitor 3.4.3 Experimental Setup for Electrowetting Force Measurement This was implemented by attaching a 9mm x 9mm glass plate to a modified tip of a Hysitron Triboindenter in Figure 10 The assumptions made in this technique are: Wetted indenter plate. Very thin fluid so that substrate contact area closely approx imates the indenter plate area. Fluid fully wets the entire substrate area. The indenter plate and substrate are parallel. The Triboindenter measures the forces on the plate normal to the substrate and in one in-plane direction. Figure 10 Schematic of Electr owetting Force Measurement and Picture of Experimental Setup Since water wets the glass plate, drops are trapped between the plate and the substrate. As the gap between the plate and substrate decreases, the drop/substrate 24

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contact area closely ap proximates the area of the top plate and is does not change significantly under an applied voltage. 3.4.4 Experimental Procedure For EWF measurements, the substrate with conducting tape attached for electrical connection is placed on the panel. The coordinates of the position near the edge along gap is measured and alignment of gap is checked. If the sample is misaligned in any of direction then sample is tilted such that gap looks perfect horizontal. Th en coordinates of the gap is found. The coordinates of gap cente r is needed for positioning the droplet The droplet position located at th e 3mm offset from the gap cen ter in all experiments until stated otherwise. Then droplet size of 50-55L is placed on the defined position. The alligator clips were connected to conducting tape attached with metal electrode on the substrate. Then the coverplate attached tip is lowered down and droplet is squeezed. The droplet is squeezed until the coverplate is we tted completely. Before every test the air scratch is done to obtain the lateral and normal force. For WEF measurements it is necessary to minimize the drift before applying the voltage across the electrodes. The normal drift data less than 300 N for a 15 seconds air scratch test is considered good for testing. Additionally, the minimal drift force s hows that the coverplate is wetted entirely. Then voltage across electrodes is ap plied and test is performed. 3.5 Electrochemical Impedance Spectroscopy 3.5.1 Introduction The use of aqueous droplet in electrowett ing actuated applications was reported earlier. For any electrochemical reaction, such as corrosion, to take s place water, oxygen 25

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and ionic flow are required. The use of the organic coating in electr owetting applications acts as a dielectric layer also. The organi c coating in general shows very poor ionic conductiv ity. However, in two conditions the organic coating, CYTOP which is in interest of this work, could possibly provide an ionic path. Firstly pores/defects on the coating can lead to the direct contact of liquid and metal and secondly the diffusion of water into the polymer in contact. The possib ility of discussed two mechanisms is very likely in electrowetting phenomena. The investigation of coating performance in situ environment requires a nondestructive technique. Electrochemical impeda nce spectroscopy (EIS) has been proven to be a powerful technique in co rrosion characterization. 3.5.2 Principle of Electrochemical Impedance Spectroscopy A metal with a flawless organic coating on its surface tends to behave similarly as an ideal capacitor with capacitance C A d C0 Equation 8. Capacitance of a metal with organic coating where C is the capacitance, is the dielectric constant of organic layer, A is the area and d is the thickness of the organic layer. The di electric constant of a polymer coating is a value typical of polymers ( ~2 to 5). In this work, the dielectric constant of organic polymer is 2.1. However, as electrolyte molecules are absorbed by the coating during service, the dielectric constant increases beca use of the much higher di electric constant of water. 26

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The dielectric constant of the polym er with an absorbed water volume fraction, f, as, W CYTff Equation 9. Change in dielectric constant wi th water absorption. where CYT is the dielectric constant of th e polymer (CYTOP in this work), w is the dielectric constant of water. Because of the high value of for electrolyte, increases with the immersion time [43]. For water, the percentage of water volume fraction, f, diffused in the polymer can be found as, f t800 Equation 10. Change in di electric constant of cyto p with water absorption. The diffusion of water molecule in th e polymer coating can be expressed by Ficks first law of diffusion as [44], Dx Equation 11. The relation between diff usivity constant and time constant where x is the thickness of the coating, D is the diffusivity coefficient of sodium chloride solution in polymer and is the characteristic time In electrowetting, the dielec tric layer is expos ed to the liquid droplet over long periods of time. If the dielectric and droplet interacts, this creates a situation where the capacitance of the dielectric la yer changes with time. This could have the adverse affects on the performance of the electrowetting. The reliability of electrowetting system due to electrochemical corrosion has been discussed previously, but the proper investigation has 27

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not been carried out so far. In this thesis the EIS for investigation of electrowetting system reliability has been explored. 3.5.3 Experimental Procedures The EIS requires there electrodes namely, working electrode, counter electrode and reference electrode. The working electrode is submerged in the elec trolyte during test where as the counter electrode provides the connection to metal beneath the coating for impedance measurement. In this work, the design of the electroch emical cell has been adopted from Sagues et al. [45]. The schematic of el ectrochemical cell is shown in Figure 11 The experimental set up is sho wn in Figure 12 A plexi glass tube of 1.5 inches in length (outer diam eter d= 3.2 cm, inner diameter = 2.5cm) is glued with epoxy (Loctite ma de, 3 minute adhesive) on the CYTOP spin coated Si wafer. The t ube is glued over the CYTOP part of the Si such that counter electrode connection can be easily made near the flat edge of the wafer which is only coated with aluminum th in film. Activated Titanium (coated with metal oxide such as Tantalum, Niobium and Zirconium oxides) wire is used for the counter and reference electrodes. The cell atta ched with the wafer is then placed over the plexi glass sheet. All three electrodes are then connected with the SS screws as shown in Figure 11 During the experiment, alligator clips of the EIS testing machine is connected w ith the screws to make connections. Data of the experiment are collected over different time period to observe the effect of diffusion of water and hence the change in capacitance of the coating. Later, using data acquisition system capacitance of the organic coating of the test sample is calculated. 28

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Figure 11 Schematic of Exp erimental Set-up for EIS Testing Figure 12 Experimental Set-up for EIS Testing 29

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CHAPTER 4 CONTACT ANGLE AND E LECTROWETTING FORCE MEASUREMENTS 4.1 Introduction In this chapter, the experimental data are discussed. The first section of this chapter details the contact angle measurem ent data. Then, electrowetting force (EWF) measurements by modified nano indenter results are discussed. Finally, the electrochemical corrosion observation by EW F measurements are investigated and detailed. 4.2 Contact Angle Measurements In electrowetting, an electric field is used to tune the interfacial energy and thus droplet shape. The change in shape of dropl et upon applying the voltage is measured by change in the contact angle. The electrowet ting system is often characterized by the contact angle measurement as a function of varying voltage. In this work, contact angle is measured to investigate the affect of different liquid type a nd voltage polarity on electrowetting and reversibility of electrowetting process. 4.2.1 Different Liquids The contact angle is measured for DI water, 1mM NaCl, and 1M NaCl solution. The contact angle is measured using sessile droplet method with droplet size 6-8 L. 30

