Capacitance Effects of a Hydrophobic-Coated Ion Gel Dielectric on AC Electrowetting

micromachines

Article Capacitance Effects of a Hydrophobic-Coated Ion Gel Dielectric on AC Electrowetting

Taewoo Lee 1 and Sung-Yong Park 2,*

1 Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore; [email protected] 2 Department of Mechanical Engineering, San Diego State University, 5500 Campanile Drive, San Diego, CA 92182, USA * Correspondence: [email protected]

Abstract: We present experimental studies of alternating current (AC) electrowetting dominantly influenced by several unique characteristics of an ion gel dielectric in its capacitance. At a high- frequency region above 1 kHz, the droplet undergoes the contact angle modification. Due to its high-capacitance characteristic, the ion gel allows the contact angle change as large as ∆θ = 26.4◦, more than 2-fold improvement, compared to conventional dielectrics when f = 1 kHz. At the frequency range from 1 to 15 kHz, the capacitive response of the gel layer dominates and results in a nominal variation in the angle change as θ ≈ 90.9◦. Above 15 kHz, such a capacitive response of the gel layer sharply decreases and leads to the drastic increase in the contact angle. At a low-frequency region below a few hundred Hz, the droplet’s oscillation relying on the AC frequency applied was mainly observed and oscillation performance was maximized at corresponding resonance frequencies. With the high-capacitance feature, the ion gel significantly enlarges the oscillation performance by 73.8% at the 1st resonance mode. The study herein on the ion gel dielectric will help for various AC electrowetting applications with the benefits of mixing enhancement, large contact angle modification, Citation: Lee, T.; Park, S.-Y. and frequency-independent control. Capacitance Effects of a Hydrophobic-Coated Ion Gel Keywords: AC electrowetting; ion gel; contact angle modulation; droplet oscillation; high capacitance Dielectric on AC Electrowetting. Micromachines 2021, 12, 320. https://doi.org/10.3390/mi12030320 1. Introduction Academic Editor: Shih-Kang Fan Using the surface tension force dominance over body forces in micro/meso scales,

Received: 21 February 2021 electrowetting has been explored as an effective means for small-scale liquid handling Accepted: 16 March 2021 without complex mechanical components such as pumps, tubes, and microchannels [1–3]. Published: 18 March 2021 When an electric potential is applied between a droplet and an electrode, the electric charges are re-distributed and modify the surface tension at the liquid-solid interface where the

Publisher’s Note: MDPI stays neutral like-charge repulsion reduces the work by expanding the surface area [4,5]. The resulting with regard to jurisdictional claims in contact angle (θ) of a liquid droplet can be mathematically estimated with the applied published maps and institutional afﬁl- electric potential (V) by using the popular Young−Lippmann equation [6]: iations. 1 cos θ = cos θ + cV2 (1) 0 2γ

where θ0 is an initial contact angle with zero potential application, γ is the surface tension Copyright: © 2021 by the authors. between two immiscible ﬂuids, and c is the speciﬁc capacitance of the dielectric layer which Licensee MDPI, Basel, Switzerland. is proportional to its relative permittivity (εr) and the inverse of the layer thickness (t). This article is an open access article Using the electrowetting mechanism, numerous applications have been demonstrated, distributed under the terms and including lab-on-a-chip [7–9], tunable optics [10–12], energy harvesting [13–15], and on- conditions of the Creative Commons chip cooling [16,17]. Attribution (CC BY) license (https:// According to the Young-Lippmann Equation (1), electrowetting performance (i.e., creativecommons.org/licenses/by/ contact angle modulation) is determined by the capacitance of a dielectric layer used. 4.0/).

Micromachines 2021, 12, 320. https://doi.org/10.3390/mi12030320 https://www.mdpi.com/journal/micromachines Micromachines 2021, 12, 320 2 of 11

