Electrowetting Actuation of Liquid Metal Wires for Reconfigurable Electronic Switches and Wire-Grid Polarizers

Electrowetting actuation of liquid metal wires for reconfigurable electronic switches and wire-grid polarizers

A thesis submitted to the Division of Research and Advanced Studies of the University of Cincinnati in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE

In the Department of Electronic and Computing Systems of the College of Engineering and Applied Science

2016

Aaron Diebold

B.S., University of Cincinnati, 2014

Dr. Jason C. Heikenfeld, Committee Chair

ABSTRACT

As the sophistication of electronic and photonic devices increases, the demand for increasingly miniaturized and small footprint components naturally grows. Indeed, recent trends indicate that the inventory of objects comprising our modern day Internet of Things will become ever more populated, so that proper interfacing will require device adaptability in order to provide efficient information transfer and communication. Otherwise, a crowding of the usable frequencies will quickly make such technologies intolerable. In addition, the variety of wearable sensors and other flexible electronics will demand flexible components which are preferably reconfigurable in order to supply the necessary frequency response or radiation pattern dictated by the specific application. Particularly in wearable technologies, we see a strong need to minimize the amount of circuitry and active device size, simply due to conventional clothing standards and physical conformability. In this respect, the two available options are to physically shrink the size of the components, or else to make the most efficient use of the least amount of space by allowing reconfiguration of the device functionality. It is the latter method which is pursued in this work.

In this work, various actuation and reconfiguration methods are presented which use liquid metal gallium alloys as the active material. A demonstration of devices utilizing electrowetting as the actuation method is then given, with associated results and discussion.

Material considerations are presented, as well as other challenges for future research. Various significant obstacles stemming from material properties of gallium alloys are overcome, providing important design and fabrication parameters for future work. These findings open new pathways to a wealth of applications where the high conductivity and fluid behavior of gallium alloys are desired, and may lead to a radical growth in potential for reconfigurable microwave electronics.

ACKNOWLEDGEMENTS

I would like to thank my friends, family and girlfriend, who have all helped me to remain sane, to relax and enjoy the challenge, and to be excited about my work while maintaining genuine interest in all things wonderfully unique.

I would also like to thank my advisor Dr. Jason Heikenfeld, whose passion for his work is highly contagious, and who has helped me to achieve a level of experience and confidence which continues to influence all aspects of my life. To push forward without fear of failure, and to lead without hesitation are qualities which stem directly from his methods, and for these realizations I am very grateful.

My thanks go also to Chris Tabor and the Air Force Research Lab who have provided the funding necessary for me to complete this research in a timely and thorough manner.

I was fortunate to have the support and guidance of Dr. Mast, whose authority in his field was indispensable in achieving the rigorous understanding that I desired. I thank him also for his help in getting me to the next stage of my educational experience.

Finally, I would like to express sincere appreciation to my colleagues and lab mates who have continually relayed their knowledge and experience, while maintaining an atmosphere both professional and enjoyable. I am always impressed at the level of motivation and achievement which prevails in the Novel Devices Lab, and I am excited to see what it will give to the world as the future unfolds.

TABLE OF CONTENTS

COPYRIGHT PAGE ...... 3 ACKNOWLEDGEMENTS ...... 4 TABLE OF CONTENTS ...... 5 LIST OF FIGURES ...... 6 Chapter 1: Introduction ...... 8 1.1 Motivation ...... 8 1.2 Research Aims ...... 9 Chapter 2: Background and Review ...... 11 2.1 EGaIn and Galinstan...... 11 2.2 Conventional Actuation Methods ...... 11 2.3 Electrowetting Theory ...... 29 2.4 Electrowetting Applications and Challenges ...... 32 2.5 Electrowetting of Liquid Metals ...... 38 Chapter 3: Electrowetting Actuation of Liquid Metal Wires for Applications such as Switches and Polarizers ...... 43 3.1 Introduction ...... 43 3.2 Background ...... 43 3.3 Materials and Device Fabrication ...... 45 3.4 Theory & Design ...... 49 3.5 Results ...... 52 3.6 Discussion ...... 59 3.7 Conclusion ...... 61 Chapter 4: Conclusions and Future Work ...... 62 4.1 Current Work ...... 62 4.2 Further Challenges ...... 63 4.3 Future Applications and Conclusion ...... 66 REFERENCES ...... 68

LIST OF FIGURES Figure 2.1: Frequency tunable and phase shifting surface using continuous injection of liquid metal slugs [8]...... 12 Figure 2.2 Reconfigurable, reversible low-pass filter enabled by selective injection of Galinstan [9]...... 13 Figure 2.3 Reconfigurable plasmonic device using concentric circular channels patterned in PDMS, allowing selective injection of EGaIn [10]...... 14 Figure 2.4 Frequency shifting liquid metal antenna exhibiting pressure-responsiveness. EGaIn is initially stabilized by the use of Laplace barriers paired with the mechanically stabilizing oxide skin [11]...... 15 Figure 2.5 Pressure responsive bandstop filter utilizing Laplace barriers, integrated into a microstrip resonator [12]...... 16 Figure 2.6 Switchable resistive networks and resonators. The actuation method is Laplace pressure shaping, which relies on vacuum supplied by an external pump and microreplicated trenches [13]...... 18 Figure 2.7 Reversibly deformable dipole antenna relying on mechanical strain for frequency tuning [16]...... 20 Figure 2.8 Flexible microstrip patch antenna filled with EGaIn. Tuning is enabled by mechanically varying the antenna's curvature [17]...... 21 Figure 2.9 Subwavelength aperture array fabricated using PDMS and EGaIn. Transmission resonances may be tuned by mechanically straining the array [18]...... 22 Figure 2.10: Charge distribution of electric double layer along liquid metal slug, enabling continuous electrowetting [19]...... 24 Figure 2.11: Liquid metal micromotor using continuous electrowetting of mercury slugs [19]. .... 25 Figure 2.12: Variable frequency slot antenna, where tuning is achieved by changing liquid metal position and capacitive loading by continuous electrowetting. Patterned channel allows for discrete positioning of Galinstan [20]...... 26 Figure 2.13: Tunable and reversible monopole antenna relying on reduction of surface tension due to electrochemical oxidation of EGaIn. Length change is accomplished by associated capillary forces [21]...... 27 Figure 2.14: EGaIn steered through branched channels by electrocapillarity paired with electrochemical oxidation to halt flow [23]...... 28 At the boundary of a droplet resting on a surface there are competing interfacial surface tensions of the three phases, the liquid droplet, the ambient gas, and the solid surface (see Fig. 2.15). Treating this as a force balance problem yields Young’s equation: ...... 29 Figure 2.16: Balance of interfacial surface tensions at the boundary of a droplet resting on a surface [14]...... 30 Figure 2.17: Variable focus liquid lens, where focal length is modulated by varying the contact angle of a droplet through electrowetting [32]...... 33 Figure 2.18: Beam steering by electrowetting. Liquid microprisms are manipulated by electrowetting on each of four side walls, tilting the planes of refraction [31]...... 33

Figure 2.19: Liquid state field-effect transistor utilizing electrowetting of a conductive fluid to displace an oil droplet. The fluid then acts as the conductive medium [25]...... 34 Figure 2.20: Transposition of colored inks by electrowetting in order to enact large surface area changes, for use in reflective displays [29]...... 35 Figure 2.21: Demonstration of bistability in an electrowetting device utilizing partial-post Laplace barriers [36]...... 36 Figure 2.22: Laplace barriers used in combination with addressable 2-D electrode arrays, allowing programmable and bistable transport and shape configurations [38]...... 37 Figure 2.23: Demonstration of contact angle change of mercury droplet under applied electrowetting voltage [39]...... 39 Figure 2.24: Variable-height micromirror actuated by electrowetting of mercury droplet [40]. .... 39 Figure 2.25: Liquid metal switch, where switching is achieved by contact-line sliding of a liquid metal droplet as a voltage is applied [41]...... 40 Figure 2.26: Frequency reconfigurable annular slot antenna, where tuning is achieved as the contact angle of a mercury droplet is changed by electrowetting, varying the loading capacitance [42]...... 41 Figure 3.1: Device diagrams and relevant dimensions for switchable polarizer...... 47 Figure 3.2: Contact angle change of EGaIn vs. AC voltage...... 52 Figure 3.3: Wetting and dewetting speeds in polarizer...... 54 Figure 3.4: Photos of operating device, including (a) initial self-loading, (b) with voltage off, and (c) with voltage on...... 56 Figure 3.5: Signal transmission for various electrode materials including Fluorine doped Tin Oxide (FTO) and Indium Tin Oxide (ITO)...... 57 Figure 3.6: Transmission results for device on and off, with the liquid metal wires (a) perpendicular and (b) parallel to the electric field polarization...... 58 Figure 3.7: Switchable EGaIn polarizer in the (a) off state and in the (b) on state...... 59 Figure 4.1: Dielectric stacks as barriers for fluid infiltration, acting to reduce defects in electrowetting dielectric films [68]...... 65 Figure 4.2: Illustration of self-healing dielectric composed of aluminum electrode covered in a hydrophobic dielectric. Use of proper electrolytes and voltage forms will anodize the aluminum at sites of breakdown, spontaneously forming replacement dielectric [71]...... 66

Chapter 1: Introduction

1.1 Motivation

Metal components are indisputably among the most common materials used in electromagnetic transmission systems due to their high electrical conductivities and low losses.

