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UNIVERSITY OF CINCINNATI

Date:

I, ,

hereby submit this original work as part of the requirements for the degree of:

in

It is entitled:

Student Signature:

This work and its defense approved by:

Committee Chair:

11/18/2009 321

29-Oct-2009

Balaji Raj

Master of Science

Electrical Engineering

Advanced Aqueous Solutions for Low Voltage and Electrolysis-Free

Electrowetting Devices

Jason Heikenfeld, PhD

Punit Boolchand, PhD

Ian Papautsky, PhD

Jason Heikenfeld PhD

Punit Boolchand PhD

Ian Pa autsk PhD

Bala i Ra

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Advanced Aqueous Solutions for Low Voltage and

Electrolysis-Free Electrowetting Devices

A thesis submitted toThe Division of Research and Advanced Studies

of theUniversity of Cincinnati

in partial fulfillment of requirement for the degree of

MASTER OF SCIENCE

In theDepartment of Electrical and Computer Engineering

of the College of EngineeringOctober 2009

By

Balaji Raj

B.S. in Computer Engineering,University of Cincinnati

Committee Chair - Dr. Jason Heikenfeld

Committee Members - Dr. Boolchand and Dr. Papautsky

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ABSTRACT

Electrowetting has come a long way since the concept was first introduced by

Gabriel Lippmann in 1875. Since the emergence of Bruno Berges concept of

electrowetting on dielectrics in the 1990s, electrowetting has seen an explosion of

interest. Recent development in electrowetting applications has led to the need for low

operating voltages for many applications. Besides a low voltage requirement, larger

contact angle modulation, reliable, and electrolysis free electrowetting are important for

all electrowetting applications.

The basis of this thesis is to compare and introduce ways to attain low voltage

electrowetting on a two-layer dielectric system. Two dielectrics are investigated, Al2O3

and Si3N4. Surfactants are then tested in the form of sodium dodecyl sulfate (water

soluble) and triton X-15 (oil soluble) to observe how the interfacial surface tension

affects electrowetting effect. Once low voltage electrowetting is observed, the

investigation of electrolysis free and reliable electrowetting is discussed. Various

surfactants are tested for dielectric failure and a catanionic surfactant is synthesized.

The catanionic surfactant is also tested for dielectric failure. Once those tests are

performed, water is replaced as a medium for electrowetting by propylene glycol. With

larger molecules for propylene glycol, dielectric failure is substantially reduced.

The results of the work described herein show the promise for low voltage andreliable electrowetting.

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ACKNOWLEDGEMENT

I wish to express gratitude towards my academic advisor, Dr. Jason Heikenfeld,

for having the confidence in me as a student in the Novel Device Laboratory. His

continued effort to guide, support, and encourage my research endeavors here at the

University of Cincinnati pushed me towards new challenges, and multidisciplinary

understanding of new concepts, and techniques associated with new technologies have

pushed me through to attain this thesis.

I would like to thank the members of my lab for their continuous support,

creativity, advice, and assistance during my thesis work.

In addition to this, I would like to express my gratitude to Dr. Punit Boolchand and

Dr. Ian Papautsky for taking the time to be part of my thesis committee.

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i

TABLE OF CONTENTS

CHAPTER 1: INTRODUCTION.......................1

1.2 Background related to this Thesis...3

1.3 In This Thesis............3

CHAPTER 2: ELECTROWETTING BASICS AND THEORY ...5

2.1 Contact angle on planar surfaces...........5

2.1.1 Interfacial Surface Tension5

2.1.2 Liquid Droplet on planar surface in air ..........6

2.1.3 Liquid Droplet on planar surface in oil ambient ...............8

2.2 Electrowetting on planar surfaces...9

2.2.1 In Air or Oil ...........................9

2.3 Limitations in electrowetting...10

2.4 Low voltage electrowetting .......12

2.5 Review of past work in Low Voltage Electrowetting .....14

CHAPTER 3: LOW VOLTAGE ELECTROWETTING DEVICES .................18

3.1 Introduction...................................18

3.2 Fabrication.................................20

3.3 Experimental results....22

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CHAPTER 4: ION AND LIQUID DEPENDENT DIELECTRIC FAILURE IN

ELECTROWETTING SYSTEMS..27

4.1 Introduction...................................27

4.2 Background and experimental results .......................................................28

4.3 Choice, Preparation and Synthesis of Test Liquids...32

4.4 Dielectric Fabrication and Preparation 35

4.5 Experimental Setup for EW and Dielectric Failure Tests .36

4.6 Results...37

4.7 Discussion and Conclusion49

CHAPTER 5: DISCUSSION AND FUTURE WORK.51

5.1 Summary of this work .51

5.2 Achievement in the context of others work..52

5.3 Future work...53

REFERENCES55

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iii

LIST OF FIGURES

Figure Page

1 Figure 1.1 Electrowetting applications that are being developed at

the (a) University of Cincinnati [12], (b) Philips Research [13], (c)

University of California Los Angeles [14], and (d) Duke University

[5].

2

2 Figure 2.1 An aqueous droplet on a planar surface in air with

Youngs angle.

7

3 Figure 2.2 An aqueous droplet on a planar surface in silicone

oil with Youngs angle of 180.

9

4 Figure 2.3 Microscopic view of droplet showing three phase

state during electrowetting.

10

5 Figure 2.4 Common limitations in electrowetting (a) dielectric

charging (b) oil charging and (c) electrolysis [17].

11

6 Figure 2.5 Electrowetting responses for varying wt. % of SDS

in 0.1 M NaCl solution [21].

16

7 Figure 3.1 Diagrams and photographs of example

electrowetting optical devices that can be implemented in

arrayed format on glass substrates [23].

19

8 Figure 3.2 (a,b) Diagrams and (c) photographs of basic

electrowetting droplet characterization in an oil ambient [23].

20

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iv

9 Figure 3.3 Electrowetting data on samples processed with

varied CYTOP 809M wt. % in the spin coated fluorosolvent

solution. The final approximate thickness, based on wt. % of the

CYTOP 809M in the spin coated solution, is also labeled on the

plot. Two samples were tested for each curve and the results

averaged [23]. 1 wt. % sodium dodecyl sulfate (SDS) in water

was used as the surfactant for this approach.

23

10 Figure 3.4 Diagrams of dielectric charging and its effect on

contact angle saturation [23].

24

11 Figure 3.5 Electrowetting data with ~50 nm CYTOP 809M

deposited on 150 nm Si3N4 films. All experiments utilized 0.1

wt. % Triton X-15 in the dodecane. AC vs. DC and the addition

of KCl vs. SDS (sodium dodecyl sulfate) were also included in

the experiments. Two samples were tested for each curve and

the results averaged [23].

25

12 Figure 4.1(a) Initial state of a sessile droplet in oil ambient. (b)

Proper charge buildup and electromechanical alteration of the

wetting response. (c, d, e) Non-ideal behavior due to dielectric

charging, oil charging, or electrolysis at dielectric defects [17].

30

13Figure 4.2 Conductivity (S/cm) and interfacial surface tension

(IFT, mN/m) of aqueous DTA-OS catanionic surfactant solution

vs concentration (wt %). The cmc value for DTA-OS was

determined to be ~0.065 wt% [17].

34

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14 Figure 4.3(a) Plot of contact angle vs. electrowetting voltage

for conventional surfactant solutions; the wt% used are above

the cmc points [17].

38

15 Figure 4.3(b) Electrowetting (EW) contact angle response for

several wt% of catanionic surfactant (DTA-OS) solution [17].

39

16 Figure 4.4 Dielectric failure current vs. voltage for various

aqueous solutions on a fluoropolymer/Al2O3 dielectric stack.

Every test, and individual positive and negative polarity voltage

sweeps, were measured at independent dielectric sites. The

data are presented without any averaging, allowing one to

appreciate that the data are fairly repeatable even for dielectric

failure tests [17].