The use of high-capacitance dielectrics is very beneficial to induce large contact angle modification at a given potential input by allowing high electrostatic energy (cV2/2) stored across a capacitor of a dielectric layer. To meet this requirement, our group recently presented an ion gel as a low-cost, spin-coatable, high-capacitance dielectric [18]. An ion gel is a solid thin-film form of an ionic liquid, being fulfilled by combining it with a structuring copolymer [19,20]. Since the ionic liquid is an essential constituent of the ion gel, it possesses similar electrical properties to the ionic liquid. Under an electric field applied, free counter-ions inside the ionic liquid compactly accumulate at the liquid-solid interface where the nanometer-thick electric double layer capacitor (EDLC) is formed [21]. This ultra-thin EDLC provides a large specific capacitance (c ≈ 10~40 µF/cm2), which provides 2 to 3 orders of magnitude higher than that of conventional dielectrics such as silicon dioxide (SiO2)[19,22]. Using such a high-capacitance feature of the ion gel, our previous study has demonstrated significant improvement in electrowetting performance under an application of a direct current (DC) voltage [18]. For typical electrowetting studies [4,23,24], a liquid droplet is equivalently modeled as a resistor and a capacitor connected in parallel, while a dielectric layer underneath a droplet is equivalently assumed as a capacitor. Under a DC situation where an ohmic current dominates, the droplet behaves as a perfect conductor and the applied electric potential fully contributes to the contact angle reduction of the droplet. A resulting contact angle modification can be reasonably estimated by using the Young-Lippmann Equation (1). However, when an alternating current (AC) voltage applied, this general understanding is no longer valid because a displacement current dominates. An alternating electric input results in several interesting AC electrowetting phenomena such as droplet oscillation [25,26] and internal flow [27] that cannot be simply explained using the Young-Lippmann Equation (1). These AC electrowetting phenomena have been advantageously used for various AC electrowetting applications, including rapid mixing without the need of extra apparatus such as pumps or mixers, less contact angle hysteresis, and mitigation of contact angle saturation [28–30]. In this paper, we present AC electrowetting responses of a water droplet dominantly affected by several unique characteristics of an ion gel dielectric in its capacitance, such as high-capacitance magnitude, thickness independency, and frequency independency in a specific frequency region. To understand the effects of such features on AC electrowetting, experimental studies were conducted with a hydrophobic-coated ion gel layer at both high- and low-frequency AC regions. For comparative studies, the AC electrowetting perfor- mances on the gel layer were further compared to the ones on conventional dielectrics such as SiO2 fabricated by a sputtering process. The study herein on the ion gel dielectric will help for various AC electrowetting applications with the benefits of mixing enhancement, large contact angle modification, and frequency-independent control.

2. Fundamentals of an Ion Gel Dielectric for AC Electrowetting Ionic liquids are generally known as room temperature molten organic salts [21,31,32]. Due to their favorable properties such as wide electrochemical window, high thermal stability, negligible vapor pressure, and large ionic capacitance, ionic liquids have been used for numerous applications of batteries, solar cells, and lubricants [33–35]. To further extend their practical applications, ionic liquids have been also fabricated in a solid thin- film form either by chemical or physical cross-linking with structuring polymers. This thin-film form of an ionic liquid is known as an ion gel. Hence, the ion gel retains similar electrochemical properties to the ionic liquid. In addition, the ultra-thin EDLC formed at an electrified surface allows a specific capacitance in a few orders of magnitude higher than that of conventional dielectrics. Based on previous studies [19,36,37], it was reported that the capacitance of the ion gel shows frequency-dependent behaviors very differently from that of conventional dielectrics. For AC electrowetting study, a typical dielectric layer is equivalently modeled as a single capacitor, while the ion gel dielectric layer can be modeled using an equivalent Micromachines 2021, 12, x FOR PEER REVIEW 3 of 11