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Figure 13 shows the change in contact angle for DI water for sample having 1.78 m thick dielectric layer. The contact angle changes from 109 to 57. The contact angle is estimated using the YoungLipmann equation and plotted with the measured contact angle values. The Young-Lipmann equation ob served to be deviating from 30V DC value. The deviation of Young Lipmann equati on is not expected at lower voltages. The observed deviation can be due to the error of thickness measurement of the dielectric layer. The adhesive tape was used during spin coating of the CYTOP dielectric layer. The tape is used to prevent th e coating of dielectric over the aluminum film near the edge where electrical connections are needed for the experiment. The spin coating of CYTOP is done at 1600 rpm for 20 second. Due to this, CYTOP could accumulate near the edge of the tape. Th e thickness measurement of the dielectric layer is done by a profilometer. The stylus of the profilome ter moves across the edge of CYTOP and aluminum to measure the thickness of CY TOP film relative to aluminum film. However, the accumulation of CYTOP during spin coating and later after annealing treatment can lead to thickness measurement error. 31

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Figure 13 The Variation of Contact Angle fo r DI Water for DC Voltage. The Thickness of Dielectric Layer is 1.78 m. The YoungLipman n Equation is Used to Estimate the Contact Angle at Different Voltages. Figure 14 shows the contact angle measuremen t f or three different liquids: DI water, 1mM NaCl and 1M NaCl. The contac t angle variation is shown in terms of difference in cosine of contact angle at vo ltage V and voltage zero. The dc voltage was applied up to 120V. The contact angle of the 1M NaCl solu tion droplet observed to be saturated at 110 V. The 1mM NaCl solution droplet was found to be saturated at 120V. The DI water does not seems to saturated t ill 120 V. The saturated contact angle for 1mM NaCl and 1M NaCl is found to be 61 and 69 respectively. However, the contact angle measurement at higher voltages needed to ju stify the saturation of contact angle. The early saturation of 1M NaCl dr oplet compares to others liqu id and reported earlier [24, 26, 46, 47]. Verhejen et al [26] suggested that the saturation of contact angle occurs when charges accumulate in dielectric layer causing formation of shield and apparently further 32

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change in contact angle is not po ssible. The earlier saturation of 1M NaCl solution is likely due to availability of more ions as compared to other liquid. Figure 14 Contact Angle Measurements for Different Liquid on a Substrate having Dielectric Thickness of 2.05 m 4.2.2 Reversibility of Electrowetting The electrowetting is expected to be a reversible phenomenon. However, many researchers reported the irreve rsibility of electrowetting pr ocesses [26, 29, 34, 48]. The reversible nature of electrowetting was inve stigated by measuring the contact angle of different liquids for increas ing and decreasing voltages. Figure 15, Figure 16 and Figure 17 show the contact angle measurements when voltage was ram ped up and down in steps. DC voltage was used for the testing. The maximum voltage is 120V for measuring the contact angle changes. Then, voltage was lowered in steps and contact angle was measured until the voltage is lowered to 0V. The hysteresis of DI water droplet is smaller than the salt solution. In the beginning (whe n voltage was zero) and end of the test (when 33

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voltage is made zero after ram ping down) the contact angle values changes by 6 and 2 for salt solution (both 1mM NaCl and 1M NaCl) and DI water respectively. The irreversibility of electrowetting is attributed to the trapped charges. The trapping of charge in dielectric layer is more in aque ous droplet Also the electrowetting is not perfectly reversible for DI water also which suggest that charges were trapped at end of test. The possibility of impure DI water use during experiment is very likely. Figure 15 Reversibility of Elect rowetting. Contact Angle Me asurements for DI Water on Sample having Dielectri c Thickness of 2.05 m 34

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Figure 16 Reversibility of El ectrowetting. Contact Angle Measurements for 1M NaCl Solution Sample having Diel ectric Thickness of 2.05 m Figure 17 Reversibility of El ectrowetting. Contact Angle Measurements for 1mM NaCl Solution Sample having Diel ectric Thickness of 2.05 m 35

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4.2.3 Polarity Dependence The electrowetting experiment is often done in two ways: applying the voltage across electrode and droplet by immersing wire electrode in liquid droplet and secondly by applying voltage across coplanar elect rode and droplet is positioned between electrodes. The polarity of elect rodes during electrowetting has been reported earlier [34, 49, 50]. Shih et al used the polarity dependency of electrowetting for pumping the liquid droplet. The authors named th e polarity dependence phenomena Asymmetric Electrowetting on Dielectric (AEWOD) . The polarity dependency of el ectrowetting has been investigated by measuring the contact angle for two possible experiment se t up in this work. First, the droplet is grounded and second, the substrate (aluminum electrode) is grounded. The contact angle is measured for different volta ge in both configurations. Figure 18 Figure 19 and Figure 20 show the contact angle measured for both above m entioned configuration for DI wate r, 1mM NaCl and 1M NaCl. The contact angle modulation for all liquids was lower when the aluminum electrode was grounded. Also the trend of contact angl e change is found to be irre gular in the aluminum grounded case as compared to the grounded droplet c onfiguration. However, the dependency of polarity found to be more in 1mM NaCl compar e to 1M NaCl liquid. This observation is inconsistent with the observed contact angle changes in earlier case. Perhaps, this irregular trend may be due to the im perfect surface morphology of substrate. 36

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Figure 18 Asymmetric Electrowetting: Polarity Dependence of Electrowetting for DI Water on a Sample having Dielectric Thickness of 1.78 m. Figure 19 Asymmetric Electrowetting: Polarity Dependence of Electrowetting for 1M NaCl Solution on a Sample having Dielectric Thickness of 2.05 m 37