41-43

17 Figure 4.5 Dielectric failure current vs voltage for(a) pure

propylene glycol and (b) propylene glycol with 1 wt. % SDS. For

comparison with (b) a water:SDS solution of similar conductivity

is plotted in (c) [17].

46-47

18 Figure 4.6 Measured contact angle relaxation (charge injection)

vs. time for several aqueous solutions biased at 28 V. Voltage

was applied at ~0.5 s, and the contact angles captured with a

VCA Optima system [17].

49

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vi

LIST OF TABLES

Table Page

1 Table 4.1. Table of surfactants utilized herein. 32

2 Table 4.2. Conductivity measurements for various liquids tested. 44

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1

CHAPTER 1

INTRODUCTION

Electrowetting traces its origins back to 1875 through the work of Gabriel

Lippmann [1]. Lippmann introduced the theory of electrocapillarity [1] from which

electrowetting has come to be. In his works, Lippmann found that capillary depression

of mercury can be varied by the application of a voltage between the mercury and

electrolyte solution (where the mercury was in contact with the electrolyte solution). For

practical applications, one major obstacle was found; the electrolytic decomposition of

water after the application of voltage in the range of only 10-3

volts [2]. In the early

1990s, however, B. Berge introduced the concept of using a thin insulating layer to

separate a conductive liquid from the metal electrode [3]. The purpose of this was to

eliminate the causes of electrolysis. This concept is now widely accepted or known as

electrowetting on a dielectric (EWOD) [3], and has caused enormous interest in

electrowetting devices.

Today, electrowetting [2] is the common term used to describe the

electromechanical [4] reduction of a liquid contact angle on a hydrophobic

dielectric/electrode substrate. A reversible contact angle change as much as >100 can

be achieved in oil ambient. Fast contact line velocities (>10 cm/s) and low electrical

energy (~1s to 10s mJ/m2) are also typical. Few other micro or electrofluidic

technologies can match this performance. There has been a global effort to use

electrowetting for applications such as lab-on-chip [5, 6, 7], optics [8, 9], and displays

[10, 11], to name a few. Some of these applications have also been developed here at

the University of Cincinnati, Philips Research Eindhoven, University of California Los

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Angeles, and Duke University (Figure 1.1). The creation of new device platforms has

outpaced the development of improved electrowetting materials. This thesis will attempt

to provide new insight and improved performance for electrowetting solutions and

dielectrics. In particular, it is a goal of this thesis to show improved low-voltage

electrowetting.

Figure 1.1 Electrowetting applications that are being developed at the (a) University of

Cincinnati [12], (b) Philips Research [13], (c) University of California Los Angeles [14], and (d)Duke University [5].

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1.2 Background related to this Thesis

A brief overview will be made on some recently published research on low

voltage electrowetting. It will be discussed in more detail in chapter 2. Low voltage

electrowetting has been achieved, as published by Berry et al. and Moon et al. Berry et

al. showed voltages as low as ~5 V with very thin dielectric. However, the materials they

use may not be feasible for commercial devices that are fabricated on glass or plastic.

Moon et al. also achieved low voltage electrowetting, ~15 V, but the electrowetting

response was marginal. Most importantly, neither group investigated the important topic

of reliability and degradation.

1.3 In this thesis

Low voltage electrowetting is desirable for virtually all applications including lab-

on-chip [5,6,7], optics [8,9], and displays [10,11]. Most electrowetting devices are

arrayed which requires the need of thin-film-transistors [15] driven at < 15 V. In order to

achieve this, high capacitance will be required which would reduce the overall

electrowetting operating voltage. Therefore, herein dielectric materials and hydrophobic

fluoropolymer surfaces will be investigated to obtain high capacitance by making dual-

layered dielectric on a substrate. Keeping the requirements of high capacitance in mind,

variables such as thickness of the dielectric, relative permittivity of the dielectric, and

material of the electrode will be looked at. Surfactants will also be explored to reduce

interfacial surface tension between the aqueous/oil interface which can further reduce

the required electrowetting voltage.

However, it is also known that some surfactants in solution can pose a problem

during the electrowetting operation. The most common problems associated with

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electrowetting are dielectric charging, oil charging and electrolysis. Some

fluoropolymers have pores in the nano scale and possibly the micro scale. Ions from

these surfactants may be small enough to form a liquid wire through the dielectric layers

causing dielectric failure in the material. This would halt or reduce the electrowetting

effect. Therefore, also explored herein is the type of surfactant that can be used such

that low voltage electrowetting is achieved without dielectric failure. Observed in the

latter chapters, polar liquids that contain ions that are physically larger in size than other

counter ions show trends of a less likely onset of dielectric failure. Even though the work

presented herein is not entirely comprehensive, it does provide clear guidance on howcarefully selecting various liquids and ionic content leads to improved electrowetting

performance.

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CHAPTER 2

ELECTROWETTING BASICS AND THEORY

This chapter will give a better understanding of electrowetting theory. Contact

angle on planar surfaces will be described first followed by electrowetting on planar

surfaces.

2.1 Contact angle on planar surfaces

First, the interfacial surface tensions of an aqueous liquid droplet on a planar

substrate will be discussed. This droplet will be placed in an air ambient and a non-polar

liquid ambient and the corresponding equilibrium state of the interfacial surface tension

at the 3 phase contact line will be discussed.

2.1.1 Interfacial Surface Tension

Interfacial surface tension is the basis for contact angle of a liquid droplet on any

given surface whether it is planar or structured, or hydrophobic or hydrophilic. By law of

thermodynamics, when a system is in equilibrium it tries to attain the lowest energy

state. For instance, when a liquid droplet is placed on a surface the droplet attains a

form of semi-spherical shape. This is due to interfacial surface tension forces acting on

it. Interfacial surface tension plays an important role in determining the shape of a

droplet due to the various molecular interactions that take place at the interface. Liquids

with a high surface tension tend to exhibit a stronger spherical shape, like water for

instance, which has a high surface tension ~75 mN/m.

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Surface tension can be expressed as follows [16]

A

W

(1)

which is the work done per unit increase in surface area where (N/m) is surface

tension, W is work done or change in energy and A is change in surface area. In this

thesis, some liquids will contain salts which do not change the surface tension of the

water. However, with the addition of surfactants, surface tension of the liquids can be

decreased by as much as an order of magnitude.

2.1.2 Liquid Droplet on planar surface in air

Most polar liquids, such as water, exhibit a hydrophobic contact angle if placed

on a non polar surface. Due to this hydrophobic surface and the high surface tension of

the liquid, a water droplet is placed on this non polar surface and exhibits a spherical

cap shape. The droplet contact angle is determined by the Youngs equation and thisequation is used to determine the thermodynamic equilibrium of the three interfacial

surface tension vectors. The three interfacial surface tension vectors are the FA

(fluoropolymer/air), WA (water/air) and WF (water/fluoropolymer), and the common

vertex of those vectors is commonly referred to as the triple point or contact line. Figure

2.1 provides a graphical representation of the triple phase contact point. Like any

system at equilibrium, the vectors must be balanced. Balancing all the vectors in place

we obtain Youngs equation [8].

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Figure 2.1 An aqueous droplet on a planar surface in air with Youngs angle.

YWAFAWF cos- (2)

where Y is the Youngs angle. Generally, liquid droplets that exhibit contact angles

larger than 90, then the planar surface is considered hydrophobic. If Youngs angle is

lower than 90, the surface is generally classified as being hydrophilic. However, for

electrowetting applications, generally a large Youngs angle is preferred and therefore

the planar surface is made such that it is hydrophobic. Most non-polar surfaces that

contain either hydrocarbons or fluorocarbons are hydrophobic in nature. Non-polar

surfaces containing fluorocarbons, however, exhibit higher degree of hydrophobicity

[16]. Therefore, fluoropolymers are commonly used and contain C-F bonds that are

covalent and are electronically screened by the presence of a large number of fluorine

atoms in the polymer chain [16]. Contact angles for water droplets on fluoropolymer

surfaces are generally between 110 and 120 in air ambient which is desirable for

electrowetting.