thermal stability, negligible vapor pressure, and large ionic capacitance, ionic liquids have been used for numerous applications of batteries, solar cells, and lubricants [33–35]. To further extend their practical applications, ionic liquids have been also fabricated in a solid thin-film form either by chemical or physical cross-linking with structuring polymers. This thin-film form of an ionic liquid is known as an ion gel. Hence, the ion gel retains similar electrochemical properties to the ionic liquid. In addition, the ultra-thin EDLC formed at an electrified surface allows a specific capacitance in a few orders of magnitude higher than that of conventional dielectrics. Based on previous studies [19,36,37], it was reported that the capacitance of the ion gel shows frequency-dependent behaviors very differently from that of conventional dielectrics. For AC electrowetting study, a typical dielectric layer is equivalently modeled as a single capacitor, while the ion gel dielectric layer can be modeled using an equivalent circuit consisting of capacitors and a resistor connected in series, as shown in Figure 1. The capacitor term (cEDLC) of the gel layer represents the compact EDLC formed at the electrified interface, while the resistor part (Rbulk) is induced by a bulk electrolyte resistance emerging from the constituent ion liquid. This indicates that the capacitance of the gel layer is solely contributed by the thickness of the EDLC, not determined by the thickness of the gel layer itself. According to the Gouy-Chapman theory [21,31], the thick- Micromachines 2021, 12, 320 ness of the EDLC is effectively estimated by using the Debye3 of 11 length (less than 10 nm for most ionic liquids), which can be used to determine the gel layer’s capacitance [31,38]. Furthermore, Lee et al. have experimentally discovered the frequency-dependent charac- circuit consisting of capacitors and a resistor connected in series, as shown in Figure1. Theteristics capacitor of term the (cEDLC gel) oflayer the gel composed layer represents of the a compact P(VDF-HFP) EDLC formed structuring at the copolymer (poly(vinyli- electrifieddene fluoride-co-hexafluoropropylene)) interface, while the resistor part (Rbulk) is induced by and a bulk an electrolyte [EMIM][TFSI] resistance ionic liquid (1-ethyl-3-me- emerging from the constituent ion liquid. This indicates that the capacitance of the gel layerthylimidazolium is solely contributed bybis(trifluoromethylsulfonyl)imide) the thickness of the EDLC, not determined by the thicknessusing impedance spectroscopy [19]. of the gel layer itself. According to the Gouy-Chapman theory [21,31], the thickness of the EDLCThe isimpedance effectively estimated (Z) was by using measured the Debye length in the (less form than 10 of nm Z for = mostZ′ + ionic iZ″, where i is an imaginary unit, liquids),and Z which′ and can Z be″ usedconstitute to determine the the real gel layer’s and capacitance imaginary [31,38 parts]. Furthermore, of the impedance, respectively. Us- Lee et al. have experimentally discovered the frequency-dependent characteristics of the geling layer this composed method, of a P(VDF-HFP) they found structuring that copolymer the ion (poly(vinylidene gel layer fluoride-co-mainly behaves as a capacitor at the hexafluoropropylene)) and an [EMIM][TFSI] ionic liquid (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide)input AC frequency below using impedancearound spectroscopy15 kHz and [19]. its The capacitance impedance value was estimated c ≈ 10 (Z)μF/cm was measured2 from in the the form imaginary of Z = Z0 + iZpart00, where (Zi″is) anof imaginary the impedance. unit, and Z0 and Above 15 kHz, the resistive re- Z00 constitute the real and imaginary parts of the impedance, respectively. Using this method,sponse they more found dominates that the ion gel layerthan mainly the behavescapacitive as a capacitor one and at the the input value AC was calculated as R = 0.1~10 frequencyΩ cm2 belowfrom around the real 15 kHz part and its(Z capacitance′) of the valueimpedance. was estimated Consideringc ≈ 10 µF/cm2 the theoretical understanding from the imaginary part (Z00) of the impedance. Above 15 kHz, the resistive response more dominatesdescribed than theabove, capacitive the one ion and thegel value dielectric was calculated possesses as R = 0.1~10 severalΩ cm2 from unique characteristics in its ca- thepacitance real part (Z0 )under of the impedance. an AC Considering input, such the theoretical as high-capacitance understanding described magnitude, thickness independ- above, the ion gel dielectric possesses several unique characteristics in its capacitance under anency, AC input, and such frequency as high-capacitance independency magnitude, thickness in a independency, specific frequency and frequency region. In following sections, independencyexperimental in a specific studies frequency will region. be discussed In following sections, to understand experimental studieshow such ion gel’s features for the will be discussed to understand how such ion gel’s features for the capacitance have an influencecapacitance on AC electrowetting. have an influence on AC electrowetting.

FigureFigure 1. An 1. experimental An experimental setup for alternating setup current for alternating (AC) electrowetting current study with (AC) the ion electrowetting gel. study with the ion gel. To investigate the gel layer effect on AC electrowetting, a 15 µL de-ionized water drop is placed on theTo ion investigate gel layer covered the by gel a thin layer hydrophobic effect layer. on AnAC equivalent electrowetting, circuit for the a gel 15 layer μL can de-ionized water drop is placed beon modelled the ion as twogel capacitors layer covered (cEDLC) serially by connecteda thin hydrophobic with a resistor (Rbulk layer.). A water An drop equivalent is circuit for the gel layer can equivalently modelled as an impedance formed with a resistor (R ) and a capacitor (c ) connected be modelled as two capacitors (cEDLC) seriallyw connected withw a resistor (Rbulk). A water drop is in parallel, while a hydrophobic layer is modeled as a single capacitor (ch). equivalently modelled as an impedance formed with a resistor (Rw) and a capacitor (cw) connected 3. Experimental Setup in parallel, while a hydrophobic layer is modeled as a single capacitor (ch). Figure1 shows an experimental setup to investigate the effects of the ion gel dielectric on AC electrowetting performance. In this study, a P(VDF-HFP) structuring copolymer and an [EMIM][TFSI] ionic liquid were selected for the gel layer fabrication on an ITO substrate. A gel solution was ﬁrst prepared by mixing a pellet of the copolymer in the ionic