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Figure 20 Asymmetric Electrowetting: Polari ty Dependence of Electrowetting for 1mM NaCl Solution on a Sample having Dielectric Thickness of 2.05 m 4.3 Electrowetting Force (EWF) Measurement 4.3.1 Predicted EWF The electrowetting force was measured using nano indenter. The theoretical model and experimental procedur es are discussed in a previous chapter. The analytical relations for EWF are expressed as equation 6. Figure 21 shows the predicted EWF for perf ect dielectric layer and floating droplet electrowetti ng configuration. Figure 22 compares the predicted EWF and m easured EWF for different liquids at various voltages. The deviat ion of measured EWF at higher voltages from the predicted EWF attributed to the saturation of electrowetting phenomena at higher voltages. 38

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Figure 21 Electrowetting Force (EWF) Prediction for Floating Drop on Coplanar Electrode and Sorted or Electrode Coated with Defect on Dielectric Layer. Figure 22 Comparison Between Measured and Predicted EWF 39

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4.4 Measured EWF Figure 23 shows the measured EWF for 1mM NaCl solution. The 75 V dc voltage w as applied for 20 seconds. The electrode, in which the droplet was positioned at 3mm offset from the center of coplan ar electrode gap, was grounded. Figure 23 Measured Electrowetting Force for 1mM NaCl Solution. 75 DC Voltage Applied during time 20 to 40 second. Ex perimental Parameters: Droplet size = 55L; the Offset Side of Electrode Grounded, Offset = 3mm (b) EWF during Voltage Applied Period. 40

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The EWF peaks at the value of 114 N for 75 V DC voltages. The force measured during voltage applied period is shown in Figure 23 (b). Interestingly, the EWF decreases during the period of voltage applied. The for ce d ecreases by 8N. The decrease in force during voltage applied period shows a decay phenomenon of the system. The change in force indicates that moveme nt of droplet towards the electrode having low voltage strongly depends on the system performance. Al so measured force s hows that at the end of voltage, the system force never reaches it s initial value. This was observed in every experiment. The residual force value for 1mM NaCl dropl et measured to be in the range of 2035N. Verheijen et al. [26] experimentally measured the contact angle of an electrowetting system and proposed that tr apped charges may account for contact angle hysteresis. The change in contact angle dur ing electrowetting syst em strongly depends on the trapped charges. The residual EWF after th e voltage applied is re lated to the trapped charges in the system. The series of experiments were carried out on different liquid to investigate the performance of electrowetting system for increasing applied voltage across the electrodes. The dc voltage pulses were increased manually. Figure 24 Figure 25 Figure 26 and Figure 27 show the performance of 1mM Na Cl 1M NaCl, DI water and 1mM Na2SO4 droplets respectively when the voltage across the electrodes was increased. The contact angle saturation phenomena in electrowetting system have been reported by many researchers [11, 23, 24, 31 ]. The contact angle saturation is also observed in contact angle measurement in th is work. The measurement of EWF at higher voltages evidently shows saturation phenomena in electrowetting system. 41

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In contact angle measurement, after sa turation voltage the c ontact angle is not changing. However, the saturation of electrow etting effect could be better explored using EWF measurement In EWF measurement meth od, the EWF increases with the voltage increase until the threshold value. After th e threshold voltage the EWF decreases with increase in voltage. The value of threshold vol tage is different for different liquids. The apparent decrease in force at higher voltages clearly indicates that performance of the electrowetting system diminishes after the threshold voltage. Figure 24 Electrowetting Force for 1 mM NaCl Solution. DC Volt age Ramped up. The Voltage Applied : 25V, 50V, 60V, 75V, 90V, 110V, 120V, 130V. 42

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Figure 25 Electrowetting Force for 1M NaCl So lution. DC Voltage Ra mped up. The Voltage Applied : 25V, 50V, 60V, 75V, 90V, 110V, 120V, 130V. Figure 26 EWF for DI Water. DC Voltage Ra mped up. The Voltage Applied : 25V, 50V, 60V, 75V, 90V, 110V, 120V, 130V. 43

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Figure 27 Electrowetting Fo rce for 1mM Na2So4 Solution. DC Voltage Ramp-up. The Voltage Applied : 25V, 50V, 60V, 75V, 90v, 110V, 120V, 130V. Additionally, the performan ce of aqueous solutions an d DI water is clearly different at higher voltages. For aqueous solutions, the EWF measurement at higher voltages shows spikes while for DI water no spikes are observed. The spiky nature of EWF indicates that electrowetting system is unstable. The possible mechanism of spiky nature in aqueous solution can be better descri bed in terms of charge injection at higher voltages. In electrowetting the change in contact angle (in this work EWF) with increasing voltage has two essential assumpti ons: first the liquid is conductor and second the dielectric layer is a perfect insulator. At higher voltages the possibility of violation of any one or both of these assumptions are fair y possible. The dielectric coating in this work is characterized for 75V dc. At highe r voltages, the leakage current in these dielectric starts which violates the perfectly insulating coating assumption of electrowetting theory. In such cases, charges can be trapped inside the dielectric layer. 44

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These trapp ed charges will decrease the effect ive voltage across the capacitor. Verhijin et al. [26] proposed a modified Young-Lipmann eq uation incorporating th e affect of trapped charges. The modified mathematical relation is: lv T ro oVV 2 )( coscos2 1 Equation 12. Modified Young-Lipman n equation by Verhijin et al. where VT is the potential drop across capacitor due to trapped charges. The decrease in voltage across the capacitor will lead to decr ease in EWF. However, the spikes observed in only aqueous solution case strongly suggest that there is a signi ficance difference in trap charge mechanism between aqueous soluti on and DI water. In aqueous solution, the available ions in the liquid can aid the trap charge amount with the leakage current charge. In counterpart, for DI water case there is no available ions to be trapped inside the dielectric layer. The greater amount of trapped charges will lead to a greatly strained dielectric matrix and system stability can be highly disoriented leading to spikes in EWF measurements. In 1999, Vallet et al. [51] studied the contact angle saturation phenomena experimentally using optical observations. Fo r the aqueous solutions, the authors observe that droplet luminesces at high voltages. They observed the light emitting at short pulses with duration of 100 ns. After correlating the light emission and current measured at the same time in system, authors found that dur ing the light emission time the current value spikes indicating the discrete discharging ev ent. For low conducting fluid, like DI water authors observed a different phenomenon at higher voltages. Authors claim that during high applied voltage, small satell ite droplets eject from the mother droplet in the case of 45