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2.1.3 Liquid Droplet on planar surface in oil ambient

If the air ambient was replaced by a non-polar liquid in the form of oil, assuming

the same non-polar (hydrophobic) surface, a higher Youngs angle will be observed.

This is because the oil has a surface tension closer to the hydrophobic surface than the

air. In fact, if the interfacial tension between the oil and the fluoropolymer is close to

zero, the Y will be close or equal to 180. This is the same case when using many

silicone oils. For alkane oils Youngs angle is closer to ~160.

Figure 2.2 provides a graphical representation of the Youngs angle in oil

ambient. If the oil is chosen to have a larger density than water, the water droplet will

sink to the fluoropolymer surface. However, there will be a very thin film of oil between

the fluoropolymer surface and the water droplet. Oil has other effects including

reduction in hysteresis, effects of gravity, interfacial surface tension, and also slowing

evaporation.

2.2 Electrowetting on planar surfaces

In this section we describe electrowetting on planar surfaces for an air or oil

ambient system.

2.2.1 In Air or Oil

Electrowetting on dielectrics (EWOD), as described by B. Berge [3], is an effect

caused due to the induced attraction of a liquid, via application of a voltage, on a

dielectric surface. The electrowetting system entails the use of a thin dielectric layer

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Figure 2.2 An aqueous droplet on a planar surface in silicone oil with Youngs angle of 180.

(fluoropolymer) placed between two conducting electrodes, the first is the water droplet

and the second is a conducting substrate like silicon or metalized glass. As soon as

voltage is applied to this system, capacitive charging takes place and there exists

charge accumulation at the water-fluoropolymer surface. The fluoropolymer layer is well

insulated so this prevents any effects of electrolysis. Between the fluoropolymer layer

and the substrate, an equal and opposite charge is induced as can be seen in Fig. 2.3.

The electrowetted angle is governed by the Youngs-Lippmann equation which

can be further simplified as the electrowetting equation. Youngs-Lippmann [2] equation

is as follows

WA

YV.d

Vcos

2r..

2

1cos 0

(4)

where V is the electrowetted contact angle, Y is the Youngs angle at V = 0, V is the

voltage applied, d is the thickness, 0 is the dielectric constant of free space, r is the

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Figure 2.3 Microscopic view of droplet showing three phase state during electrowetting.

dielectric constant and WO is the water/air or water/oil interfacial surface tension.

The device structure, between the water droplet and the electrode, acts like an

equivalent capacitor and the capacitance is dependent on the thickness and dielectric

constant of the dielectric material. When a voltage is applied between this electrowetting

system, charge accumulation takes place close to the water droplet-dielectric interface.

This charge accumulation is directly proportional to the applied voltage and capacitance

per unit area of the dielectric. Due to this charge accumulation, there is an

electromechanical force in the horizontal in-plane that alters the balance of the

interfacial surface tension forces. This additional force is labeled in Figure 2.3.

Increased wetting is due to this electromechanical force in the horizontal plane and is

the component responsible for electrowetting.

2.3 Limitations in Electrowetting

In an ideal setup, electrowetting would generally work without any flaws based on

theoretical calculations as explained in the previous section. However, this is not the

case as there are limitations in electrowetting. These limitations usually occur

depending on the material choice one uses for the dielectric or the type of oil used in an

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Figure 2.4 Common limitations in electrowetting (a) dielectric charging (b) oil charging and (c)

electrolysis [17].

electrowetting test or even the aqueous solution used. The most common limitations are

dielectric charging, oil charging, and electrolysis. Figure 2.4 gives a better depiction of

this.

Figure 2.3 shows an ideal electrowetting experiment where applied voltage will

drop across the entire dielectric and accumulation of applied charge will only take place

in the aqueous phase. Charge injection is something that needs to be avoided for

desirable electrowetting to take place. This can be observed in Figure 2.4a. Dielectric

charging can cause fatigue in the fluoropolymer [18] and often provides unpredictable

electrowetting response. More importantly, this can lead to contact angle saturation and

increased contact angle hysteresis [19].

Another limitation is charge injection in the oil and not within the dielectric layer.

Oil with low-breakdown field or containing some form of soluble charged particles, seen

in Figure 2.4b can show signs of electrowetting saturation or droplet relaxation.

Dodecane or tetradecane oil seems to be the best choice of oil for electrowetting and

show no signs of saturation or droplet relaxation. If there are pin-holes or other major

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defects that exist in the dielectric, electrolysis can occur and at low voltages, formation

of gas bubbles can be observed in the aqueous phase, as seen in Figure 2.3c.

In the upcoming chapters, these limitations will try to be reduced to achieve the

best electrowetting response for various applications. In particular, it is the goal of this

thesis to develop low voltage electrowetting but with materials such that the limiting

phenomena in Figure 2.4 are avoided.

2.4 Low voltage electrowetting

Low voltage electrowetting is desired for most electrowetting applications. Fromthe electrowetting equation, equation (5), the following conditions would generally be

desired for low voltage electrowetting:

decreasing the dielectric thickness, d;

decreasing the interfacial surface tension between the aqueous ambient and

other ambient like air or oil;

choosing an appropriate dielectric such that is has a high-K dielectric constant.

The general trend of achieving low voltage electrowetting is to decrease dielectric

thickness. Usually a single thin dielectric is coated on a substrate followed by a

hydrophobic fluoropolymer layer. Both these layers have capacitance in series and it

results in an equivalent capacitance which is generally dominated by the lower value ofcapacitance. Capacitance can be increased in accordance to the following equation:

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d

.A.C

r0 (7)

Here C is the capacitance, A is the area of the dielectric, 0 is the dielectric permittivity,

r is the dielectric constant and d is the thickness of the dielectric.

However, with this approach, there are some disadvantages that could hamper

the complete potential of increasing capacitance. Dielectric failure tends to increase for

thinner dielectrics. Charges tend to accumulate at the fluoropolymer surface and inject

into the fluoropolymer [20]. This charge injection is one of the reasons for electrowettingsaturation. This has been explained in the Limitations of Electrowetting section.

The next variable that would have to varied in the electrowetting equation in

order to obtain low-voltage electrowetting would be the interfacial surface tension

between the water/air interface or water/oil interface. This can be carried out by the

introduction of a surfactant in the water (water soluble surfactant) or even in oil (oil

soluble surfactant). The effects of surfactants on interfacial surface tension will be

observed in the latter chapters. Reduction in the interfacial surface tension between

water/air or water/oil interface will result in lowering operating voltage for electrowetting.

However, the oil density must be matched to the water density to reduce the effects of

vibration or gravity on the system. With lower surface tension there is also a longer

duration for a water droplet to rise back to its original state when applied voltage is

removed. This can be explained by the water/oil interfacial surface tension which results

in less force to pull back the water droplet after applied voltage is removed.

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One more area that could be explored to achieve low voltage electrowetting is

the high-rof the dielectric material chosen. Most fluoropolymers or other hydrophobic

materials that show a large initial contact angle (Youngs angle) usually have a smaller

dielectric constant, in the range of ~2-3. Most high-rdielectric layers are not

hydrophobic as they have strong polar domains which results in high surface energy

and wetting of polar liquids such as water. Therefore the use of stand-alone high- r

dielectrics for electrowetting are not suitable, resulting in the requirement of an upper

hydrophobic fluoropolymer. Therefore the best practice is to use a multiple layer

dielectric to achieve low voltage electrowetting. This will be seen in the next section andchapters that follow.