The call for miniaturization paired with a crowding of the usable frequency spectrum by wireless devices has been answered in part by growing research into reconfigurable electronics and multipurpose components. Even more fundamentally, the passive nature of many electromagnetic structures insists that sophisticated switching methods be used in order to change their geometry and thus dictate the desired frequency response or lack thereof. Though this has largely consisted of rigid actuating schemes, fluidic components, such as metals which are liquid at room temperature, come to mind due to their flexibility and ready malleability.

These properties in turn yield durable and reliable components that have potential for reconfigurability unseen in other materials.

For example, the multitude of antenna shapes and variety of their effects provide evidence for the subtle influence of geometry. More recently, metamaterials and metasurfaces have been fabricated which rely on periodic arrays of exotic structures, often in a planar geometry. Small variation in geometrical parameters may yield large changes in their response, so that we find ample justification for researching actuation methods that allow us to actively modify such parameters.

Much of the above can arguably be achieved using more conventional semiconductor arrays, among other designs. These existing technologies are prone to fall short in such areas where the high conductivity of metals are required, i.e. where applications demand high radiation efficiencies and low losses in order to achieve the strongest possible signal. Similarly,

8 highly-conductive materials are crucial in high-current transmission lines in order to prevent thermal damage. Switchable, durable components in such transmission lines offer increased control and signal reliability. Thus, we see that a demand for liquid metal electronics exists in a slew of areas.

Taking the above into account, we must note that contemporary liquid metal electronics do indeed exist which utilize a multitude of reconfiguration schemes. Certain elements of these designs force us to refine our criteria. While many reconfigurable/switchable liquid metal devices have been demonstrated (discussed later), most have not made it into consumer electronics due to their utilization of the toxic liquid metal Mercury. Others have fabricated effective devices containing gallium alloys as the active material. These are non-toxic, yet offer additional challenges due to oxidation properties. Reconfiguration schemes utilizing these alloys have been demonstrated, but a reliable method to actuate them by electrowetting has yet to be seen, due to material barriers. As will be discussed later in this work, it would be advantageous to overcome these obstacles in order to reconcile the highly-developed field of electrowetting with the strong potential of non-toxic liquid metal gallium alloys.

1.2 Research Aims

It is the goal of this thesis to establish a method to reliably actuate non-toxic liquid metal gallium alloys using the method of electrowetting. To “reliably” actuate gallium alloys requires that the liquid metal be kept oxide-free either through chemical removal of the oxide or by provision of an oxygen-free environment, or both. Therefore, the first task of this work is to lay out experimental constraints and methods which allow this, and to indicate the associated device lifetime. These methods must be in line with the physical principles which define electrowetting.

Once such a possibility is established and the proper constraints acknowledged, the next task is to outline design constraints. This includes modelling power requirements and associated device capabilities, as well as inspection of material compatibility and limitations.

The final task is to demonstrate the fabrication and operation of a functioning device which successfully utilizes electrowetting to actuate gallium alloys, followed by characterization of the device response, speed, and lifetime. The device presented in this work is a switchable linear polarizer, the design elements of which may also be applied as a liquid metal switch.

In order to justify these goals, Chapter 2 first outlines the existing technologies which seek to achieve similar functions, and examines where improvements are deemed necessary.

We then introduce the theory of electrowetting, and important applications which have been explored, as well as how these technologies may potentially be applied in combination with liquid metal components. Finally, we take an honest look at the challenges associated with applying this method, including the constraints it places on device design and operation.

Chapter 3 describes the theory, fabrication, and characterization of a liquid metal switchable polarizer which, in its reduced form, may also operate as a liquid metal switch.

Unresolved challenges are indicated here, to be fully explored in Chapter 4. Potential applications are also discussed in Chapter 4, along with the associated obstacles to their realization.

Chapter 2: Background and Review

2.1 EGaIn and Galinstan

Though a safe alternative to mercury, EGaIn (75% Ga, 25% In) and Galinstan (68.5%

Ga, 21.5% In, 10% Sn) have several unique properties which dictate design methods and

applications. Under ambient conditions of greater than about 1 ppm oxygen, a stabilizing oxide

skin develops nearly instantaneously, with a thickness of about 0.5 nm under vacuum [1] to 2.5

nm under high humidity conditions [2]. Gallium alloys have different characteristics depending

on their level of oxidation. EGaIn is a low-viscosity liquid ሺ߭ൌͳǤͻͻൈͳͲିଷܲܽݏሻ[3] with very low

ିଵଷ vapor pressure ሺ݌௩ ൏ͳͲ ݌ݏ݅ሻ[4]. It is highly conductive, though roughly an order of

ௌ magnitude less than that of copper ቀߪ ൌ ͲǤ͵Ͷ ൈ ͳͲ଻ ቁ[5]. When oxidized, they are non- ௠

Newtonian, viscoelastic fluids which easily wet most surfaces. In its clean state EGaIn is

normally a non-wetting Newtonian fluid. Surface tension values are known to differ as well,

offering a value (for EGaIn) of about 624 N/m in its oxidized state and about 435 N/m in its clean

state [5]. The oxide is mechanically stable to the point of being able to sustain exotic droplet

shapes, and will not rupture up to a yield stress of 0.5 N/m [6]. The oxide is composed of mostly

ܩܽଶܱଷ and ܩܽଶܱ oxides. This can be removed electrochemically by acidic or basic solutions.

HCl is often used, and has been shown to yield a surface composition of ܩܽܥ݈ଷ, ܫ݊ܥ݈ଷ and

ܪଶܱ[7].

2.2 Conventional Actuation Methods

2.2.1 Pneumatic and Pressure-Driven Actuation

Pressure-driven transport is probably the most conventional method for fluid actuation,

as manual and automated syringe injection has been the state-of-the-art for many technologies

11 for some time. It can be simple in that functionality requires essentially only inlet and outlet ports in order to displace existing fluids by a desired fluid, and the required pressure may be supplied externally either by a human user or pump.

This technique was applied in [8] to enable a tunable frequency selective and phase shifting surface which takes advantage of the continuous transport enabled by syringe-driven actuation. The principle relies on an array of sub-wavelength capacitive patches which rest on a dielectric substrate, below which sits an inductive grid. Capillary tubes which contain Galinstan slugs and lubricating Teflon solution run beneath each row of capacitive patches, operating essentially as fluidic varactors. Applying pressure allows for continuous displacement of the

Galinstan slugs, which functions to continuously vary the surface impedance of the capacitive layer, resulting in a continuous change in pass band as well as phase response for a select range of frequencies.

Figure 2.1: Frequency tunable and phase shifting surface using continuous injection of liquid metal slugs [8].

Where the previous method takes advantage of the continuous transport of liquid metal droplets, others utilize injection and removal of liquid metal as a switching scheme for electromagnetic effects.

In [9], dumbbell-shaped structures are etched into the ground plane of a microstrip transmission line. Injection of Galinstan into chambers bounding these structures lowers the effective inductance of the lattice and consequently raises the cutoff frequency of the transmission line. Hence, the device acts as a tunable low-pass filter. A carrier fluid acts to remove the oxide skin from the liquid metal, allowing it to retain fluidic properties so that it may be reversibly tuned by removing the Galinstan as desired.

Figure 2.2 Reconfigurable, reversible low-pass filter enabled by selective injection of Galinstan [9].

A similar method is employed in [10], where molded PDMS (Polydimethylsiloxane) defines a bullseye pattern of voids consisting of concentric circles. This structure is adhered to a gold foil containing a single subwavelength aperture. As EGaIn is selectively injected into different annular voids, the temporal response of the transmission profile varies under the influence of secondary oscillations supplied by surface plasmon polaritons generated on the surface of the liquid metal. The device is made reconfigurable by coating all surfaces with a fluorosilane solution, preventing adhesion issues notoriously associated with the gallium oxide skin. In this way, the annular voids may be filled and emptied as desired.

Figure 2.3 Reconfigurable plasmonic device using concentric circular channels patterned in PDMS, allowing selective injection of EGaIn [10].

Others have taken advantage of the mechanically stabilizing nature of the oxide skin to develop pressure-responsive devices. Dickey et al. in [11] demonstrated a frequency shifting

14 antenna enabled by this feature. Two volumes of EGaIn are segregated initially by “Laplace barriers,” i.e. posts which locally increase Laplace pressure with the effect that fluid beneath a desired pressure is restrained. This is combined with the stabilizing oxide skin which impedes fluid flow until a certain threshold pressure is reached which ruptures the skin. Once this threshold is reached, the segregated volumes of liquid metal are allowed to merge, effectively doubling the length of the antenna and effecting a corresponding change in resonant frequency.