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D I water. Interestingly, these high voltage events with liquid droplet in electrowetting found to be the same as for the saturation voltage. During EWF measurements, the droplet is sandwiched between substrate and plate such that the entire plate is wetted. Given the very small dimension of plate and thickness of droplet after sandwiching, the possibi lity of small droplets is very unlikely. In EWF measurements, possibly this mechan ism of spiking current after saturation voltage is similar to spiking of EWF values at higher voltages for aqueous solution. In case of DI water, the ejection of small droplets as suggested by Vallet et al. [51] at higher voltages is not possible during EWF measurement due to very restricted space for droplet ejection. 4.4.1 Polarity Dependence of Electrowetting System The experiments are performed to investig ate the affect of voltage polarity on EWF. Figure 28 show the EWF measurements on 55L 1mM NaCl droplet. The 75V dc voltage is applied across the electrode. At the beginning of the expe riment (t=0 s), the electrode having the offset position of droplet was grounde d. At time t=10 s, 75V DC voltage pulse was applied across the electrode At t=30 s the voltage across electrode was made zero by turning off the voltage source output. 46

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Figure 28 EWF for 1mMNaCl. 75V DC Pulse was Applied. The Elect rode with Offset Droplet Position was Negative at the Beginning of the Test. During Second DC Pulse the Polarity was Reversed Making the Electrode wi th Offset Droplet as Positive and so on. Then, the polarity of the electrode was changed manually. In all experiments the polarity of voltage across electrode changes likewise. Th e EWF measurement for 1mM NaCl indicates that the magnitude of EWF during any polarity combination is almost the same. However, the EWF during the voltage ap plied varies for different polarity. In the beginning ( Figure 28 ), when the electrode having offset positioned droplet is grounded shows a small increase in EW F during the period of voltage applied. However, in the next dc pulse with opposite polarity, a st eep decrease in EWF (from 144N to 135 N) can be observed. Later, in all dc pulses EW F is decreasing during the voltage applied. The decay in EWF after first dc pulse can be attributed to the charge trapped inside the 47

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diele ctric layer. Additionally, increase in residual force with every dc pulse is due to the charge trapping. At the end of the e xperiment the residual force is ~ 35N. Figure 29 shows the EWF for DI water with change in polarity for dc pulse. D uring the first dc pulse with the electrode, EWF increases during the voltage applied period like the 1mM NaCl droplet However, the EWF observed to be almost constant for later pulses of dc voltage during voltage appl ied period. The amount of the residual force at the end of the experiment is ~ 25N. The lesser amount of residual force for DI water as compared to 1mM NaCl drop is consistent with the theory of no available ions to be trapped inside the dielectric layer in DI water, opposite to 1mM NaCl case. Figure 29 EWF for DI Water. 75V DC Pulse was Applied. The Electrode with Offset Droplet Position was Negative at the Beginning of the Test. During Second DC Pulse the Polarity was Reversed Making the Electrode with Offset Droplet as Positive and so on. Figure 30 shows the EWF measurement for 1mM Na2SO4 droplet with change in polarity in applied dc pulse. The EWF measurement for 1mM Na2SO4 shows a spread 48

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value. The EW F pick for different pulse is different. The one similarity with other liquid discussed (DI water and 1mM NaCl) is the in crease in EWF during the first dc pulse. The scattered value in the EWF may be due to the substrate surface morphology. Also, the sample for this test had more than 24hr ol d dielectric coating. The possible adsorption of moisture from lab environment ma y have caused scattered EWF. Figure 30 EWF for 1mM Na2SO4. 75V DC Pulse was Applied. The Electrode with Offset Droplet Position was Negative at the Beginning of the Test. During Second DC Pulse the Polarity was Reversed Making the Electrode wi th Offset Droplet as Positive and so on 4.5 Electrochemical Corrosion The reliability of MEMS devices and p ackaging has been researched for more than one decade. The use of thin metallic film in micro devices has been well established. The very common metal used in micro devices is aluminum. Often, a thin film of dielectric layer is coated over the metal film to avoid the electrochemical corrosion. The failure of dielectric layer, either from def ect or voltage breakdown, can initiate corrosion 49

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of metal film. The reliability of micro de vices with respect to corrosion has been researched by many [52-55]. The thin dielectric film coat ed metal electrode is genera lly used in electrowetting with aqueous solution as liquid droplet Al so the demand of low voltage electrowetting claims the use of very thin film of diel ectric layer. Given the small dimension of electrode, makes the electrowet ting system very prone to electrochemical corrosion. Recently, Nanayakkar et al [56] explored th e use of ionic fluid instead of aqueous solution in electrowetting systems. In this work, the electrochemical co rrosion has been observed during EWF measurement of electrowetting system. In el ectrowetting system the electrochemical corrosion is initiated by the failure of dielectric layer. The failure of dielectric can occur either due to voltage breakdown or exposure of defects on dielectric to electric field. Often it is a combination of factors since dielectric breakdow n typically occurs in the vicinity of defects. Figure 31 Figure 33 and Figure 38 show the EWF measurement where defects were observed after testing. In all cases 75 V dc pulse with change in polarity has been used to measure the EWF. Figure 31 show the EWF measurement for 1mM NaCl solution. W hen first 75V dc pulse is applied to the system, the measured EWF is similar to the other cases discussed. During first dc pulse, the electrode having offset droplet position was grounded. When the voltage polarity was reversed, the force sign changes which should not be observed according to equati on 4. This can be discussed in terms of electrochemical corrosion theory. When the el ectrode having offset droplet position is grounded, it behaves like an a node. When the voltage across the electrodes is applied, the 50

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droplet tends to m ove to the small area el ectrode. Droplet movement should be in the same direction also, when the voltage polarit y is changed. However, when the voltage polarity is reversed, the force sign changes indicating the change in nature of the droplet motion. This opposite nature of motion of liquid is only possible if the large area electrode has the larger voltage across it. Th is could occur if the small area electrode was degraded. In this work, aluminum has been used as the electrode material. Aluminum is a very reactive material and forms oxide very quickly once exposed to water or air. This suggests that when the voltage polarity cha nges, the electrode ( now cathode) passivates. The passivity of aluminum is due to the form ation of an oxide. The formation of passive layer has been observed after the te sting by examining the test spot. Figure 33 and Figure 38 show the case of 1mM NaCl and DI water droplet when the voltag e polarity changes similar to earlier discussed case. The spikes as in EWF measurements indicate the trapping of charges in dielectric layer leading to failure. The change in sign of the force has si milar reason as discussed above. Figure 32 and Figure 34 show the damaged spot on the substrate due to electrochem ical corrosion for EWF measurements corresponding to Figure 31 and Figure 33 respectively. 51