2.5 Review of past work in Low Voltage Electrowetting

In brief, some important findings from the past will be looked at here and ways to

overcome some of the problems of low voltage electrowetting will be addressed in this

thesis. Berry et al. [18,21] had reported very low voltage results (< 5V) using

fluoropolymer CYTOP on thermally grown SiO2. A surfactant was used in the form of

sodium dodecyl sulfate (SDS), in water, to lower the interfacial surface tension. This

would allow for lower electrowetting voltages and greater contact angle modulation.

Their group varied the thicknesses of the both the SiO2 and the CYTOP layer. The

oxide thicknesses that were tested were 11 nm, 30 nm and 100 nm. The thicknesses of

the CYTOP were 6 nm, 12 nm, 20 nm, 28 nm, and 50 nm. On top of these variations in

thicknesses, the SDS was tested with varying wt. % for each sample. It was observed

that operating voltages were as high as 14 V and as low as 3 V. The surfactants were

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tested with various wt. % in order to change in the interfacial surface tensions that

would affect the operating voltage and electrowetting response. It was also briefly

mentioned that contact angle saturation was observed and due to fluoropolymer

charging or dielectric failure. The best results were obtained for the following scenario:

6 nm CYTOP spin coated on 11 nm SiO2 with varying wt. % (0.03, 0.1, 1, 2) with

operating voltages as low as 3 V. Figure 2.5 will provide a clearer picture of

their results.

Although the results seem promising, there still exists contact angle saturation due to

surface charging or dielectric failure. Also, the use of SiO2 as a thermal oxide is not a

good option for low cost fabrication on glass [18,21] and thereby reduces it potential use

for commercial electrowetting applications such as optical arrayed devices. Furthermore

this group reported no results on reliability, and as will be seen in this thesis it is unlikely

the materials they use could result in reliable electrowetting devices.

Moon et al. [22] had reported electrowetting as low as 15 V. They had tested four

various cases. One included using a single layer of amorphous fluoropolymer (Teflon

AF, DuPont) with thickness of 100 nm on a silicon substrate. The second case involved

coating 12 m parylene on a silicon substrate followed by 20 nm Teflon AF. The next

case was depositing a barium strontium titanate film (~70 nm) using metal organic

chemical vapor deposition (MOVCD) on a Pt/Ti/Si wafer followed by a thin layer (20 nm)

of Teflon AF. Finally, SiO2 layer (~ 100 nm) was grown on a silicon substrate followed

by a layer of 20 nm Teflon AF. The following were observed for the various cases:

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Figure 2.5 Electrowetting responses for varying wt. % of SDS in 0.1 M NaCl solution [21].

Case 1 - ~ 15 for ~100 nm Teflon AF. Dielectric failure observed at larger

potentials due to electrolysis.

Case 2 - ~ 40 for ~12 m Parylene and ~100 nm Teflon AF but voltages up

to 200 V required.

Case 3 - ~ 40 observed at 15 V for ~70 nm barium strontium titanate and ~20

nm Teflon AF dielectric layer but titanates have been shown to have tendencies

for increased chance of charge trapping leading to hysteresis or even

delamination [23].

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Case 4 - ~ 40 observed at ~20 25 V after which contact angle saturation

was observed for ~100 nm SiO2 and ~20 nm Teflon AF.

The third case showed the lowest electrowetting voltage (15 V) and largest

contact angle modulation (~ 40). However, the reported contact angle modulation is

insufficient for optical devices such as micro prisms [22]. Also, as will be shown herein,

other factors must be considered if reliable low voltage electrowetting is to be achieved.

In this thesis, low voltage electrowetting devices on substrates are achieved on

films that are fully compliant for devices such as electrowetting optics [8,9]. In addition

to that, dielectric, liquid, and ionic surfactants were studied and optimized for greatly

improved reliability.

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CHAPTER 3

LOW VOLTAGE ELECTROWETTING DEVICES

3.1 Introduction

In the previous chapter, it was highlighted that low voltage electrowetting is

required for most commercial applications. Typically such voltage should be < 15 V,

especially for arrayed devices that are operated with thin-film-transistors [15]. Figure 3.1

shows some examples of arrayed devices developed at the University of Cincinnati. It

was therefore hypothesized that multi-layered dielectrics and surfactants would be

required to decrease the operating voltage of an electrowetting device.

A basic electrowetting system consists of a liquid/dielectric/electrode capacitor.

Here, electrowetting is performed via application of a voltage across this capacitor. The

dielectric layer is itself hydrophobic or coated with a hydrophobic fluoropolymer. This

hydrophobic nature provides a liquid with a high surface tension, such as water, a

Youngs angle that is > 110 in an air ambient and a Youngs angle > 160 in oil

ambient. The preference of oil ambient compared to an air ambient is for the following

reasons: (1) it reduces any effect of gravity, (2) alleviates contact angle hysteresis, and

(3) provides the largest range of contact angle modulation. Youngs angle as mentioned

in chapter 2 is given by the following equation:

Y

WOFOWFcos (2)

TheYoungs angle is determined by the in-plane balance of interfacial surface tensions

(, mN/m) of the water (W), oil (O), and fluoropolymer (F) phases.

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Figure 3.1 Diagrams and photographs of example electrowetting optical devices that can be

implemented in arrayed format on glass substrates [23].

In this electrowetting system, when a voltage is applied it creates an

electromechanical force per unit length (mN/m) near the three-phase point. Figure 3.2

shows the resulting decrease in the liquid contact angle due to electrowetting. This

device structure and oil ambient of Figure 3.2 will be the structure used for low-voltage

electrowetting.

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Figure 3.2 (a,b) Diagrams and (c) photographs of basic electrowetting droplet characterization

in an oil ambient [23].

3.2 Fabrication

In general, electrowetting films can be fabricated on silicon, glass and even

plastic substrates such as polyethylene-naphthalene as long as the maximum

processing temperatures are kept between ~ 120 to 180 C. The main substrate layer

can be an electrode such as Silicon (Si) or for applications related to optics, a

transparent Indium Tin Oxide (ITO, In2O3:SnO2) can be deposited on a glass or plastic

substrate. In order to achieve low-voltage electrowetting, equation (5) in chapter 2, it

can be understood that energy needs to be stored in the dielectric capacitor. It is known

that even the best fluoropolymers are insufficient for low voltage electrowetting

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whatever the thickness may be. As fluoropolymer thickness decreases, the thickness

can reach the size of a defect such as a pore or other electrically conductive pathway.

So in its standalone state, fluoropolymers will not achieve the desired high-capacitance

for low voltage. Therefore, it was hypothesized that using a multi-layered hydrophobic

dielectric would achieve low-voltage electrowetting.

For this work, ~100 nm Aluminum Oxide (Al2O3, ~9) and ~150 nm Silicon

Nitride (Si3N4, ~7) were investigated, and they both have breakdown fields that exceed

2 MV/cm. In the past, other materials have been utilized such as Silicon Dioxide (SiO2)

[18,22] and Barium and Strontium Titanate (BaTiO3, SrTiO3) [20,22]. However, this workand past observation have shown that SiO2 can suffer from electrochemical attack

(blistering) and the titanates showed increased occurrence for charge trapping

(hysteresis) or even delamination. For applications such as lab-on-chip, these problems

can be overlooked, but for optics it could be problematic due the requirement of > 106

device cycles. Therefore further reliability testing may be required for all materials used

here. Two approaches were taken using the same fluoropolymer.

The first approach involved using an inorganic dielectric in the form of Aluminum

Oxide (Al2O3) which was deposited on a Si substrate with atomic layer deposition

(Cambridge Nanotech Savannah 100 atomic layer deposition tool). Excellent

dielectric properties with ~100 nm Al2O3 were observed with deposition temperatures

ranging to as low as 120 C. This was followed by spin coating and thermal curing of

various wt. %, 0.5, 1.0 and 2.0 wt. % of CYTOP 809M in fluorosolvent. The varied wt. %

of CYTOP 809M was explored to determine how variation in fluoropolymer thickness

with a constant single dielectric layer of Al2O3 would affect low-voltage electrowetting.