Though the effect is irreversible, it may find applications in wireless sensing and security, for example.

Figure 2.4 Frequency shifting liquid metal antenna exhibiting pressure-responsiveness. EGaIn is initially stabilized by the use of Laplace barriers paired with the mechanically stabilizing oxide skin [11].

This technique is exploited in [12] as the tuning mechanism for a microstrip bandstop filter. Injected EGaIn forms the initial state of the transmission line, and further flow is impeded by an array of Laplace barriers as well as the stabilizing oxide film. As further pressure is applied, the skin ruptures and the liquid metal is allowed to flow past the first row of posts until it is blocked by another set of more narrowly-spaced posts. This may be repeated by applying further pressure, resulting in a three-state bandstop filter.

Figure 2.5 Pressure responsive bandstop filter utilizing Laplace barriers, integrated into a microstrip resonator [12].

A final method, termed Laplace pressure shaping, relies on evacuation of ambient air by a micro pump so that atmospheric pressure drives actuation of the liquid metal, and is

16 demonstrated in [13]. The device consists of liquid metal droplets sandwiched between a top plate and a bottom plate patterned with channels. The dimensions of these channels dictate the curvature that must be attained by the droplet in order for the liquid metal to wet into the channels. This in turn determines the required pressure according to the definition of Laplace pressure [14]:

ͳ ͳ ο݌ ൌ ߛሺ ൅ ሻ ܴଵ ܴଶ where ȟ݌ is the pressure difference across the boundary of the droplet, ߛ is the interfacial surface tension between the droplet and the ambient, and ܴଵ and ܴଶ are the principal radii of curvature of the droplet. The configurations attainable by such a device are determined merely by the patterning of the bottom plate, which may be achieved in many ways. Examples include switchable resistive networks and resonators. This technique yields fast switching speeds, up to ͵Ͷܿ݉ݏିଵ, and continuous tuning has been demonstrated by employing a slight design modification [15], consisting of tapered channels which requires increasing pressure for continued actuation.

Figure 2.6 Switchable resistive networks and resonators. The actuation method is Laplace pressure shaping, which relies on vacuum supplied by an external pump and microreplicated trenches [13].

Some limitations associated with the above techniques are now presented. Though control by syringe or external pump is simple and effective as a prototyping method, the use of such peripherals is a bulky hindrance in the actual application of microdevices. The available

18 functionalities of such devices are also limited in these cases by the amount of pressure which can be applied, and associated material problems may arise under high pressures. For example, Laplace pressure shaping relies on atmospheric pressure to promote actuation. This implies that the minimum attainable channel width is about ͳͲߤ݉, unless additional pressure is supplied [13]. Also, we see that some pressure-responsive tuning techniques, such as those incorporating Laplace barriers, yield only irreversible actuation. Finally, though complicated geometries may be achieved with the above methods, the requirement of physical channels bars any potential for robust reconfigurability.

2.2.2 Mechanical Actuation/Deformation

Closely related to several of the pressure-responsive devices outlined above, mechanically-actuated devices rely on the flexibility of Gallium alloys to provide strain- dependent tuning of electromagnetic response. These have applications as conformal and stretchable electronics, such as those desired for use in textiles and electronic paper.

Such stretchable liquid metal electronics typically involve a gallium alloy encased in a patterned elastomeric casing. Dickey et al. demonstrate [16] a deformable dipole antenna employing this architecture. Stretching the antenna changes its length and, thus, its frequency response. The dipole operates from 1910 – 1990 MHz, radiating at about 90% efficiency. This frequency range is achieved by length variation from 54 to 66 mm. The tuning method is completely reversible, and the oxide skin is exploited to promote self-healing. Any leak in the device will be spontaneously sealed by the stabilizing oxide.

Figure 2.7 Reversibly deformable dipole antenna relying on mechanical strain for frequency tuning [16].

In [17], a flexible microstrip patch antenna is presented which operates using similar principles. Two parallel conducting planes, separated by and encased in PDMS, define the ground and radiating planes of the patch antenna. Fabrication is facilitated by the use of

Laplace barriers so that the liquid metal properly fills the device in a serpentine fashion, eliminating trapped air and uneven fills. The reflection coefficient of the antenna may then be tuned by varying the curvature mechanically.

Figure 2.8 Flexible microstrip patch antenna filled with EGaIn. Tuning is enabled by mechanically varying the antenna's curvature [17].

Nahata et al. employ such methods in [18] to fabricate a plasmonic aperture array with tunable lattice spacing. A PDMS mold is created and EGaIn injected into it to define a 15 x 15 periodic array of holes in the liquid metal. Upon stretching in the x direction, with incident THz radiation polarized along the x direction, a shift in the anti-resonant peaks is observed, such that the associated wavelength varies linearly with the increase in lattice spacing. In addition, EGaIn is shown definitively to support surface plasmons, verifying its potential as a material for flexible plasmonic devices.

Figure 2.9 Subwavelength aperture array fabricated using PDMS and EGaIn. Transmission resonances may be tuned by mechanically straining the array [18].

As with the pressure-tunable technologies discussed above, such mechanically-tunable devices offer a simple, effective and reversible actuation method. Versatile reconfigurability cannot be achieved, though, as the attainable geometries are constrained by the pattern of the cast PDMS. In addition, the range of tunability is defined by the elastic limit of the elastomer.

It would be desirable to employ the precision, speed and versatility of electrostatic

actuation in the case of liquid metals. One challenge with this is that the electrostatic pressure

required to rupture the oxide skin on gallium alloys, which allows flexibility and tunability in the

above devices, yields unfeasible power requirements. Methods for overcoming this challenge

and the associated electrical actuation techniques are presented in the following sections.

2.2.3 Continuous Electrowetting

The phenomenon of electrocapillarity, famously discovered by Lippman, consists of the

lowering of the surface tension of a liquid metal droplet submerged in an electrolyte by

establishing an electrical potential across the interface. If a potential gradient exists along the

droplet, motion of the droplet can be induced [19], and this result is termed continuous

electrowetting (CEW).

Submerging a liquid metal droplet in an electrolyte causes a charge to accumulate at the

interface, establishing an electrical double layer, which electrically isolates the droplet from its

surroundings. This leads to a relationship between interfacial surface tension, ߛ and the applied

voltage, ܸǡ given by Lippman’s equation:

஼ ߛൌߛ െ ሺܸെܸሻଶ , ଴ ଶ ଴

where ߛ଴ is the maximum surface tension achieved at ܸൌܸ଴, and ܥ is the capacitance per area

of the electrical double layer.

At equilibrium, the positively charged liquid metal droplet is surrounded by a uniformly

negatively charged electrolyte, all encased in a channel. As a voltage is supplied between

electrodes on either side of the droplet, in contact with the electrolyte, a small current will flow

between the droplet and the channel walls. This causes a potential gradient along the length of

23 the liquid metal droplet, resulting in a surface tension gradient. This is the desired mechanism for inducing motion of the droplet.

Figure 2.10: Charge distribution of electric double layer along liquid metal slug, enabling continuous electrowetting [19].

A well-known example of this is a liquid micromotor developed at UCLA, using mercury as the liquid metal. The linear transport is given continuous driving functionality by wrapping a channel into a circular track formation. Issues with transport efficiency arise in CEW due to polarization of electrodes, limiting the long-term current output. This challenge is overcome by alternating the role of successive electrodes as driving electrodes, and thus regularly switching their polarity, which eliminates polarization effects. The micromotor offers a maximum rotational

24 speed of 420 r/min (using a 2-mm-diameter loop), using a voltage of 2.8 V and an average current of only 10 ߤܣ [19].

Figure 2.11: Liquid metal micromotor using continuous electrowetting of mercury slugs [19].

Various other difficulties associated with this technique are resolved in [20] to enable fabrication of a variable-frequency slot antenna. The first challenge is a result of the necessary low-friction environment in CEW, so that unintended slug displacement prevents precise positioning of the droplet. This is resolved by patterning the channel to provide selective areas where the droplet may minimize its surface area. Equivalently, the channel is patterned to provide variations in Laplace pressure, such that changes in surface tension must be induced

(by CEW) to compensate (see Fig. 2.11). The second challenge arises due to electrochemical oxidation which occurs on gallium alloys when exposed to the potentials required for CEW. This oxidation causes undesired adhesion to the channel walls and prevents fluid motion. The solution demonstrated by the group is to supply an alternating potential with DC bias. This

25 provides the necessary actuating potential while alternatively oxidizing and reducing the metal surface, inhibiting significant buildup of the oxide skin. The group then applies these techniques to the development of a frequency-tunable slot antenna which relies on capacitive coupling of a liquid metal slug to a copper feed line. Moving the slug varies the electrical length of the feed line, which shifts the resonant frequency due to reactive loading of the antenna. The tunable bandwidth is reported as 15.2%.