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Figure 31 Electrochemical Corrosion: EWF Me asurement at 75V DC Polarity Change for 1mM NaCl Droplet. Change in Sign of th e Force and Spikes in EWF Spectrum Due to Electrochemical Corrosion. Figure 32 The Damaged Surface on Substrate due to Electrochemical Corrosion for EWF Measurement Corresponding to Data from Figure 31 52

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Figure 33 Electrochemical Corrosion: EWF Me asurement at 75V DC Polarity Change for 1mM NaCl Droplet. Change in Sign of th e Force and Spikes in EWF Spectrum Due to Electrochemical Corrosion. Figure 34 The Damaged Surface on Substrat e due to Electrochemical Corrosion. The Damaged Surface on Substrate due to Electrochemical Corrosion for EWF Measurement Corresponding to Data from Figure 33 Figure 35 and Figure 36 show the scanning electron microscope (SEM) image of the dam aged spot corresponding to EWF measurement Figure 33 for two different 53

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m agnifications. Figure 37 show the Energy Dispersive Sp ectroscopy (EDS) analysis of the corroded spot. The high presence of fluorine (F) in the EDS spectrum indicates the precipitation of fluorine from CYTOP after the electrochemical corrosion. Figure 35 SEM Image of Damaged Test Spot Due to Electrochemical Corrosion. Magnification: x200 Figure 36 SEM Image of Damaged Test Spot due to Electrochemical Corrosion. Magnification: x2200 54

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Figure 37 EDS Analysis of Damaged Test Spot. Figure 38 Electrochemical Corrosion: EWF Me asurement at 75V DC Polarity Change for DI Water. Change in Sign of the Force and Spikes in EWF Spectrum due to Electrochemical Corrosion The defects observed due to electrochemical corrosion are circular in shape, like doughnuts. When the dielectric layer fails, the el ectrons diffuse in the dielectric layer. The diffusion of electrons creates path for the flow of water molecule and oxygen which 55

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for ms oxide as soon as it comes in contact with aluminum film. The aluminum layer degrades by electrochemical reaction and possibly creates the pressure beneath the dielectric (CYTOP) layer. Later, the delimitation of dielectric layer exposes the aluminum thin film and corrosion starts at the dielectric/al uminum interface. Figure 39 The Corrosion of Al Film and Delimitation of CYTOP 56

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CHAPTER 5 ENVIRONMENTAL EXPOSURE OF THE ELECTROWETTING SYSTEM 5.1 Introduction In this chapter, the effect of environm ental exposure on electrowetting system is studied. The changes in contact angle on a substrate with time immersed in liquid substrate have been carried out. In the second part of this chapter, the electrochemical impedance spectroscopy results are presented. 5.2 Contact Angle Measurement with Environmental Exposure Electrowetting actuated devices are often exposed to the working liquid or other ambient environments. The performance of th e electrowetting could degrade due to the environmental exposures. Electrowetting experi ments are being conducted in air or liquid medium. Different types of Oil are the only medium reported so far as liquid medium. The use of olive oil and Fomblin vacuum oil is being reported in recent years [22, 23, 29], which reduces the cont act angle hysteresis. While air is often used in place of a oil as the environmental medium, the samples are also repeatedly exposed to aqueous drops that are manipulated via electrowetting. These drops often contain concentrations of various salts to improve conductivity. This work considers changes due to exposure to the aqueous fluids that are present in virtually all applications. 57

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The substrates were imm ersed in a test fluid and removed at varying intervals of time for testing. After testing samples were immersed for additional exposure. The test substrate used in this experiment was proces sed similar to the contact angle measurement testing. Three types of immersion liquid we re tested, DI water, 1mM NaCl and 1mM Na2 SO4. The substrates were immersed in all liquids such that only the di electric layer coated was in touch with the liquid. The test fluid is prevented from contacting the dielectricfree zones with exposed aluminum film ar ea to overcome the possible corrosion of the bare metal. The contact angles were measured at regular intervals with voltage change to observe the effect of immersi on on electrowetting performance. Figure 40 Figure 41 and Figure 42 show the contact angle measurement for 1mM NaCl, 1mMNa2 SO4 and DI water respectively. The contact angle for the substrate imme rsed in 1mM NaCl shows a significant change when the immersion time is 30 min. The contact angle at 75V for immersion time t=0 and t=30 min is almost the same as shown in Figure 40 but the difference in trend can be easily observed. The contact angl e changes significantly after 2 hours of im mersion time. Interestingly, the contact angle at zero volta ge also decreases indicating the change in dielectric layer composition. Th e magnitude of change in the contact angle with a given applied voltage decreases as the immersion time increases. The same observation is made in the case of 1mM Na2SO4 ( Figure 41 ) also. However, the change in contact angle with DI water as imm ersion liquid is minimal ( Figure 42 ). In the literature, th e change in contact angle over time has been reported previously [57-61]. Lee et al. [ 57] reported the change in cont act angle of different type 58

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Figure 40 Change in Contact Angl e for 1mM NaCl Droplet over Time of liquids, polar, nonpolar liquids etc. on the fluropolymer over time Authors claim that the decrease in contact angle over time is due to the interaction of ions with the polymer over time. Li et al [59] reported the change in contact angle of water droplet with time on the poly vinyl (PVC) membrane. Authors claimed that the penetration of water inside the membrane changes the membrane properties in terms of wettability. Interestingly, the change in contact angle is more for the firs t 2 hours and then become slower. The cross linking between ions in polymer after pene tration changes the polymers surface energy [59-61]. Wang et al. [58] experimentally meas ured the contact angle of water on polymer by sessile drop method. The dr oplet was kept on surface and contact angle was measured at regular time intervals. The time-dependence in contact angle measurement was mainly attributed to the surface reconstruction wh en water drops were deposited on polymer surfaces. The starting contact angle was contributed by the hydrophobic component on 59

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polym er surface and the equilibrium contact angle mainly by the hydrophilic component of polymer. The change in contact angle with liqui d immersion for CYTOP has not been reported earlier. However, the mechanism of change in contact angle for other organic polymers will possibly be the same as the chan ge in contact angle of CYTOP in this work. Figure 41 Change in Contact Angle for 1mM Na2SO4 with Immersion Time 60