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To obtain optimal surface smoothness for CYTOP 809M, annealing >160 C for ~15 min

was determined. Better smoothness reduces the effect of electrowetting hysteresis.

However, no substantial degradation was observed in electrowetting response at lower

annealing temperatures. Dodecane oil was used for all electrowetting experiments and

the electrowetting liquid was an aqueous solution that will be discussed later.

The second approach involved using a different inorganic dielectric, Si3N4 and

~150 nm Si3N4 was deposited on a glass/ITO electrode by plasma enhanced chemical

vapor deposition (external commercial source). This was followed by depositing a thin

layer of a fluoropolymer, CYTOP 809M ( ~2), via spin coating and thermally cured at ~180 C on the substrate. ~ 50 nm of 1 wt % solution of CYTOP 809M was spin coated

and annealed for this second approach. Both the first and second approaches resulted

in a high-capacitance double-layered dielectric.

3.3 Experimental Results

The aqueous solution tested was 1 wt. % sodium-dodecyl-sulfate, a surfactant, in

water. This solution lowers the interfacial water/oil surface tension (eq. 2) (WO) by ~10X

to ~5 mN/m [17,23] thereby reducing required voltage predicted by eq. 2. Figure 3.3

shows electrowetting results with varied CYTOP 809M thickness as deposited on Al2O3

layer. A DC potential was applied for these electrowetting tests. From the observed

data, it shows that an optimal thickness for CYTOP 809M is ~50nm for desired low

voltage electrowetting. Anything above or below 1 wt. % of CYTOP 809M appeared to

result in charging of the fluoropolymer surface. The effects of charging are diagrammed

in Figure 3.4 and can be experimentally observed from the data in Figure 3.3. In

summary:

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Figure 3.4 Diagrams of dielectric charging and its effect on contact angle saturation [23].

Based on the experimental observations of Figure 3.3, one may ponder as to

why the thicker 100 nm CYTOP 809M does not show charging until higher voltages, yet

also did not attain the minimum contact angle. One way of explaining it could be that the

fluoropolymer CYTOP 809M has a low relative permittivity r ~2, and at ~100 nm the

CYTOP 809M limits the overall capacitance compared to the ~100 nm Al2O3 (r ~9).

This results in a lower electromechanical force that would drive the electrowetting

process. Since electrowetting has d1/2 dependence, based on the electrowetting

equation discussed in chapter 2, and because ideally CYTOP 809M breakdown voltage

has a ddependence, thicker dielectrics (fluoropolymer CYTOP 809M in this case) with

lower permittivity should exhibit lower final contact angles at higher voltages. However

this is not the case as seen from Figure 3.3. The results in Figure 3.3 were found to be

repeatable, and the reason for the poorer performance of the CYTOP 809M (~100 nm)

is not understood at this time.

The next sets of experiments were performed on the glass/ITO/Si3N4 substrate,

the CYTOP 809M was fixed at ~50 nm thickness and various surfactants tested. Figure

3.5 shows the results of the experiments. All experiments used 0.1 wt. % Triton X-15 oil

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Figure 3.5 Electrowetting data with ~50 nm CYTOP 809M deposited on 150 nm Si3N4 films. All

experiments utilized 0.1 wt. % Triton X-15 in the dodecane. AC vs. DC and the addition of KCl

vs. SDS (sodium dodecyl sulfate) were also included in the experiments. Two samples were

tested for each curve and the results averaged [23].

soluble surfactant in dodecane, and either 1 wt. % sodium dodecyl sulfate or 1 wt. %

KCl. The experiments included the comparison of AC vs. DC as well as the addition of

SDS vs. KCl with respect liquid conductivity. The validity of the AC tests were ensured

by using ionic KCl in the absence of SDS. Therefore electrowetting capacitor charging

would not be RC time-constant limited. As seen from the Figure 3.5, lower contact

angles are observed with AC bias. There are two main reasons for this:

At 1 kHz, AC bias, there isnt sufficient time for the CYTOP 809M layer to charge

and cause saturation.

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At AC bias, there is a lower chance of hysteresis since at the microscopic level,

the contact line remains in continuous motion and therefore has a higher chance

to surpass microscopic defects that cause hysteresis (contact line pinning).

Another observation can be made in Fig. 3.5, where combining SDS surfactant in

aqueous phase and Triton X-15 in oil reduces the final electrowetting contact angle to

60 with only 8V operation. This is due to the reduction in WO and therefore less surface

tension force to counter act the electrowetting effect. In conclusion, CYTOP 809M

thickness was optimized and surfactants explored to achieve electrowetting well below

15 V. However, reliability of such systems was not certain, and is the topic of the next

chapter.

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CHAPTER 4

ION AND LIQUID DEPENDENT DIELECTRIC FAILURE IN

ELECTROWETTING SYSTEMS

4.1 Introduction

In order to attain low voltage electrowetting, based on the electrowetting equation

[2], it is understood that lowering the interfacial surface tension between the oil and

aqueous phases can help achieve this. The use of ionic surfactants can help achieve

lower interfacial surface tension; however, the ionic content present can increase

electrolysis at dielectric defects and possibly cause electrochemical corrosion of

electrodes. Another way to obtain low voltage electrowetting would be to decrease the

thickness of the dielectric thereby increasing the capacitance. This approach, however,

increases the risk of device failure via current flow due to the increased electric field

across the dielectric. It is therefore not surprising that the first commercial electrowetting

products used high voltage and thick dielectrics, such as the high voltage ~60 VAC

liquid lenses (Varioptic SA) with a thick (~3-5 m) Parylene dielectric. For reliable low

voltage electrowetting, a better understanding of the nature of the dielectric failure in

electrowetting systems is required. Dielectric failure modes in electrowetting might be

distinct from failure in solid-state systems. It was suspected that using liquid electrodes,

due to their unique ability to propagate through complex dielectric defects, could result

in increased dielectric failure. In this chapter it will be seen that the larger the physical

size of the polar liquid molecule, the lower the chances of dielectric failure. Similar

results will be seen for cations and anions that are of distinct size. Positive and negative

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voltage sweeps will be implemented to reveal any trends in dielectric failure. The work

herein, although not comprehensive, provides initial guidance, and shows careful

selection and synthesis of liquid and ionic content and how it leads to improved

electrowetting reliability.

4.2 Background and experimental results

Choice of materials and experiments will be first reviewed via some background.

It should be noted that the use of a polar liquid such as deionized water is not a

practical option for electrowetting in areas that include bioapplications such as lab-on-

chip as the ionic content is crucial to the health of proteins, cells, DNA, etc. and in

electrowetting optics an AC voltage is preferred due to minimization of contact angle

hysteresis [25]. The DC electrowetting response is poor for low-conductivity liquids such

as propylene glycol and to properly accumulate charge near the contact line, ionic

content must be added. An assumption can be made that for all or most real-world

electrowetting applications, ionic content is a requirement. Another assumption that will

be made is the need for a two-layer dielectric stack (Figure 4.1a) for low-voltage

electrowetting devices due to the naturally porous nature of the fluoropolymer film. This

two layer approach allows any conductive pores in the stand-alone thin fluoropolymer to

terminate against the strong insulation properties of the inorganic dielectric. However,

even these inorganic oxide or nitride dielectrics show marginal improvement in dielectric

reliability. Any possibility of self-healing failure [26] is barred for a system using a liquid

electrode. Close attention must therefore be given for selection of ionic content to allow

reliable, low voltage electrowetting. Other than dielectric failure, the electrowetting effect

can degrade for several other reasons. This has been briefly discussed in chapter 2. In

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the ideal case of electrowetting, it is assumed that all the ions or charges remain in the

liquid and solid electrodes. Figure 4.1 shows that ions that appear anywhere else but

the solid or liquid electrodes can cause a decline in electrowetting response. It can be

observed in Figure 4.1c that fluoropolymer charging [19] can cause contact angle

saturation (relaxation). The same can be said about inadequate insulation [27] of the oil,

as seen in Figure 4.1d, which results in a degraded response to electrowetting [27].