Figure 2.12: Variable frequency slot antenna, where tuning is achieved by changing liquid metal position and capacitive loading by continuous electrowetting. Patterned channel allows for discrete positioning of Galinstan [20].

Though electrocapillarity can be achieved for any liquid metal, properties unique to gallium alloys have allowed researchers to demonstrate novel phenomena similar in nature.

Dickey et al. have shown in [21] that the oxide acts as a surfactant to the liquid metal, lowering its surface tension dramatically from over 500 mN/m to nearly zero in some cases. By using a pH-neutral electrolyte, the group has demonstrated electrochemical oxidation and reduction of a liquid metal droplet in order to electrically tune the surface tension over this range. Using this technique they were able to demonstrate a reconfigurable monopole antenna in [22] where length change was induced by electrochemical deposition of oxide and subsequent capillary injection. Reversibility is achieved by reduction of the liquid metal surface, resulting in an increase of surface tension and withdrawal from the capillary. The monopole exhibits a large change in length from 4 mm to 75 mm, corresponding to a range of resonant frequencies from

0.66 GHz to 3.4 GHz, i.e. a tuning ratio of 5.2:1. The total radiation efficiency was measured to range from 41% to 70%.

Figure 2.13: Tunable and reversible monopole antenna relying on reduction of surface tension due to electrochemical oxidation of EGaIn. Length change is accomplished by associated capillary forces [21].

In addition to this application, Dickey’s group demonstrated in [23] a method to steer liquid metal through branched channels by using both CEW and electrochemical oxidation. By applying a cathodic potential to the liquid metal relative to a counter electrode, the liquid metal will be steered preferentially towards the counter electrode according to traditional CEW.

Applying an oxidative potential to the liquid metal deposits oxide on the surface which, although known to significantly lower the surface tension, eventually halts the flow as the growing oxide layer creates a mechanical impediment to flow. By this method complicated patterns and reversible reconfiguration may be achieved within the constraints of channel geometry.

Figure 2.14: EGaIn steered through branched channels by electrocapillarity paired with electrochemical oxidation to halt flow [23].

The CEW approach offers many benefits including reliability due to material compatibility

(oxide prevention and removal), low power requirements, moderate speeds and good control of transport. There are several characteristics which nevertheless act as a detriment to the desired applications. Most notably, the need for electrolytic environments introduces lossy conductive components into the device which may hinder signal ratios due to power dissipation. Also, in all cases some form of predefined channel is required, limiting the potential number of configurations. An alternative, known as electrowetting on dielectric, excels in several areas

28 where CEW is disadvantageous while offering its own material challenges, as will be discussed in the next section.

2.3 Electrowetting Theory

Electrowetting on dielectric (EWOD) is an extension of the phenomenon pioneered in

Gabriel Lippman’s classic experiment regarding electrocapillarity. The effect is qualitatively a reduction of a conductive droplet’s effective surface tension under the application of a capacitive potential, though in reality the true contact angle does not change. The following derivation is adapted from [14].

At the boundary of a droplet resting on a surface there are competing interfacial surface tensions of the three phases, the liquid droplet, the ambient gas, and the solid surface (see Fig.

2.15). Treating this as a force balance problem yields Young’s equation:

ఊೞೡିఊೞ೗ ܿ݋ݏߠ௒ ൌ ఊ೗ೡ

where ߛ௦௩, ߛ௦௟ and ߛ௟௩ are interfacial tensions between the solid-vapor, solid-liquid and liquid- vapor interfaces, respectively. ߠ௒ is Young’s angle, i.e. the equilibrium contact angle of the droplet.

Figure 2.16: Balance of interfacial surface tensions at the boundary of a droplet resting on a surface [14].

We first imagine placing a liquid metal droplet into an electrolytic solution. If a small

voltage ܸ݀ is applied to the droplet, an accumulation of an electric double layer on the surface

will result consisting of charges from the metal and counter-ions from the electrolyte. This

spontaneous process yields a reduction of the effective surface tension:

௘௙௙ ݀ߛ௦௟ ൌെߩ௦௟ܸ݀

where ߩ௦௟ is the counter-ion surface charge density. We then integrate this equation, assuming

all counter-ions are located a distance ݀ு from the droplet surface, so that the electric double

layer has a capacitance given by

ఌబఌೝ ܿு ൌ ௗಹ

denoting the dielectric constant of the liquid by ߝ௥. Integrating the change in effective surface

tension gives us the relationship between electrical potential and the effective surface tension:

௘௙௙ ௏ ෨ ఌబఌೝ ଶ ߛ௦௟ ሺܸሻ ൌߛ௦௟ െ ׬ ߩ௦௟ܸ݀ ൌߛ௦௟ െ ሺܸ െ ܸ௣௭௖ሻ ௏೛೥೎ ଶௗಹ

ܸ௣௭௖ is the potential of zero charge which arises spontaneously when a material is placed in an electrolyte solution. Substituting this value into Young’s equation gives us the following equation relating the contact angle of the droplet to the applied voltage:

ఌబఌೝ ଶ ܿ݋ݏߠ ൌ ܿ݋ݏߠ௒ ൅ ሺܸ െ ܸ௣௭௖ሻ ଶௗಹఊ೗ೡ

Above very modest voltages using this configuration, electrolysis sets in. An alternative configuration is thus used where the droplet is separated from the bottom electrode by a thin dielectric layer, where ߝௗ is the relative permittivity of the dielectric with thickness ݀. As the capacitance of this layer is much greater than that of the electric double layer, the total capacitance per unit area may be taken as:

ఌ ఌ ܿൌ బ ೏ ௗ

Assuming ܸ௣௭௖ is zero in this configuration, this leads to a reduction of the effective surface tension as:

௘௙௙ ௏ ఌ ఌ ௏మ ߛ ሺܸሻ ൌߛ െ ׬ ߩ ܸ݀෨ ൌߛ െ బ ೏ ௦௟ ௦௟ ଴ ௦௟ ௦௟ ଶௗ

Inserting this revised value into Young’s equation gives the following basic equation for EWOD:

మ ఌబఌ೏௏ ܿ݋ݏߠ ൌ ܿ݋ݏߠ௒ ൅ ଶௗఊ೗ೡ

We see that the reduction in contact angle of a conductive droplet depends on the relative permittivity and thickness of the dielectric layer, the surface tension of the droplet, and the applied voltage. This model has been shown to be valid up to a saturation point [24], a point which is currently under investigation.

Such a configuration requires the ambient fluid to be electrically insulating in order to establish the necessary electromotive force. The design challenges implicated by this are included in the next section.

2.4 Electrowetting Applications and Challenges

Since the fundamental demonstration of contact angle modulation by EWOD, many capabilities have been realized which illustrate its importance in the field of microfluidics. In combination with digital reconfiguration schemes, EWOD offers unprecedented precision and versatility in the control of droplets. Exploiting these capabilities has yielded applications spanning switching components and electronics [25], biofluid mixing and dispensing for lab-on- a-chip [26][27], reflective displays [28][29] and beam steering [30][31], among many others.

One of the most well-known commercial applications of EWOD uses the simplest case of contact modulation to vary the curvature of a liquid droplet, resulting in a variable-focus liquid lens [32]. Similarly, beam steering may be achieved by modifying the contact angle differently with respect to different surfaces, thus changing the surface profile of a droplet, resulting in variable electrowetting microprisms [31].

Figure 2.17: Variable focus liquid lens, where focal length is modulated by varying the contact angle of a droplet through electrowetting [32].

Figure 2.18: Beam steering by electrowetting. Liquid microprisms are manipulated by electrowetting on each of four side walls, tilting the planes of refraction [31].

Fundamental electronic components may be realized using electrowetting. An example is a liquid-state field-effect transistor which operates by switchable displacement of an insulating oil by a conductive fluid, which then operates as the conductive medium [25].

Figure 2.19: Liquid state field-effect transistor utilizing electrowetting of a conductive fluid to displace an oil droplet. The fluid then acts as the conductive medium [25].

In [33], Kim et al. derived the relationship between electrostatic contact angle modulation and electromotive pressure for a droplet confined in a channel, using a parallel plate geometry.

This type of control enables the pulling of droplets into a channel, and transport of discrete droplets may be achieved by controlled addressing of the driving electrodes. Such precision also allows pixel-level control of fluid shape. The technique was extended in [34] by theoretical treatments of the creation, transport, cutting and merging of liquid droplets by electrowetting.

Using these design parameters and actuation techniques, various groups have demonstrated digital microfluidic biosensors [26][27] which enable sample dispensing, mixing and analysis in a single device.

Droplet actuation and control of droplet shape has also allowed display capabilities,

offering devices with large potential owing to their flexibility, reflectance, contrast ratios, and efficiency. Actuation methods include displacement of colored oils by an electrowetted fluid [28]

and transposition of pigment dispersions to effect large surface area changes in the visibly

34 active material [29]. Such devices have been shown to be able to function at video speeds [35],

offering a competitive technology for the field of displays and e-readers.