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Figure 42 Change in Contact Angl e for DI Water with Immersion Time The change in contact angle is higher for 1mM NaCl as compared to the 1mM Na2SO4. The mechanism of the interaction of i ons present in liquid and polymer can be attributed to the change in contact angle over time. Possibly the diffusion of ions into CYTOP layer is responsible for the cha nge in contact angle with time. Additionally, diffusion of ions in the dielectric layer is also accountable for the poor electrowetting performance over time. Figure 43 show the time constant of contact angle decay for different liquid .The time c onstant for DI water, 1mM Na2SO4 and 1mM NaCl is found to be 0.20 hours, 0.90 hours and 1.20 hours. 61

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Figure 43 Time Constant of Steady Decay in Electrowetting Response due to Liquid Exposure Figure 44 compares the contact angle modulati on for a voltage change of 75 V for all three types of liquids. Cl early, the decrease in cont act angle modulation for 1mM NaCl liquid is significant (from 34 to 20 degrees over 24 hrs). 62

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Figure 44 The Chan ge in Contact Angle Modulation for Different Fluids with Immersion Time. 5.3 Electrochemical Impedance Spectroscopy The electrochemical impedance measuremen t is carried out to investigate the change in capacitance of dielectric layer (CYTOP) over time. Experiments were performed on the three different samples fabricated by the method discussed in chapter 3. The experimental parameters were kept iden tical for all three different samples. The experimental specifications were: maximu m frequency of 100 KHz minimum frequency of 1 Hz and 1 mV rms AC voltage. The electrol yte used in all experiment runs were 1mM NaCl solution. The experiment was perfor med for 4 hours to observe the change in capacitance. The data were collected over diffe rent periods of time after the electrolyte was poured into the cell. The sample sp ecifications are listed in Table 1. 63

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Table 1 Sample Specifications Sample Number Thickness (m) 1 1.93 2 1.68 3 1.73 The percentage increase in the capacitance of the CYTOP thin film with immersion time is plotted in the Figure 45 Figure 45 Percentage Increase in Capacitance with Exposure Time The maximum increase (~5.40%) in the ca pacitance with immersion time of 4 hours was observed for the sample 2 with resp ect to the initial capacitance. The minimum 64

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incre ase (1.77%) was observed for the sample 3. The marginal difference in enhancement of the capacitance between samples is possibly attributed to the moisture observation from lab air prior to experiment. Figure 46 show the increase in capacitanc e with water adsorption. The time constant for water diffusion is calculated and found to be 0.75 hour. The similar time constant value of contact angle decay ( Figure 43 ) for 1mM NaCl indicates the same m echanism of water adsorption with time. Figure 46 Time Constant of Water Mo lecule Diffusion in Dielectric Layer The change in the capacitance with time is believed to be due to the diffusion of water molecules into the CYTOP film. Th e characteristic time (in equation 12) is found by fitting the data to a first order expone ntial response. From the time constant, the diffusivity constant of electrolyte for each sample is calculated. Based on the capacitance 65

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change, the diffusivity constant of electrolyte in CYTOP can be estimated from equation 11. The diffusivity coefficient fo r the electrolyte were ranged from 1.37 x 10-11 cm2/s to 2.1 x 10-12 cm2/s. Literature report on the diffusivity coefficient for the CYTOP film is not available. However, an other fluorocarbon polymer of similar nature (Teflon) has been studied before for the diffusion of aqueous NaCl solution [62]. The diffusivity constant is reporte d to be in the range of 10-11 cm2/s to 10-12 cm2/sec [62]. Also calculations were made for the di electric constant of the CYTOP film and water absorption based on the EIS data obtained. For calculation of dielectric constant actual area of dielectric layer (= 4.9cm2) was used. These dielectric constant values were calculated after the first set of data are observed (immersion time t= 2min 35sec). The calculate values are listed in Table 2. Table 2 Dielectric Constant and Water Uptake for Samples Sample Number Dielectric Constant Calc ulated Water uptake (percentage volume) 1 1.51 0.10 2 1.52 0.11 3 1.53 0.035 The graphical comparison of samples with respect to increase in capacitance with immersion time, dielectric constant calc ulated and water uptake is shown in Figure 47 66

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Figure 47 Comparison of Different Samples with Respect to Capacitance after the First Data Collected of the Experiment (t =155 seconds); Percentage Incre ase in Capacitance at 4hrs and Dielectric Constant Calculated at t=155 seconds. The capacitance of the CYTO P film in contact with 0.1mM NaCl electrolyte was found to increase moderate ly after 4 hours of immersi on. The increase ranged from 1.77% to 5.4% over the initial value. The in itial value of the capacitance (~3.5 nF) was approximately consistent with th e surface area of the sample (~5 cm2), the thickness of the coating (~1.8 m) and the expected value of the dielectric constant (~2.1 based on values reported in the literature [63]).The va lue of the dielectric constant of CYTOP calculated from the EIS data wa s on average 1.52, nearly the same for all samples (range: 1.51 to 1.53). The calculated percentage water uptake in the CYTOP layer for different samples ranged from 0.03% to 0.11% after 4 hours of immersion time. Water adsorption by the organic coating can degrade the electr owetting system performance substantially. More experiments are needed to have insightful knowledge of the corrosion in microfludic devices. These initial EIS tests were conducted successfully and suggest that 67

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im pedance measurements can be a powerful te chnique to investigat e the performance of electrowetting actuated microfluidic device s where CYTOP films are used as the dielectric layer. 68

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69 CHAPTER 6 CONCLUSIONS AND FUTURE WORK 6.1 Conclusions The electrowetting process is proved to be a powerful method for the tuning of surface energy in accordance with electrical energy. The electrowetting system performance depends on the li quid type, voltage, environmenta l conditions and quality of the dielectric layer. These parameters of interest have been investigated by various experimental techniques. However, the conventional contact angle measurement technique does not reveal the change in system performance spontan eously. The contact a ngle measurement has been done for different liquids. The contact angle measurements exhibited the polarity dependence of electrowetting. The performance of the electrowetting sy stem has been investigated by direct measurement of the electrowetting force. A lumped system model has been developed and the electrowetting force for different diel ectric layers and droplet s was predicted. The change in capillary force due to electrical energy was directly measured by using a nanoindenter. The EWF measurements have been done for different liquids and the performance has been discussed. At higher voltages, the charge is being trapped in dielectric layer and performance degrades significantly. The degrad ation in performance at higher voltages is more in salt solution as compared to DI water. The electrowetting

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70 system performance for the case when the diel ectric layer is defected was well captured by EWF measurements. The environmental exposure of the electr owetting system has been investigated by measuring the contact angle over time and electrochemical impedance spectroscopy. The change in the dielectric layer with water adsorption degrades the system performance. The contact angle modulation changes by as high as 45% for continuous liquid exposure period of 24 hours .The ch ange in dielectric constant with liquid immersion has been measured by impedan ce spectroscopy. The dielectric constant changes due to water absorption in dielectric layer and highest change observed is 5.3% for immersion time of 4 hours. Initial resu lts conclude that the electrochemical impedance spectroscopy can be used to study the electrochemical corrosion in electrowetting system for microfluidic applications. 6.2 Future Work The various experimental t echniques should be extended to investigate the overall optimization of electrowetting system. The future work includes: The EWF measurements more AC voltage to investigate the electrowetting system performance. The parameters of interest for experiments will be various rms AC voltages and frequencies. The EWF measurements for substrate immersed in liquid solutions. The EWF data will be related to the contact angle and electrochemical impedance spectroscopy measurements for a better understanding of electrochemical corrosion in electrowetting systems.