Micromachines 2021, 12, 320 4 of 11

liquid at a ratio of 1:4 by weight. Acetone was added at a wt% ratio of 1:4 to further dilute the mixture. Then, the copolymer was fully dissolved in the acetone-mixed solution at 60 ◦C for 2 h. The mixture solution was then spin-coated on an ITO-coated glass substrate to obtain a thin-film gel layer. The layer thickness was controlled by varying a spin speed. The coated gel layer was cured at 75 ◦C for 4 h, after leaving it for 24 h at room temperature to ensure the vaporization of acetone. A Teflon solution (AF1600, Dupont, Wilmington, DE, USA) was subsequently spin-coated on top of the cured gel layer, followed by baking it at 75 ◦C for 16 h to deposit a 150 nm thick hydrophobic layer. A 15 µL deionized water droplet was placed on a hydrophobic surface and a platinum (Pt) wire was inserted into the droplet. A sinusoidal AC signal was applied between the Pt wire and the bottom ITO electrode. Its AC frequency (f ) and root-mean-square voltage (VRMS) were variably controlled by a waveform generator (WW5061, Tabor Electronics, Nesher, Israel) and a high- voltage amplifier (9100A, Tabor Electronics, Nesher, Israel), respectively. AC electrowetting behaviors of a de-ionized water droplet were recorded using a high-speed camera (Fastcam Mini AX200, Photron, Tokyo, Japan) for further characterization. To highlight the effects of the gel layer on AC electrowetting, the experimental results with the gel layer were further compared to the ones on a hydrophobic-coated SiO2 layer deposited by sputtering at 150 W and 0.5 Å/s without a post-annealing process. All experiments were repeated four times for each experimental condition and the mean values were presented with the error bars for each case.

4. Results and Discussion AC electrowetting phenomena are typically characterized in two input frequency regions, i.e., a high-frequency region above 1 kHz and another region at low frequencies below a few hundred Hz. Following sections will discuss about AC electrowetting responses of a water droplet inﬂuenced by the ion gel in both frequency ranges.

4.1. High-Frequency AC Electrowetting with the Ion Gel

A high-frequency AC electrowetting study was firstly conducted on a SiO2 layer as a conventional dielectric. At the high-frequency region above 1 kHz where a displacement current becomes more dominant, a droplet acts in a capacitive manner due to limited dipole response time and thus the formation of an electric field across the droplet cannot be negligible [39,40]. As a result, the contact angle change is mainly observed under a high-frequency AC input. Figure2 presents the contact angles experimentally measured while the applied AC frequency (f ) was varied from 1 to 100 kHz. Without the voltage application, the droplet was placed on a hydrophobic surface with its initial contact angle ◦ of θ0 = 116.0 (indicated as a black dashed line in Figure2). When VRMS = 42.4 V and ◦ f = 1 kHz, it underwent the contact angle reduction to θ = 103.5 on a 50 nm SiO2 layer ◦ (i.e., angle change, ∆θ = θ0 − θ = 12.5 ). With an increase in f, less contact angle change is observed. At f = 10 kHz, it returned almost close to an initial state of θ0. For thicker layers of SiO2, such a trend in the angle change becomes slower due to lower capacitance of the dielectric layers. For example, a 300 nm thick SiO2 layer allows the contact angle ◦ change as small as ∆θ = 7.5 at f = 1 kHz, although the same VRMS = 42.4 V was applied. As further increasing f, the contact angle approached the initial state of θ0, similarly to other thicknesses of a SiO2 layer. The experiment above was repeated for the gel layer and several interesting results were observed. When the same VRMS = 42.4 V and f = 1 kHz were applied, the ion gel dielectric enabled the contact angle reduction to 89.6◦ (Figure2). This shows the significantly enhanced angle change ∆θ = 26.4◦ due to its high-capacitance characteristic, more than 2-fold improvement, compared to that of a 50 nm SiO2 layer. Another interesting observation is the contact angle almost constantly remained as θ ≈ 90.9◦ on the ion gel layer at the frequency range from 1 to 15 kHz (Figure2). Such AC electrowetting responses result from the unique feature of almost frequency-independent capacitance for the ion gel at the given frequency range [19,36,37]. Consequently, the contact angles correspondingly Micromachines 2021, 12, x FOR PEER REVIEW 5 of 11