Figure 2.20: Transposition of colored inks by electrowetting in order to enact large surface area changes, for use in reflective displays [29].

Bistability becomes a critical issue for such devices when occupation of different states requires excessive power consumption, i.e. when a desired configuration is out of equilibrium under zero voltage. Such a case is often encountered when more than a momentary device response is required. As mentioned above in the case of pressure actuation, the use of Laplace barriers has enabled metastable fluid volumes by increasing local Laplace pressure to hinder dewetting [11]. These structures are similarly utilized in [36], where partial-post Laplace barriers are fully investigated and optimized for use in bistable electrowetting displays. The effect of these barriers is termed “virtual confinement” due to the fact that fluids may be indefinitely supported as though by channels. The freedom offered by such virtual channels is a feature unavailable to other actuation methods, indicating that EWOD may be a preferred route to sophisticated reconfiguration.

Figure 2.21: Demonstration of bistability in an electrowetting device utilizing partial-post Laplace barriers [36].

Reconfiguration of discrete droplets has also been investigated in many cases [37][38] using programmable 2-D electrode arrays. Different electrode pixels may be selectively activated by various addressing methods in order to determine the wetting pattern, which may be arbitrarily user-defined. This technique, termed “digital microfluidics,” has been combined with the Laplace barriers mentioned above to enable versatile shape change by electrowetting.

In this way, bistable fluid configurations may be achieved which are limited only by electrode pixel resolution.

Figure 2.22: Laplace barriers used in combination with addressable 2-D electrode arrays, allowing programmable and bistable transport and shape configurations [38].

Achieving actuation of liquid metals by EWOD introduces several challenges. Upon inspection of the electrowetting equation, it is evident that voltage requirements for EWOD actuation scale with surface tension. The high surface tension of liquid metals (> 500 mN/m [5]) implies that relatively high voltages will be required (see next section). Such voltage levels yield high power consumption, especially if methods are not employed to enforce zero-power bistability. These high voltages are also often near the breakdown voltage of dielectric films fabricated at thicknesses typical for aqueous electrowetting. The result is that thicker dielectric films must be incorporated into the design, yielding even higher voltage requirements. It should be noted that, according to the electrowetting equation, a decrease in necessary voltage is achieved by using dielectric materials with higher values of ߝ௥. In practice, such high-permittivity

37 materials are often unreliable due to porosity and defects, so that aqueous fluids easily penetrate and cause breakdown. It has yet to be seen whether this is true in the case of liquid metals.

2.5 Electrowetting of Liquid Metals

Despite such high expected power requirements, several groups have demonstrated electrowetting of mercury as a viable option in device design. Feinerman et al. have shown reversible electrowetting of mercury on Teflon®-coated Parylene films [39]. A contact angle change of 30 degrees was demonstrated with voltages as low as 25 V using a 0.2 ߤ݉ thick film of tantalum oxide, which possesses a relative dielectric constant of 25. In a different paper [40], the group reports a piston-motion micromirror which relies on contact angle change of a mercury droplet by electrowetting to change the height of a micromirror which rests on top of the droplet. The group reports a maximum displacement of 60 ߤ݉, requiring merely 75 V. The group also states that electrowetting of a gallium alloy was achieved by removing oxide with nitric acid followed by immersion in vacuum pump oil, though this method was not effective enough to gather reliable data.

Figure 2.23: Demonstration of contact angle change of mercury droplet under applied electrowetting voltage [39].

Figure 2.24: Variable-height micromirror actuated by electrowetting of mercury droplet [40].

In [41], a fast (60 ߤݏ latency) microswitch is demonstrated which utilizes contact line sliding of a mercury droplet as the switching mechanism. Many design parameters are addressed and optimized in this work, such as switching speed, confinement geometry, and uncertainty allowances. The signal rise and fall times are measured to be 5 ߤݏ, and the common issues of switch bounce and contact degradation are shown to be absent or minimized.

Figure 2.25: Liquid metal switch, where switching is achieved by contact-line sliding of a liquid metal droplet as a voltage is applied [41].

An application related to those presented in previous sections, using alternative actuation methods is presented in [42]. In this work the author integrates an EWOD platform onto an annular slot antenna. Lowering the contact angle of a mercury droplet increases the

40 contact area, which varies the loading capacitance. The result is a tuning mechanism for the frequency response of the slot antenna, allowing a variation in operating frequency from 11 to

13 GHz. The operating voltage used in the work is approximately 140 V.

Figure 2.26: Frequency reconfigurable annular slot antenna, where tuning is achieved as the contact angle of a mercury droplet is changed by electrowetting, varying the loading capacitance [42].

The effect of oxide on the mechanical properties of gallium alloys has been discussed above. The viscoelastic response and highly wetting nature of the oxide, though advantageous in certain designs, renders electrowetting ineffective to the point that actuation cannot be achieved. The oxide may be (theoretically) prevented by working in an oxygen-deficient environment. For example, in [43] Kim et al. characterize the gallium alloy Galinstan by performing all measurements in a glove box with less than 0.5 ppm oxygen levels. They were able to accurately measure surface tension values using the pendant drop method after qualitatively observing droplet characteristics under varying oxygen levels. EWOD contact angle measurements are also recorded in this work, indicating that reliable EWOD may be achieved under a proper inert atmosphere. Similar characterization has not been performed for the binary

41 alloy EGaIn, though EGaIn has been seen empirically to oxidize at a significant rate under such conditions (see next chapter).

Experiments have shown such oxide prevention to be difficult to infeasible, so that the conventional method is to actively remove the oxide by immersing the metal in either an acidic or a basic solution, or in an ambient containing acid vapor. Whereas this method is acceptable in many applications such as CEW or Laplace pressure shaping, the requirement of an electrically insulating environment bars the use of such electrolytic materials in EWOD. Thus, practical and reliable devices using electrowetting of non-toxic gallium alloys have been nearly prohibited.

In [44], a tunable RF MEMS resonator is presented, where resonant frequency tuning is achieved by electrowetting a Galinstan droplet. Operating voltage is kept below 100 V, while a frequency range of 12 to 18 GHz is achieved, with quality factor within the range of 1400 to

1840. Nothing is mentioned in this work regarding oxidation of the Galinstan droplet, so that sustained integrity and reliability of the device is doubtful.

Methods for overcoming these challenge and associated design parameters are presented in the next chapter, along with a device demonstration which indicates the potential of such a versatile actuation method for the reconfiguration of non-toxic liquid metal.

Chapter 3: Electrowetting Actuation of Liquid Metal Wires for Applications such as Switches and Polarizers

3.1 Introduction

In the previous chapters, the demand for safe liquid metal electronics was discussed, as well as the potential benefits encouraging the use of such a material, such as flexibility and reconfigurability. In this discussion, various conventional actuation methods were examined, and their associated strengths and challenges indicated. The question of versatile, user-defined reconfiguration has been approached, and a natural avenue to such an end has been identified as electrowetting, which offers robust reconfiguration and added benefits when certain design modifications are included. Until now, reliable electrowetting of gallium alloys has been barred by material challenges, most notably surface oxidation and high surface tension. In this chapter, these challenges are again reviewed, and a novel material is introduced which allows reliable electrowetting by simultaneously removing the oxide skin and providing an insulating ambient.

The electrical response of EGaIn in this oil is characterized, and a partial confinement design is presented which significantly reduces voltage requirements. Theoretical background is provided which justifies this design, and material compatibility issues are fully explored. Application demonstrations are included in the form of a switchable wire-grid polarizer and a liquid metal switch.

3.2 Background

Reconfiguration schemes for electronic and wave transmission components often suffer from rigid and lossy components, as well as limited tunability or functionality options. For example, digitally configurable electrode array antennas may require switching diodes to

43 achieve effective length change, therefore incorporating lossy semiconductor elements into the radiating structure [45][46][47][48]. Others require clever placement of parasitic elements relative to a fixed antenna structure, offering modest variation of the frequency response or radiation pattern [49][50]. In contrast, liquid metals, such as eutectic Gallium Indium (EGaIn), offer the potential for true physical reconfigurability[6], for physical flexibility [17][51], and conductivity allowing for high radiation efficiencies [16]. To this end, recent work has demonstrated various effective actuation methods. Laplace pressure shaping [13][15] and pressure-driven flow [18], though capable of fast actuation, require additional peripherals such as pumps or syringes. Others have demonstrated continuous electrocapillarity or redox tuning of surface energy [22][21][52][20] as possible routes. These devices nicely eliminate the need for peripheral controls, and can operate using only several volts. However, for reconfigurable electronic or electromagnetic applications, these same techniques can be limited in switching speed, require immersion in electrically lossy electrolytic solutions, may have limited operation cycles, and require physical side-wall confinement such as capillary or rectangular microchannels.

A potentially attractive alternative for liquid metal actuation is electrowetting which, in theory, can resolve the challenges that exist for electrocapillarity or redox tuning approaches.