Electrolysis, seen in Figure 4.1e, can also degrade electrowetting response by

damaging the electrowetting surface or by causing a voltage drop across the liquid

electrode. It was therefore postulated that in order to decrease the effects ofelectrowetting degradation, larger size ions should be used as they will generally have a

lower chance of penetrating the insulating materials such as the dielectric or oil phase.

Tests will be performed for both dielectric failure and dewetting (saturation) for a variety

of ionic solutions. All tests will be performed with only DC voltage. Once reliable tests

are performed for DC voltages, AC voltages can be tested to reduce propagation of ions

through the dielectric layer.

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Figure 4.1(a) Initial state of a sessile droplet in oil ambient. (b) Proper charge buildup and

electromechanical alteration of the wetting response. (c, d, e) Non-ideal behavior due to

dielectric charging, oil charging, or electrolysis at dielectric defects [17].

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4.3 Choice, Preparation and Synthesis of Test Liquids

Herein two polar liquids were tested, deionized (DI) water and propylene glycol. It

is known that H2O partially dissociates into H3O+ (hydronium) and OH- (hydroxide) at

300 K. Under an applied electric field, further dissociation occurs [28]. Even though

deionized water can be marginally conductive for DC electrowetting, it is often

considered impractical in many devices or devices biased with AC voltage. Propylene

glycol was also tested as it contains larger size molecules. Small inorganic ions

(hydrodynamic radii ~1-2 ) were also tested using inorganic salts such as NaCl and

KCl. Larger ions were explored using an anionic surfactant, sodium dodecyl sulfate

(SDS) and a cationic surfactant dodecyltrimethylammonium chloride (DTAC). These can

be compared in Table 4.1. Nonionic surfactants were not tested due to their unlikely

impact on dielectric. Any aqueous surfactant was also chosen that contains a large

molecule for both the anion and the cation. The following process was used to custom-

synthesize a catanionic surfactant, dodecyltrimethylammonium octanesulfonate (DTA-

OS). 1g of DTAC and 1g of sodium octanesulfonate (SOS) were each separately

dissolved in methanol (99.8%, M-TediaCorp.) to make two clear solutions containing

0.867 g (3.787 x 10-3 mol) of docecyltrimethylammonium cation and 0.894 g (4.6296 x

10-3 mol) of the octanesulfonate anion. These two solutions were then combined at

room temperature and a final clear solution was formed.

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Table 4.1. Table of surfactants utilized herein.

Surfactant Structure

SDS

SOS

DTAC

DTA-OS

The ion-exchange reaction is instantaneous and goes to completion. This solvent

was then evaporated leaving behind ~2 g of solid residue which consisted of inorganic

salt (NaCl) and catanionic surfactant (DTA-OS). The residue was then dissolved in ~ 75

mL of DI water and placed in a dialysis tube (Fisherbrand) having a porosity of 12000

14000 Da and dry cylinder diameter of 28.6 mm. This porosity should allow organic

ions, to a lesser degree, to diffuse through it compared to other individual inorganic ions

in solution, and micelles or multimolecule phases to an even lesser degree of diffusion.

Conductivity measurements were then measured using an electrode of a conductivity

meter (OAKTONCON6/TDS6 measured in S/cm, 0.5%). This electrode was placed

in the dialysis tube, and the tube was sealed and submerged in a continuous flow of DI

water. Conductivity was measured at 30 min intervals until a time of 6 hrs at which time

the conductivity became constant which indicated that most of the inorganic salt had

diffused out of the tube. The remaining solution containing DTA-OS was removed from

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the dialysis tube and heated to evaporation. 0.968 g of solid residue was obtained after

evaporation. From earlier tests performed, it was determined that NaCl and sodium

octane sulfonate do not dissolve in chloroform (99.9%, M-Tedia Corp.). To verify the

complete removal of NaCl and identify any excess unreacted sodium octane sulfonate,

~0.2 g of the residue was redissolved in chloroform. The resulting solution was cloudy

signaling the presence of NaCl salt and/or sodium octanesulfonate. The cloudy

precipitate was removed by passing it through a filter paper. The remaining filtrate was

heated to evaporate the chloroform and obtain ~0.135 g of solid DTA-OS surfactant. To

further confirm the purity of the DTA-OS, a 0.05 wt. % aqueous solution was tested forCl- ions by adding ~ 5 drops of 0.1 M silver nitrate (AgNO 3) in ~20 mL of DTA-OS

solution. The test was negative i.e. no precipitate of silver chloride (AgCl) was observed.

Since the critical micelle concentration (cmc) for DTA-OS was not available, it had to be

experimentally determined by measuring the conductivity vs. concentration (wt %) of the

DTA-OS solution. The graph of Figure 4.5 are conductivity (, S/cm) and surface

tension ( in air, mN/m) vs concentration (wt. %) of the DTA-OS solution. Due to

reduced electrophoretic mobility for larger micelles, lamellar sheets, and precipitates

[29,30,31], conductivity increases more slowly above the cmc point. The cmc was found

to be ~ 0.065 wt. % (1.54 mM). This is lower than the cmc for SDS (~0.24 wt. %, 8.32

mM) which is expected from a catanionic surfactant as both the component ions (DTA-

and OS-) are amphiphilic. The cmc for DTA-OS was further confirmed by measuring thesurface tension vs. concentration (wt. %) of the DTA-OS solution (Figure 4.2). In order

to measure the surface tension of the DTA-OS solution, the pendant drop function of the

VCA Optima contact angle measurement system was utilized.

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Figure 4.2 Conductivity (S/cm) and interfacial surface tension (IFT, mN/m) of aqueous DTA-

OS catanionic surfactant solution vs concentration (wt. %). The cmc value for DTA-OS was

determined to be ~0.065 wt.% [17].

4.4 Dielectric Fabrication and Preparation

As mentioned earlier, thicker the dielectric materials like Parylene C (~3-5 m)

can exhibit excellent resistance to dielectric failure. However, this comes at the cost of

requiring a high operating voltage (~50 - 100 V). Therefore, for low voltage (

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aluminosilicate glass substrates that are coated with a transparent conducting

electrode. Indium tin oxide (SnO2:In2O3, 100 /sq, ~50 nm thickness, display grade)

seemed to be the most suitable transparent electrode and was purchased from PG&O

Inc. 100 nm Al2O3 (r ~ 9.1) was coated on this transparent conductive electrode via

atomic layer deposition using a Cambridge Nanotech Savannah 100 ALD System at

250 C. Two precursors were used namely trimethylaluminum (Sigma-Aldrich) and DI

water. There was also a precursor pulse time and N2 purge time used. They were 0.015

s and 8 s respectively. A thin layer of fluoropolymer was then spin-coated. The

fluoropolymer used was a 1 wt % solution of Asahi CYTOP 809M in fluorosolvent Ct.Solv. 180. The spin cycle consisted of a 500 rpm spread for 15 s and a 1000 rpm spin

for 45 s. The next step was an annealing step and the sample was annealed at 180 C

for 30 min. The resulting fluoropolymer thickness was ~50 nm. Another fluoropolymer in

the form of DuPont Teflon AF was not investigated due to past observations that

showed the surface to suffer from charge injection for thin films (

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viewed through the transparent side walls of the acrylic box. A tungsten cat whisker

probe tip (0.5 m in diameter) was inserted into the polar droplet in order to provide

electrical bias to solution and the other end of the probe tip was connected to a Trek

amplifier (model 603 A) which was further coupled to a Tektronix AFG 310 function

generator. 1 V/s step of voltage was applied to the system by using a LabVIEW

program and the droplet profile was taken by synchronized video capture. The

SnO2:In2O3 was connected to electrical ground.