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71 The use of different metal elect rodes other than aluminum in electrowetting system. The use of noble metal such as gold can potentially reduce the corrosion in electrowetting system. The use of different dielectric f ilms other than CYTOP in electrowetting process. The Teflon a nd Parelyne will be used as the dielectric layer and performance of the electrowetting system will be investigated. The more detailed experiment us ing electrochemical impedance spectroscopy to investigate the electrochemical corrosion in electrowetting system.

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73 [10] Matsumoto, H., and Colgate, J.E ., 1990, "Preliminary investigation of micropumping based on electrical control of interfacial tensi on," Proceedings. IEEE Micro Electro Mechanical Systems., Napa Valley, CA, USA, pp. 105. [11] Mugele, F., Baret, J. C., and St einhauser, D., 2006, "Microfluidic Mixing through Electrowetting-Induced Droplet Oscillations," Applied Physics Letters, 88(20) pp. 204106. [12] Chen, J. Y., Kutana, A., Collier, C. P., 2005, "Electrowetting in Carbon Nanotubes," Science, 310(5753) pp. 1480. [13] Berge, B., and Peseux, J., 2000, "Variable Focal Lens Controlled by an External Voltage: An Application of Electrowe tting," The European Physical Journal.E, Soft Matter, 3(2) pp. 159. [14] Krupenkin, T., Yang, S., and Mach, P., 2003, "Tunable Liquid Microlens," Applied Physics Lette rs, 82(3) pp. 316. [15] Kuiper, S., and Hendriks, B. H. W., 2004, "Variable-Focus Liquid Lens for Miniature Cameras," Applied P hysics Letters, 85(7) pp. 1128. [16] Hayes, R. A., and Feenstra, B. J., 2003, "Video-Speed Electronic Paper Based on Electrowetting," Nature, 425(6956) pp. 383. [17] Heikenfeld, J. C., Smith, N. R., S un, B., 2008, "Flat Electrowetting Optics and Displays," Proceedings of SPIE--the Inte rnational Society for Optical Engineering, 6887pp. 688705. [18] Heikenfeld, J., 2008, "Flat Electrow etting Optics and Displays," Proc. SPIE Int. Soc. Opt. Eng., pp.6887. [19] Roques Carmes, 2004, "Liquid Beha vior Inside a Refl ective Display Pixel Based on Electrowetting," Journal of Applied Physics, 95(8) pp. 4389. [20] Beni, G., 1981, "Dynamics of Electro wetting Displays," Journal of Applied Physics, 52(10) pp. 6011. [21] Cooney, C. G., 2006, "Electrowe tting Droplet Microfluidics on a Single Planar Surface," Microfluidics a nd Nanofluidics, 2(5) pp. 435. [22] Kuo, J. S., 2003, "Electrowetting-Induced Droplet Movement in an Immiscible Medium," Langmuir, 19(2) pp. 250. [23] Mugele, F., 2005, "Electrowetting: From Basics to Applica tions," Journal of Physics. Condensed Matter, 17(28) pp. R705.

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74 [24] Quinn, A., 2005, "Contact Angle Satu ration in Electrowetting," Journal of Physical Chemistry B, 109(13) pp. 6268. [25] Seyrat, E., 2001, "Amo rphous Fluoropolymers as Insulators for Reversible Low-Voltage Electrowetting," Journal of Applied Physics, 90(3) pp. 1383. [26] Verheijen, H. J. J., 1999, "Rev ersible Electrowetti ng and Trapping of Charge: Model and Experiments," Langmuir, 15(20) pp. 6616. [27] Yeo, L. Y., 2006, "Electrowetti ng Films on Parallel Line Electrodes," Physical Review. E, Statisti cal, Nonlinear, and Soft Matter Physics, 73(1) pp. 11605. [28] Yuejun, Z., 2007, "Micro Air Bubbl e Manipulation by Electrowetting on Dielectric (EWOD): Transporting, Splitting, Merging and Eliminating of Bubbles," Lab on a Chip, 7(2) pp. 273. [29] Zhiliang, W., 2007, "Reversible Elec trowetting of Liquid-Metal Droplet," Journal of Fluids Engineering, 129(4) pp. 388. [30] Verheijen, H. J. J., 1999, "Contact Angles and Wetting Velocity Measured Electrically," Review of Scientif ic Instruments, 70(9) pp. 3668. [31] Schaffer, E., and Po-Zen Wong, 1998, "Dynamics of Contact Line Pinning in Capillary Rise and Fall," Physi cal Review Letters, 80(14) pp. 3069. [32] Peykov, V., 2000, "Electrowetting: A Model for Contact-Angle Saturation," Colloid Polymer Science, 278(8) pp. 789. [33] Baird, E., 2007, "Comparison of Electrowetting on Dielectric and Dielectrophoresis Force Dist ributions," AIAA Paper, [34] Seyrat, E., and Hayes, R. A ., 2001, "Amorphous Fluoropolymers as Insulators for Reversible Low-Voltage Electro wetting," Journal of Applied Physics, 90(3) pp. 1383. [35] Bahadur, V., 2007, "Electrowetting-Based Control of Static Droplet States on Rough Surfaces," Langmuir, 23(9) pp. 4918. [36] Moon, H., 2002, "Low Voltage Elect rowetting-on-Dielectric," Journal of Applied Physics, 92(7) pp. 4080. [37] Cahill, B. P., Giannitsis, A. T., Land, R., 2008, "Optimization of Electrowetting Electrodes: Analysis of the Leakage Current Characteristics of various Dielectric Layers," pp. 79. [38] Raj, B., Smith, N. R., Christy, L., 2008, "Composite Dielectrics and Surfactants for Low Voltage Electrowetting Devices," pp. 187.