Electrowetting in other applications has allowed for automated transport and mixing of biofluids

[26], microprism arrays [30], variable-focus lenses [32], and reflective displays [29]. The phenomenon relies on variation of droplet contact angle by electromechanical force.

Electrowetting of liquid metal has been investigated previously [39][40][53][41], but was demonstrated using toxic Hg instead of non-toxic eutectic GaIn (EGaIn).

Our own initial attempts with electrowetting of liquid metals have revealed two prominent obstacles. Firstly, for Hg or EGaIn, there is a prohibitively large contact angle change required to cause reversible capillary wetting/dewetting into and out of a channel which, due to the high

44 surface tension of liquid metal (>500 mN/m[5]), results in excessive voltage requirements (>1 kV), often beyond or near the breakdown limit of practical dielectric films. Further complicating matters, when attempting to use non-toxic EGaIn, rapid oxide formation occurs on the EGaIn surface which renders electrowetting inoperable. Simply using an inert gas is impractical as even < 1 ppm oxygen levels over time will cause oxide formation. Using an acidic background for chemical removal [54][6] of the oxide skin that forms is also unusable with electrowetting, because an aqueous annulus forms near the electrowetting contact line, which will electrowet instead of the liquid metal.

We report here fully integrated devices that resolve all of these previous challenges for liquid metal electrowetting, and furthermore, we demonstrate functional devices in electronic switches and in electromagnetic polarizers that can be enabled or erased on demand (Fig. 1). The enabling innovations include a geometric microchannel design which greatly reduces the required contact angle range needed for actuation, and use of a novel acidic and electrically insulating oil which surrounds the EGaIn liquid metal [55]. The switch demonstration provides a very high on/off ratio of ʹǤͲͻͷ ൈ ͳͲଵଶ and requires only liquid-to-liquid breakable contact which could lengthen lifetime compared to mechanical relays. The switchable wire-grid polarizer provides an average signal attenuation of 12.91 dB in the on state and 1.46 dB in the off state, over the range of 8-9.2 GHz, with a switching speed of about 12 ms.

This work bridges the gap between two areas of vast potential in the field of microfluidics, with future applications including shape-changing antennas and metamaterials, as well as electrode array addressing for user-defined reconfiguration [56][57].

3.3 Materials and Device Fabrication

Electrowetting Characterization: EGaIn was used as purchased (Aldrich, Gallium-Indium eutectic, >99.99% trace metals basis). Contact angle characterization with voltage was performed on glass substrates coated with In2O3:SnO2 (Kaivo, < 10 Ω/square), 3.8 ߤ݉ of chemical vapor deposited Parylene C (Specialty Coating Systems, ߝ௥ ൌ͵Ǥͳ), and a top monolayer of hydrophobic FluoroPel PFC 1601V (Cytonix). The monolayer was created using a surface grafting technique, described elsewhere [58]. Electrowetting was performed in an open bath of specially formulated acidic silicone oil. Synthesis and characterization of this oil has been performed in detail by our group and is reported elsewhere [55]. More details on the oil will also be provided in the experimental results section. Contact angle change was driven by a 1 kHz square wave voltage. Contact angle measurements were performed using a VCA Optima contact angle analysis system.

Device Demonstration: The polarizer fabrication consists of a bottom plate supporting channels aligned with patterned bottom electrodes, and a top plate with thin ridges patterned perpendicular to the bottom channels (Fig. 1). The channels help to confine the liquid metal droplets, while the ridges create areas of higher Laplace pressure, which cause the droplets to dewet preferentially into the areas of lower Laplace pressure (see Theory section and Fig. 1).

For the polarizer experiments, the In2O3:SnO2 was found to be too electrically conductive/lossy, such that transmitted signal strength was low and the observed signal changes due to polarization could not be observed. Alternative electrode materials with minimal transmission losses were therefore chosen according to measurement (see results section, Fig.

5).

Figure 3.1: Device diagrams and relevant dimensions for switchable polarizer.

The bottom plate of the polarizer was fabricated by first spin-coating positive photoresist

(Microposit S1818) onto a glass slide, then patterning with 500 mJ/cm2 of i-line energy in the negative of the electrode pattern. The electrode pattern consists of a series of parallel stripes,

475 ߤ݉ in width, connected by a single contact pad at the bottom. A thin layer of gold was then sputtered onto the glass/photoresist structure and patterned using the lift-off technique.

Next, channels were formed on the bottom plate by spin-coating 72.3% solids SU-8

3000 series (Microchem) negative photoresist at 1800 rpm, yielding a thickness of 40ߤ݉, followed by a soft bake at 100° C for 15 minutes. Channels with width 958 ߤ݉ were aligned and exposed as above, and then a post exposure bake was performed for 5 minutes at 100° C.

Finally, the photoresist was developed in PGMEA, and the entire structure hard baked at 180° C for 30 minutes.

Next, a 3.5 ߤ݉ thick coating of Parylene C and monolayer of FluoroPel PFC 1601V were deposited as previously described for the electrowetting test plates.

The top plate consists of Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate

(PEDOT:PSS) (Aldrich, 1.3 wt% dispersion in H2O) as the electrode material. This was spun on at 2500 rpm, and then baked at 120° C for 15 minutes. This was repeated twice in order to achieve the desired conductivity. The sample was then heated under vacuum for 30 minutes at

110° C, in order to remove any water absorbed into the PEDOT:PSS, which is hygroscopic.

Ridges of thickness 10 ߤ݉, width 1.46 mm, and spacing 2.46 mm were patterned by spinning 50% solids SU-8 3000 series negative resist at 1000 rpm onto the cured PEDOT:PSS, then baking and patterning as described above. This structure was coated as above with a monolayer of FluoroPel PFC 1601V to achieve hydrophobicity.

Polarization measurements consisted of a signal from a Hewlett-Packard 8684B Signal

Generator being fed through an X-band waveguide and emitted by a Budd Stanley E-plane

48 sectoral horn antenna. Signal amplitude values were obtained using an IFR AN930 spectrum analyzer. High speed photos were taken using a Fastec Troubleshooter camera.

3.4 Theory & Design

Electrowetting involves an apparent change in contact angle of an electrically conductive fluid under an applied voltage [14]. The so-called electrowetting equation relates interfacial surface tensions to the equilibrium contact angle of a sessile drop:

ଶ ߝ଴ߝ௥ܸ ܿ݋ݏߠ௏ ൌ ܿ݋ݏߠ௒ ൅ ሺͳሻ ߛ௖௜ݐʹ

where ߝ଴ is the permittivity of free space, ߝ௥ is the relative permittivity of the dielectric used, ߛ௖௜ is

the interfacial surface tension between the conducting fluid and the insulating ambient, ݐ is the thickness of the dielectric coating, ܸ is the applied voltage between the bottom and top electrode, and ߠ௒ is the resting Young’s contact angle under zero applied potential.

Laplace pressure ο݌, the pressure difference across the boundary of a liquid, is related to the shape of a droplet by the Young-Laplace equation[14]:

ͳ ͳ ο݌ ൌ ߛ௖௜ ൬ ൅ ൰ሺʹሻ ܴଵ ܴଶ

where ܴଵ and ܴଶ are the principal radii of curvature of the droplet.

In electrowetting, the electrostatic driving pressure on a conducting droplet in a channel of height ܪ may be expressed as [33]:

ߝ ߝ ܸଶ ο݌ ൌ ଴ ௥ ሺ͵ሻ ݐܪʹ ௘

By inspection of Figure 1, a droplet confined in a channel has a horizontal radius of

curvature equal to half of the channel width ݓ when the electric field is zero. This is a reasonable assumption for EGaIn given its very high surface tension which should result in a

180° Young’s angle on the fluoropolymer surface. In experimental photos and data, the only reason that the Young’s angle may appear or be listed lower is because of the effects of gravity and the finite measurement capabilities of contact angle measurement by photography.

Continuing our theoretical analysis, we see therefore that ܴுଵ ൌݓȀʹ. Likewise, the vertical radius of curvature under no applied potential is equal to half of the total cell height, which is equal to the sum of the height of the channels on the bottom plate,݄, and the thickness of the ridges on the top plate, ܿ: ܴ௏ଵ ൌሺ݄൅ܿሻȀʹ.

When enough electrostatic pressure is applied, the EGaIn droplet will wet under the ridge and form to the electrode, giving associated radii of curvature ܴுଶ ൌ݀Ȁʹ and

ܴ௏ଶ ൌ݄Ȁʹ.

Thus, each droplet will wet as desired (Fig. 1) once the supplied electrostatic pressure added to the resting initial Laplace pressure exceeds the Laplace pressure of the droplet in its wetted, merged state. Mathematically, this condition can be expressed as:

ʹߛ ʹߛ ߝ ߝ ܸଶ ʹߛ ʹߛ ௖௜ ൅ ௖௜ ൅ ଴ ௥ ൐ ௖௜ ൅ ௖௜ ሺͶሻ ݀ ݄ ൅ܿ ݓ ʹሺ݄൅ܿሻݐ݄

It is important to note that the previous discussion, and eq. 4, underpins how we reduce the required electrowetting voltage for such a high surface tension conducting fluid (~10ൈ greater than water). Simply, because the EGaIn is confined, it is presented with a starting Laplace pressure which aids our goal of electromechanically changing its shape into features such as wires. As noted in the introduction, electrowetting a liquid metal into a capillary or open-face rectangular channel would require a contact transition to less than 90°, which in our experience is often prohibitive in terms of voltage and dielectric reliability.