The second set of experiments was the dielectric failure tests. These tests were

performed in air (not in oil) as the Youngs contact angle (Y) of the polar droplet is

160 in oil. The reason for a lower Youngs angle is due to the

reduction in the electrical influence of increasing capacitor area with voltage

(droplet/dielectric/SnO2:In2O3). Another possibility is using a completely fixed area

capacitor [32]. This was not explored as it eliminates high-field generation near the

sharp contour of the contact line and this effect must be included if dielectric failure

measurements are to be considered as meaningful. For each dielectric failure test, the

voltage was swept from 0 to 30 V at 1 V/s. Both the positive and negative polarity

voltage sweeps were independently measured at fresh sample locations, else failure in

one polarity could damage the dielectric and influence failure in the other polarity. The

dimension of the contacting droplet was ~2 mm in diameter for each experiment. The

tests were carried out three times for each polarity and for each solution. Three current-

voltage plots were made and plotted independently in order to show any variations in

the voltage for dielectric failure.

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4.6 Results

First, electrowetting data were plotted. These can be observed in Figure 4.3 and

they confirm that the ability of surfactants to decrease ao and the required

electrowetting voltage. In order to obtain the ao, the pendant drop method was used in

oil ambient. The ao of DI water was measured to be ~53 mN/m and 1 wt. % SDS was

measured as ~5.7 mN/m. It can be deduced from Figure 4.3b that the higher the wt % of

DTA-OS, the lower the initial contact angle (Y). This is probably due to the lower

interfacial surface tensions at the aqueous solution-fluoropolymer interface and the

aqueous solution-oil interface.

The plots of Figure 4.4 show the results of the dielectric failure tests performed

with various solutions. In Figure 4.4a, no dielectric failure is observed on the DI water

for both positive and negative polarities of applied voltage. If there was a dielectric

defect present on the surface, due to the low conductivity of DI water (Table 2) any

measurable electrolysis could be considered irrelevant. It would be more meaningful if a

measurement was made by adding small inorganic ions (salt) to DI water. As seen in

Figure 4.4b, dielectric failure is observed when an aqueous 1 wt. % solution of KCl is

tested. Since it is known that atomic layer deposition produces high-density, pin-hole

free Al2O3 films, the dielectric failure observed could not be from the result of ion

conduction through large defects such as pinholes. With standard metal electrodes, the

dielectric failure electric field (Ebd) is >30 V/100nm for Al2O3. It is therefore clear that

with a liquid electrode, the small inorganic ions and water are able to propagate through

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Figure 4.3(a) Plot of contact angle vs. electrowetting voltage for conventional surfactant

solutions; the wt.% used are above the cmc points [17].

even the smallest pathways in the dielectric. A brief discussion can be about the polarity

dependence of dielectric failure in Figure 4.4b.

In general, it is accepted that mono-valent anions such as Cl - and OH- typically

adsorb at the neutral polymer surface. It has even been validated that there exists

preferential anion adsorption for electrowetting on fluoropolymers [33]. The adsorbed

ions generally do not move easily under very high electric field and if they do move, they

are not as mobile as the ions are positioned further away from the surface. Via Debye

screening, the surface adsorbed anions locally deplete additional anions. Therefore, this

could be a

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Figure 4.3(b) Electrowetting (EW) contact angle response for several wt.% of catanionic

surfactant (DTA-OS) solution [17].

source of explanation as to why stand-alone fluoropolymer film exhibits water

electrolysis at lower voltages for positive bias. Positive ions comprise the mobile Debye

sheath inside a small pore. This might help explain the dependence of failure in Figure

4.4b where is shows clearly that the K+ ions are more easily driven into the pathway for

dielectric failure. However, it must be clarified that in the experiments, afluoropolymer/Al2O3 stack was used and the Al2O3 favors surface adsorption of cations

instead of anions [34], and this could alter the conclusions made earlier. Further

exploration of this was not performed herein and could be an involved investigation in

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itself. From further observation of the next set of data, it will be determined that size of

ions has a much stronger effect on dielectric failure than ion polarity. In this present

work, concluding on ion-size dependence of failure is the primary goal.

A commonly used anionic surfactant, SDS, was tested. It has been consistently

seen that most low voltage devices at the University of Cincinnati [9] show electrolysis

when a positive bias is applied to SDS solution but not observed when a negative bias

is applied. Therefore, to better understand this phenomenon, a more thorough

investigation is presented herein. Figure 4.4c shows that small inorganic Na+ ions

penetrate the dielectric easily, under a positive bias, whereas the larger dodecyl sulfate

(DS-) ion is clearly less able to penetrate the dielectric, under negative bias. One might

wonder if this is a manifestation of the Stern layer and Debye screening effect proposed

for the polarity dependence of failure for the KCl solution (Figure 4.4b). In order to

confirm ion size dominance for dielectric failure, a cationic DTAC surfactant was tested.

Figure 4.4d shows that the failure trend is similar to but opposite to that of Figure 4.4c,

where in the positive bias the large dodecyltrimethylammonium (DTA+) group is unable

to penetrate the dielectric. In the negative bias, it can be seen that the Cl- ions penetrate

the dielectric surface more easily. From the comparison of current-voltage results for

SDS and DTAC, both of which are 1 wt. % in solution and have similar conductivity

(~1000 S/cm, Table 4.2), it supports the argument that ion size is indeed a dominant

factor for dielectric failure in electrowetting devices.

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Figure 4.4 Dielectric failure current vs. voltage for various aqueous solutions on a

fluoropolymer/Al2O3 dielectric stack. Every test, and individual positive and negative polarity

voltage sweeps, were measured at independent dielectric sites. The data are presented without

any averaging, allowing one to appreciate that the data are fairly repeatable even for dielectric

failure tests [17].

The next solution explored was the catanionic DTA-OS surfactant. It was

explored as it may provide the essential electrical conductivity for electrowetting devices

but have a greatly reduced chance of dielectric failure. The main factors that could help

support this are that this surfactant contains both cation and anion that are large due to

the long alkane chain attached to them, and a lower cmc point making the solution more

electrically resistive. The dielectric failure results shown in Figure 4.4e do indeed

confirm that the DTA-OS is free from failure in both the positive and negative polarities.

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exhibit least drag for ion flow. Since it may be difficult to design a material that would

have the ability to completely prevent water penetration, an alternate approach will have

to be taken. It was then postulated that another solvent be used in the form of propylene

glycol ( 99.5%, Fisher Chemical) due to its larger liquid molecule size. This would

reduce the chance of liquid penetration into the dielectric. Pure propylene glycol was

found to be very electrically resistive, and from initial tests to even allow any form of

electrowetting to take place with DC voltage. Therefore no dielectric failure could be

observed. This can be observed in Figure 4.5a. When a surfactant, in the form of 1 wt.

% SDS, was added to propylene glycol the conductivity increased to ~31S/cm. There

was also strong DC electrowetting response (143 to 57 for 0 to 16 V bias). It should

also be noted that for the propylene glycol:SDS solution, no dielectric failure was

observed in the positive bias (Figure 4.5b) unlike that which was observed for the

water:SDS (Figure 4.4c). But lack of dielectric failure here could be due to the lower

conductivity of the propylene glycol:SDS solution therefore a similar conductivity of

water:SDS solution was made and tested (Figure 5.5c). Dielectric failure was still

observed in the positive bias for water:SDS. It is therefore likely that propylene glycol is

less likely to create a liquid wire compared to water through a dielectric. Therefore for

increased reliability of some electrowetting devices (non-aqueous), propylene glycol

might be a more useful liquid. However, an additional co-solvent (medium size

molecule) will be required to reduce the liquid viscosity for fast fluid motion [36]. Otherfactors affecting liquid wire formation include surface adsorption along the walls of a

pore and it could be

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Figure 4.5 Dielectric failure current vs voltage for(a) pure propylene glycol and (b) propylene

glycol with 1 wt. % SDS. For comparison with (b) a water:SDS solution of similar conductivity is

plotted in (c) [17].

different for propylene glycol as compared to water. But this will not be explored and

may be of interest in future studies.