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75 [39] Banpurkar, A. G., Duits, M. H. G ., Van Den Ende, D., 2009, "Electrowetting of Complex Fluids: Perspec tives for Rheometry on Chip," Langmuir, 25(2) pp. 1245. [40] Baviere, R., 2008, "Dynamics of Droplet Trans port Induced by Electrowetting Actuation," Microfluidic s and Nanofluidics, 4(4) pp. 287. [41] Walker, S. W., and Shapiro, B ., 2006, "Modeling the Fluid Dynamics of Electrowetting on Dielectric (E WOD)," Journal of Microelectromechanical Systems, 15(4) pp. 986-1000. [42] Berthier, J., Dubois, P., Clementz, P., 2007, "Actuation Potentials and Capillary Forces in Electrowetting Based Microsystems," Sensors and Actuators, A: Physical, 134(2) pp. 471-479. [43] L.L Shreir R.A.Jarman G.T. Burtein, 1998, "Corrosion Volumee 2," Butterworth Heinemann, Oxford, [44] Craig R. Barrett, William D.Nix,Alan S.Tetelman, 1973, "The Principles of Engineering Materials," Pr entice Hall, New Jersey, [45] Sags, A. A., Wolan, J. T., Fex, A. D., 2006, "Impedance Behavior of Nanoporous SiC," Electrochimica Acta, pp. 1656. [46] Adamiak, K., 2006, "Capillary and El ectrostatic Limitations to the Contact Angle in Electrowetting-on-Diel ectric," Microfluidics and Nanofluidics, 2(6) pp. 471. [47] Drygiannakis, A., 2009, "On the C onnection between Dielectric Breakdown Strength, Trapping of Charge, and Contact Angle Saturation in Electrowetting," Langmuir, 25(1) pp. 147. [48] Berry, S., 2007, "Irreversible Elect rowetting on Thin Fluoropolymer Films," Langmuir, 23(24) pp. 12429. [49] Shih-Kang, F., 2007, "Asymmet ric Electrowetting-Moving Droplets by a Square Wave," Lab on a Chip, 7(10) pp. 1330. [50] Wang, T., 2006, "Droplets Osc illation and Continuous Pumping by Asymmetric Electrowetting," 2006pp. 174. [51] Vallet, M., Vallade, M., and Berge, B., 1999, "Limiting Phenomena for the Spreading of Water on Polymer Films by Electrowetting," The European Physical Journal.B, 11(4) pp. 583.

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76 [52] Shea, H. R., Gasparyan, A., Ho Bun Chan, 2004, "Eff ects of Electrical Leakage Currents on MEMS Reliability and Performance," IEEE Transactions on Device and Materials Reliability, 4(2) pp. 198. [53] Yang, P., and Chern, J., 1993, "Des ign for Reliability: The Major Challenge for VLSI," Proceedings of the IEEE, 81(5) pp. 730. [54] Herrmann, H. J., 1990, "Fracture Pa tterns and Scaling Laws," Physica A, 163(1) pp. 359. [55] Frankel, G. S., Russak, M. A., Ja hnes, C. V., 1989, "Pitting of Sputtered Aluminum Alloy Thin Films," Journal of the Electrochemical Society, 136(4) pp. 1243. [56] Nanayakkara, Y., 2008, "A Funda mental Study on Electrowetting by Traditional and Multifunctional Ionic Li quids: Possible use in Electrowetting on Dielectric-Based Microfluidic Applications ," Analytical Chemistry, 80(20) pp. 7690. [57] Lee, S., 2008, "The Wettability of Fluoropolymer Surfaces: Influence of Surface Dipoles," Langmuir, 24(9) pp. 4817. [58] Wang, X., 2005, "Dynamic Behavior of Polymer Surface and the Time Dependence of Contact Angle," Science in Ch ina Series B: Chemistry, 48(6) pp. 553. [59] Li, G., 2007, "Time-Dependence of Pervaporation Performance for the Separation of ethanol/water Mixtures through Poly(Vinyl Alcohol) Membrane," Journal of Colloid and Interface Science, 306(2) pp. 337. [60] Carey, D., 2000, "Entropically In fluenced Reconstruction at the PBDox/water Interface: The Role of Physi cal Cross-Linking and Rubber Elasticity," Macromolecules, 33(23) pp. 8802. [61] Berglin, M., 2003, "Fouling-Release Coatings Prep ared from Alpha Omega -Dihydroxypoly(Dimethylsilo xane) Cross-Linked with (Heptadecafluoro-1,1,2,2Tetrahydrodecyl)Triethoxysilane," Journal of Colloid and Interface Science, 257(2) pp. 383. [62] A.L lordanski. A.L. Shterezon, Yu. V. Moissev and G.E Zakiov, 1979, "Diffusion of Electrolyte Sin Polymer," Ru ssian Chemical Reviews, 8(48) pp. 781. [63] http://www.agc.co.jp/english/chem icals/shin sei/cytop/cytop.htm


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Text (Electronic thesis) in PDF format.
3 520
ABSTRACT: Electrowetting is the change in apparent surface energy in the presence of an electric field. Recently, this phenomenon has been used to control the shape and location of individual droplets on a surface. However, many microfluidics researchers have acknowledged unexplained behaviors and performance degradation. In this work, electrowetting systems are characterized with different methods. The electrowetting response is measured by measuring contact angle for different applied voltages. A novel technique for direct measurement of Electrowetting Force (EWF) using nano indenter is proposed in this work. The EWF measurements show that, for aqueous solution the EWF is more as compared to DI water. Additionally, the electrowetting system is found to be more susceptible for degradation when aqueous solution is used. The performance degradation due to defective dielectric layer is also investigated by measuring the electrowetting force. Degradation of EWOD systems with environmental exposure over time is further studied experimentally by contact angle and electrochemical impedance spectroscopy (EIS) measurements. The time constant of 'contact angle decay' with environmental exposure is found to be similar to the time constant of electrolyte diffusion in dielectric layer.
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Mode of access: World Wide Web.
System requirements: World Wide Web browser and PDF reader.
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Advisor: Nathan B. Crane Ph.D
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Contact Angle
Corrosion
Impedance
Reliability
CYTOP
690
Dissertations, Academic
z USF
x Mechanical Engineering
Masters.
773
t USF Electronic Theses and Dissertations.
4 856
u http://digital.lib.usf.edu/?e14.3074