In our design, voltage requirements due to variation in channel height were minimized by making dewetting ridges wide and thin, since dewetting reliability increases both with ridge width and thickness, but electrostatic pressure requirements increase only with ridge thickness.

Volume conservation placed a constraint on ridge width, so that channels were made wider than the patterned electrodes in order to accommodate the necessary length change of each droplet.

Lastly, the above theory and next section of experimental results reveal the ability to create high on/off ratio electromechanical switches. Such switches would only have liquid-to- liquid contact, which could avoid known degradation issues in solid contact switches [59]. Using a value of ͵ǤͶ ൈͳͲ଺ܵ݉ିଵ for the conductivity of EGaIn [6] and the device dimensions detailed above, we predict a contact resistance of only 0Ǥ Ͳͷ͵ͳȳ in the on state, and an off-state resistance of ͳǤͳͳ ൈ ͳͲଵଵȳ, using resistivity values for the acidic oil given in [55] and measured using a 100 Hz AC signal at 355 V. This yields a high current on/off ratio of ʹǤͲͻͷ ൈ ͳͲଵଶ.

3.5 Results

Reliable electrowetting of EGaIn was first demonstrated using the standard approach of measuring contact angle variation with applied potential, as predicted by equation (1). The electrowetting response data shown in Figure 2 has several curves corresponding to varying

HCl concentrations in the silicone oil [55]. The black curve corresponds to the theoretical response predicted by the electrowetting equation, using a surface tension of 624 mN/m.

Figure 3.2: Contact angle change of EGaIn vs. AC voltage.

To make the oil which enables electrowetting, HCl is added to silicone oil until the oil is saturated, i.e. until further HCl is no longer integrated into the structure, yielding a maximum molarity of 1.5M [55]. We define percent concentrations as wt% HCl in the oil system.

At 0.4% and 0.6% concentrations, the experimental curves adhere closely to the theoretical curve. For the purpose of simplicity, our tests were performed in open air, so that

52 after the initial oxygen-free oil is dispensed, at some point there will be a diffusion rate of oxygen through the oil to the EGaIn that will exceed the diffusion rate of HCl to the EGaIn. In a closed system, this would not occur. Therefore, the results of Fig. 2 were obtained within 5 minutes of experimental setup, to ensure that the EGaIn remained oxide free.

At 0.8% HCl concentration, we see a departure of the voltage response from the predicted values at high voltages. This is most likely due to charging effects in the oil due to increased HCl and water content. This divergence was accompanied by visible oscillations of the droplet.

The 1% concentration curve terminated above 200 V due to droplet oscillations becoming so significant that the droplet jumps from the probe. This represents an upper bound for the range of practical concentrations when electrical insulation is required. It should be noted that we fully expect that in a sealed system, a larger range of acid concentrations would be permitted.

Figure 3.3: Wetting and dewetting speeds in polarizer.

In order to quantify switching speeds for the polarizer demonstration (Fig. 1), droplet edge velocity was measured using a high speed camera. Fig. 3 plots the length change of the EGaIn as a function of time. For these plots a voltage of 330 V was utilized, corresponding to an expected minimum contact angle of 110° based on the data of Fig. 2. A maximum length change of about 78% is observed over 12 ms, corresponding to the switch on time. A similar response is observed in the case of dewetting, or switching off. This implies a wetting speed of

10.3 cm/s, which is typical of speeds seen in electrowetting [14] with much greater changes in

54 contact angle (here, the contact angle change is smaller, but the surface tension is much higher).

Photos of the fully functioning wire-grid polarizer device are provided in Fig. 4, with prominent device features indicated by dotted white lines. Mercury was used in this example, due to its reliability. One of the challenges with arrayed electrowetting devices, such as a wire- grid polarizer, is dosing discrete volumes of fluids. In our previous work, we have developed self-assembly dosing techniques for electrowetting [60] and electrofluidic displays [61][29].

Here, we again demonstrate a self-assembly approach to initially dose the EGaIn into the device. As shown in Fig.4a, self-assembled dosing of the device is enabled by the effect of the electrowetting electrodes in combination with a reservoir at the bottom of the device which spans all of the stripe electrodes. Simply, the liquid metal is pulled into the channels by electrowetting (Fig. 4a), and when the voltage is removed, is deterministically split into discrete droplets by the greater Laplace pressure imparted by the top SU-8 ridge (Fig. 4b). The transition between the electrowetted wire-grid polarizer state of Fig. 4c and the discrete droplet array of

Fig. 4b was reversible and fast (~12 ms, Fig. 3).

As mentioned in the materials and fabrication section, before the device could be tested as a wire-grid polarizer, a replacement for the In2O3:SnO2 (<10 Ω/square) was needed because of excessive transmission loss for the GHz test frequencies used. Figure 5 illustrates the transmission profiles for various materials tested. As expected, the most resistive materials exhibited the least amount of attenuation. For this reason, a thin layer of sputtered gold (<50 nm) was employed for the bottom electrodes due to ease of fabrication. PEDOT:PSS conductive polymer was used as the top electrode due to its high resistivity (low attenuation), availability in dispersion form, and potential application in flexible electronics. Most importantly, compatibility with EGaIn required that the bare top electrode be of a non-alloying material.

Conductive polymers, such as PEDOT:PSS, are a natural candidate in this regard.

Figure 3.4: Photos of operating device, including (a) initial self-loading, (b) with voltage off, and (c) with voltage on.

After substituting the above described materials into the device, polarizer attenuation (using

Hg) was measured in the X-band using standard gain horn antennas. The device was actuated with 330 V AC, using a frequency of 100 Hz, in order to allow for proper capacitive discharge while still preventing dielectric charging [62]. Results are depicted in Figure 6 for the polarizer both off and on, with LM “wires” oriented both perpendicular and parallel to the polarization of the electric field. When aligned perpendicular to the electric field polarization, there was no observed signal attenuation, which indicates that losses due to reflection are negligible. When aligned parallel to the electric field polarization, the device exhibits an average attenuation of

12.91 dB in the on state and 1.464 dB in the off state over the range 8-9.2 GHz. As comparison, a patterned copper polarizer of similar dimensions offered an average signal attenuation of

14.51 dB over the same range.

Figure 3.5: Signal transmission for various electrode materials including Fluorine doped Tin Oxide (FTO) and Indium Tin Oxide (ITO).

Device actuation was then tested with EGaIn in a glovebox held at 0.5 ppm oxygen, low enough to theoretically prevent oxidation [43]. In practice, droplets oxidized within minutes.

Slight design modifications were thus made in order to hold the plates together by nuts and

57 bolts, creating a moderate seal. Inlet and outlet ports were also machined into the substrates, so that the liquid metal and oil could simply be injected directly into the device. Reversible actuation was thus enabled, as shown in Fig. 7, though device lifetime is still limited due to inescapable oxygen diffusion. It is thus seen that reliable and reversible actuation of EGaIn in practice requires the assembly of a sealed device in an inert atmosphere in concert with acidic oil, as used in this work.

Figure 3.6: Transmission results for device on and off, with the liquid metal wires (a) perpendicular and (b) parallel to the electric field polarization.

Figure 3.7: Switchable EGaIn polarizer in the (a) off state and in the (b) on state.

3.6 Discussion

If the mechanism demonstrated in this paper is to be widely employed, several remaining challenges should be mentioned. To promote device longevity, devices should be assembled and sealed in a low-oxygen environment, in order to prevent acid evaporation, oxygen diffusion and the resulting oxidation of EGaIn. We have found that certain acids react with EGaIn in undesirable ways [55], such that lower concentrations are preferred, or else alternative formulations must be used to achieve longer device lifetimes. Also, lower acid concentrations will decrease the amount of acid which partitions into the dielectric materials, which is preferred from a reliability perspective.

We note that electrowetting of Galinstan was enabled in [43] by preventing oxide formation through use of an oxygen-deficient glove box. We have seen oxide formation occur on a time scale of minutes even under such conditions, so that some form of oxide-removing material will be necessary in any reliable device. Electrowetting of Galinstan was reported in

[40] and [44], though little to no information was given on methods for long-term oxide removal.

We suspect that the acidic oil approach of this paper is necessary, unless another approach is developed which can remove and/or prevent EGaIn oxide.

Some improvements can be expected if one were to shrink device dimensions. As switching speed is determined partially by ridge width, a decrease in this quantity would result in an increased switching speed. Of course, smaller device dimensions would dictate a higher- frequency polarization response than that observed here.