Most of the tests presented thus far primarily deal with dielectric failure of

electrowetting devices. It was later decided to explore ion-size relationship towards the

onset of electrowetting saturation (charge injection) [25]. One way to observe saturation

due to charge injection is to look at the time-dependent contact angle saturation. The

time-dependent contact angle relaxation for DI water and SDS can be observed in

Figure 4.6. Applied voltages are well beyond the saturation (28 V). The data presented

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shows an ion dependence on the contact angle relaxation. The least relaxation is shown

by the case that drives the largest ion (dodecyl sulfate, C12SO4 at -28 V) into the

dielectric. Other factors will have to be considered but this preliminary study supports

previous conclusions made. Relaxation behavior can be affected by interfacial tension

and time-dependent interfacial tension [37]. It can also be said that OH- ions in the SDS

solution should generally have the ability to inject into the dielectric unless the dodecyl

sulfate somehow screens similarly charged ions at the pore entrances. After comparing

the data from Figure 4.5c and Figure 4.6, it can be suggested that ion-dependent

dielectric failure may have similar implications on understanding contact anglerelaxation due to charge injection. The similarities between dielectric failure and

saturation (charge injection) are not unexpected.

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Figure 4.6 Measured contact angle relaxation (charge injection) vs. time for several aqueous

solutions biased at 28 V. Voltage was applied at ~0.5 s, and the contact angles captured with a

VCA Optima system [17].

4.7 Discussion and Conclusion

Based on all the data presented here on electrowetting failure mode, it can be

deduced that it is highly dependent on ion size. This work provides initial guidance to

electrowetting practitioners, even though the materials tested are far from

comprehensive. The dielectric failure voltages reported in these findings far exceed the

actual required electrowetting voltage. It should not be assumed that without dielectric

failure, reliable and long-lived electrowetting operation is always guaranteed. It is nearly

always the case that the further one operates from the dielectric failure voltage of a

dielectric (for solid and liquid electrodes), the less the electrical degradation of the

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dielectric or electrode. Therefore, it has been suggested that the use of large ions in

electrowetting liquids can help improve reliability as the ions eliminate the root problem

(charging penetration) rather than simply delaying visible charging effects. Other factors

must be considered, however, such as electrochemical reactivity of the dielectric and

electrode. In this work, no such degradation was observed as a result of the small

electrical currents measured but prior aging studies [23] have shown radially

propagating electrochemical degradation that usually starts at the point of electrical

failure. Other areas that could be investigated in the future include surface adsorption

and Debye screening inside dielectric pores. With the work reported here, there provesto be positive commercialization prospects for low-voltage (

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CHAPTER 5

DISCUSSION AND FUTURE WORK

5.1 Summary of this work

The basis of this work was to attain low voltage electrowetting by choosing the

appropriate dielectrics for active-matrix driven electrowetting applications developed at

the University of Cincinnati. The first aim was to obtain electrowetting at low DC

voltages, specifically under 10 V, by choosing the appropriate thicknesses for the

dielectric. Two types of dielectric were used, Al2O3 and Si3N4. Both dielectrics included

~50 nm CYTOP 809M and exhibited good electrowetting response under 10 V. Fromthe data compiled, the dielectrics and surfactants used do indeed show low voltage

electrowetting but in the DC regime, saturation (via charge penetration) takes place.

Therefore, the electrowetting solution needed to be investigated further.

Once the type of dielectric to be used was established, which was the Al2O3 due

to the high quality of atomic layer deposition, the next step was to observe the effects of

various surfactant solutions under different polarities of voltage. Positive and negative

voltage sweeps were applied to various aqueous solutions to observe any trends in

dielectric failure. After testing the various surfactant solutions, dielectric failure was not

observed for the synthesized catanionic surfactant. Reason being that catanionic

surfactants contain long alkyl chains for both the cations and anions which means that

the surfactant contained ions that were too large to cause dielectric failure. Therefore,

from the data compiled and results observed, it has been suggested that the use of

large ions in electrowetting liquids can help improve reliability as the ions eliminate the

root problem (charging penetration) rather than simply delaying visible charging effects.

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After testing the surfactants, a proposal was made to use liquid electrodes due to their

unique ability to propagate through complex dielectric defects. The use of propylene

glycol was suggested as a medium compared to water due to its larger molecular size.

From the results, propylene glycol showed no dielectric failure as a standalone or with

SDS surfactant solution. This provides further evidence that relatively smaller

dissociated water molecules (H3O+) and (OH-) also contribute to the formation of a

conductive pathway through the dielectric. From this, it is safe to say that propylene

glycol could be a better alternative to water for electrowetting as it does not exhibit

dielectric failure.

5.2 Achievement in the context of others work

From the data gathered here it is fair to say that dielectric failure can be avoided

with the dielectrics used and the surfactants tested herein. We shall now briefly

overview the benefits of the findings here for low voltage electrowetting in comparison to

the work of other groups that are trying to experimentally attain low voltage

electrowetting. In brief, the Al2O3 is a better dielectric atomic layer deposition reduces

the chances of having any pores in the dielectric layer. Unlike the option of thermally

grown SiO2 [18,21] as a dielectric layer which is not a feasible option for low cost

commercial applications, or the barium strontium titanate [22] grown via metal organic

chemical vapor deposition, the Al2O3 is more viable. Titanates have been shown to

increase the chances of charge trapping and at times even fluoropolymer delamination

[23]. Other groups, such as Berry et al. and Moon et al., have not focused their work on

effects of saturation due to surfactants for low voltage electrowetting. Even though

these groups make mention of electrowetting saturation, no further details were

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provided for the cause of it. Given the results of the work herein, we do see the

dielectric failure effects for SDS surfactant solution with a positive DC bias. Therefore a

better alternative in the form of a catanionic surfactant, such as the DTA-OS that was

synthesized, one would have to assume that it would be better for reliable electrowetting

since the surfactant contains long alkyl cation and anion chains. However, synthesizing

this surfactant is a time consuming process and requires experimental precision. There

are very few, if any, commercial catanionic surfactants in the market. Therefore for

industry, this might not be the most desirable option available. In turn, the use of

propylene glycol can be suggested as a medium instead of water for electrowetting as itis easily attainable and showed no signs of dielectric failure, with or without SDS.

5.3 Future Work

From the knowledge of surfactants and dielectrics, it can be claimed that they

can certainly be applied to most electrowetting applications for low voltage

electrowetting. However, there is always scope for improvement when it comes down to

material characterization for low voltage electrowetting. One area that could be studied

in order to enhance reliability and longevity of dielectric layers is using composite

dielectrics instead of the standard double-layered approach that was proposed in this

thesis. Using the composite dielectric approach, the following is assumed. Most

dielectrics have pores in the micro- or nano- scale. By using composite dielectrics, if

there exists some form of charge penetration through the first layer, the dielectrics that

follow will have a mismatch therefore not allowing charge penetration to the metal

electrode since the pores wont follow through the entire composite dielectric. An

example of this process would be using Al2O3 and a photo-resist with good dielectric

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properties such as SU-8. This is currently being studied at the University of Cincinnati,

where 4 layers of these dielectrics are deposited or spin coated on a substrate. First, a

thin layer, ~50 nm, of Al2O3 is deposited on a metal substrate or metal electrode

substrate. This is followed by spin coating SU-8 to attain a thickness of ~100 nm.

Finally, a hydrophobic dielectric layer, such as CYTOP, is spin coated on the composite

dielectric layer. If this particular setup shows no dielectric failure for any surfactant

used, be it cationic or anionic, then electrowetting response tests will have to be

performed on them. Once desirable low voltage electrowetting response is observed,

this might be an area to transition to.

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