Regarding device design, sputtered gold was used as the bottom electrodes due to its ease of patterning. The 1.464 dB signal loss in the parallel, off state is most likely caused by this gold layer. Substitution of patterned PEDOT:PSS would adequately reduce these off-state losses. Methods of patterning PEDOT:PSS have been demonstrated in [63][64].

It was confirmed that device functionality is very sensitive to the volume ratio of acidic oil to EGaIn, as this value dictates the rate at which the oxide is removed. Best results were achieved by removing channels from the design, thus increasing the volume available to the acidic oil. Nevertheless, acid depletion will always remain a problem. A proposed method to remedy this issue is enabled by proper sealing techniques in combination with an integrated reservoir of HCl, such that acid may be supplied in a controlled manner and the concentration be maintained at levels appropriate to supersede oxygen diffusion.

Finally, the large voltage requirements for device operation may discourage use in some applications. However, in the actuated (polarizer) state the electrical power is quite low at

8.5ܹ݉Ȁܿ݉ଶ.

3.7 Conclusion

Reliable electrowetting of the non-toxic liquid metal EGaIn has been performed for the first time in this work, using a newly-developed oil which prevents oxidation while remaining electrically insulating. A switchable polarizer utilizing this actuation technique has been demonstrated which offers an average signal attenuation of 12.91 dB over the frequency range

8-9.2 GHz, with switching speeds of about 12 ms. We also discuss how such a design may be applied to the fabrication of an electro-mechanical switch with high on/off signal ratios of

ʹǤͲͻͷ ൈ ͳͲଵଶ. This method lays the groundwork for future devices allowing arbitrary physical reconfiguration of liquid metal components. This has large potential in areas where increased efficiency requires active components to be segregated from the actuation components, such as frequency-shifting antennas, electronic switches, variable impedance transmission lines, and switchable metamaterials.

Chapter 4: Conclusions and Future Work

4.1 Current Work

In this work, various methods for actuation of liquid metals have been described, as well as associated applications. Contemporary techniques include continuous electrowetting and electrocapillarity, mechanical actuation, and pneumatic or pressure-driven actuation.

Applications include a liquid metal micromotor, switchable resistive networks, tunable slot antennas and frequency selective surfaces, among others. Successful electrowetting of gallium alloys is hindered by a mechanically robust oxide skin, the removal of which traditionally requires ionic and intrinsically conductive materials. In addition, the high surface tension of gallium alloys often results in prohibitively high voltage requirements, approaching the limitations of available materials. Here, a novel acidic silicone oil is employed to provide an insulating environment which prevents oxide formation, and remains electrically insulating. In addition, a device mechanism is designed and demonstrated which significantly lowers voltage requirements for electrowetting of liquid metals. The design is applied in the form of a switchable wire-grid polarizer which provides an average signal attenuation of 12.92 dB in the frequency range of 8 – 9.2 GHz. In addition, a mechanical switch is demonstrated which provides signal on/off ratios of ʹǤͲͻͷ ൈ ͳͲଵଶ.

Though the above methods resolve several issues associated with electrowetting of gallium alloys, widespread implementation and long-term functionality require further device modifications and precautions, described in the next section.

4.2 Further Challenges

Reliable functionality of devices implies the need for long-term stability and many switching or actuation cycles. As oxidation of a liquid metal droplet is a dynamic process, diffusion of oxygen into the acidic oil will increase the rate of chemical reaction and will slowly deplete the available materials, therefore lowering the acid concentration and the effectiveness of the oil. Therefore, droplet oxidation remains the limiting factor for this technology. Practical use of this technique will require minimization of ambient oxygen levels as well as oxygen levels in the silicone oil. Most importantly, proper and effective sealing techniques must be employed in order to minimize or prevent oxygen diffusion into the oil and HCl diffusion out of the oil.

Some results have been shown in [55] which indicate that similar effects may be attained by using acids with lower vapor pressure, such as bromic acid. Similarly, acid depletion should be minimized by proper device design. Delivery routes of the oil must be incorporated geometrically in the design, so that the amount of oxidized metal does not exceed the amount of HCl available for its removal. In all of these cases, methods for replenishing acid content would allow prolonged device functionality.

Though acidic oil has been shown to allow liquid metal actuation, material corrosion must be considered for long-term operation. Though the oil may be considered a very dilute solution (<1.5 M [55]), chemically inert materials will surely prove beneficial over time. Such a requirement places further constraints on device design, yet has not been fully investigated in this work.

Voltage must also be minimized if this technology is to be integrated into widespread use. As explained above, though voltage requirements have largely been reduced by appropriate device design, they remain high relative to those used in conventional electrowetting applications and consumer electronics. The most common ways to reduce

63 voltage requirements, as dictated by the electrowetting equation, are to reduce surface tension, to reduce the thickness of the dielectric layer, and to use a dielectric with a higher relative permittivity, ߝ௥. The feasibility of each of these methods will now be discussed.

Surface tension of an aqueous fluid is conventionally reduced by the addition of a surfactant, such as sodium dodecyl sulfate. A corresponding method in the case of liquid metals is the use of thiols, which have been shown to reduce the effective surface tension of liquid metals, though the exact mechanism responsible is uncertain, and is currently under investigation [65]. Furthermore, elasticity or dewetting response of the droplet has been seen to be hindered by the use of thiols, so that a trade-off is evident between surface tension reduction and response speed.

Reduction of dielectric thickness will further reduce voltage requirements for electrowetting. Though many successful designs have been reported which utilize these tactics

[66][67], such devices have been seen experimentally to degrade and break down over time due to pinholes and defects inherent to the materials [68]. As thin layers are more vulnerable to electrostatic infiltration of aqueous fluids used in such devices, long term use has demanded thicker layers in order to encourage reliability. In addition, dielectric charging due to prolonged voltage application is ameliorated by using thicker layers.

Similarly, required voltage may be lowered by increasing the dielectric constant of the dielectric film. Existing materials such as tantalum pentoxide (ߝ௥ > 20 [69]) and barium titanate

(ߝ௥ > 1000 [70]) would offer significant voltage reduction. These may be deposited, for example, by atomic layer deposition. Experimentally, such materials are seen to be unreliable in terms of durability and film quality, so that pinholes and defects promote breakdown after a modest number of actuation cycles [68].

Breakdown due to defects and pinholes has been shown to be greatly reduced by using a two-layer barrier approach [68], in which two or more dielectric materials are layered as a composite dielectric stack. Any defects present in each material will presumably not coincide spatially, so that a tortuous path is created for aqueous materials, greatly hindering transport of the materials to the electrode and the subsequent electrolysis which ensues. Another method has been employed [71] which functions as a self-healing dielectric, repairing areas of infiltration or breakdown nearly instantaneously. This relies on the anodic growing of aluminum oxide by the applied electrowetting voltage and a near-neutral electrolytic fluid. Using an aluminum electrode, self-healing commences when dielectric breakdown occurs and the electrolytic fluid infiltrates the opening, making direct contact with the aluminum electrode. These healed areas will continually regenerate, so will practically never break down. This method requires DC voltages or AC voltages with a DC offset, so that dielectric charging may remain an issue.

Figure 4.1: Dielectric stacks as barriers for fluid infiltration, acting to reduce defects in electrowetting dielectric films [68].

Figure 4.2: Illustration of self-healing dielectric composed of aluminum electrode covered in a hydrophobic dielectric. Use of proper electrolytes and voltage forms will anodize the aluminum at sites of breakdown, spontaneously forming replacement dielectric [71].

The above issues concerning dielectric quality and reliability have been largely observed in aqueous electrowetting applications. Whether similar issues are evident in the case of liquid metal actuation has yet to be investigated, and may provide a route for reliable low-voltage electrowetting of liquid metals.

4.3 Future Applications and Conclusion

With provisions for further improvements, reliable electrowetting of non-toxic gallium alloys has been enabled by the incorporation of an electrically insulating acidic oil. By way of this technique, applications may now be enabled which utilize mechanisms exploited in

66 contemporary electrowetting systems like displays and lab on a chip devices. Components such as frequency-selective surfaces and metamaterials, which often rely on free-electron behavior of metals, may be made tunable by direct reconfiguration of radiating metal components. Similarly, electrowetting of liquid metals offers many routes to antenna reconfiguration, either by utilizing the liquid metal directly as the antenna structure and sweeping through arbitrary structural geometries, or by using the liquid metal as switching or parasitic components. In addition, as has been discussed above, variation of transmission properties such as capacitive loading and impedance matching may be achieved in a very straightforward manner by varying the contact area of liquid metal droplets, so that a large variety of applications may be imagined.

To realize the most versatile reconfiguration technique available for electrowetting, components such as programmable electrode arrays and Laplace barriers must be incorporated into device design, in order to allow variation of wetting patterns and bistability, as discussed in the electrowetting applications section. Such modifications could be directly incorporated into the designs examined in Chapter 3 with little effort, and liquid metal components could soon be manipulated at the will of the user.

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