enhanced angle change Δθ = 26.4° due to its high-capacitance characteristic, more than 2- fold improvement, compared to that of a 50 nm SiO2 layer. Another interesting observation is the contact angle almost constantly remained as θ ≈ 90.9˚ on the ion gel layer at the frequency range from 1 to 15 kHz (Figure 2). Such AC electrowetting responses result from the unique feature of almost frequency-independent capacitance for the ion gel at the given frequency range [19,36,37]. Consequently, the contact angles correspondingly show an almost negligible variation, although f was largely varied from 1 to 15 kHz. How- ever, this nominal variation in the angle change has not been observed for SiO2. At f = 15

Micromachines 2021, 12, 320 kHz, the contact angles present almost close to the initial5 of 11 state of θ0 (i.e., Δθ ≈ 0˚) for all thicknesses of the SiO2 layer, while the gel layer is still able to achieve the contact angle reduction as large as Δθ = 23.8° (Figure 2). Above 15 kHz, the gel layer dominantly works showin a an resistive almost negligible manner variation, due although to limited f was largely response varied from time 1 to in 15 kHz.polarization and thus its specific However, this nominal variation in the angle change has not been observed for SiO2. At ◦ f capacitance= 15 kHz, the contact value angles sharply present almost drops close with to the initialf, correspondingly state of θ0 (i.e., ∆θ ≈ 0 approaching) θ0 at 100 kHz (Fig- for all thicknesses of the SiO2 layer, while the gel layer is still able to achieve the contact angleure reduction 2). These as large experimental as ∆θ = 23.8◦ (Figure results2). Above in 15high-fr kHz, theequency gel layer dominantly AC electrowetting were further com- workspared in a to resistive the mannerDC case. due to Since limited responsean applied time in polarizationDC bias andvoltage thus its fully contributes to the angle speciﬁc capacitance value sharply drops with f, correspondingly approaching θ0 at 100 kHz (Figurechange,2) . These much experimental larger results angle in high-frequencymodification AC electrowettingcan be achieved were further for the DC case than all AC cases. comparedFor example, to the DC case.the Sincegel anlayer applied allowed DC bias voltage the angle fully contributes change to theas angle large as Δθ = 32.8° at the DC case change, much larger angle modiﬁcation can be achieved for the DC case than all AC cases. For(Figure example, 2), the gelwhile layer allowedjust Δ theθ angle= 26.4° change at as1 largekHz as ∆(Figureθ = 32.8◦ at 2). the However, DC case even this angle reduction (Figureperformance2), while just at∆θ =1 26.4kHz◦ at on 1 kHz the (Figure gel2 layer). However, is still even larger this angle than reduction the one (Δθ = 20.8°) under the DC performance at 1 kHz on the gel layer is still larger than the one (∆θ = 20.8◦) under the DC casecase for afor 50 nma 50 SiO 2nmlayer. SiO2 layer.

FigureFigure 2. High-frequency 2. High-frequency AC electrowetting AC performance.electrowetting Droplet’s performance. contact angle was measuredDroplet’s contact angle was measured on various dielectric layers such as 8.3 µm ion gel and 50, 100 and 300 nm SiO2 covered with a 150on nm various Teflon layer, dielectric while an applied layers AC such frequency as 8.3 was μ variedm ion from gel 1 to and 100 kHz. 50, Due100 to and the 300 nm SiO2 covered with a 150 high-capacitancenm Teflon dominancelayer, while of the ion an gel applied up to 15 kHz, AC the frequency contact angle modulation was varied is far superior, from 1 to 100 kHz. Due to the high- comparedcapacitance to SiO2. Thedominance AC electrowetting of the performance ion ge wasl up further to 15 compared kHz, withthe DC contact cases where angle modulation is far superior, much lower contact angles are observed. All experiments were conducted at a fixed voltage of 2 VcomparedRMS = 42.4 V. to SiO . The AC electrowetting performance was further compared with DC cases where much lower contact angles are observed. All experiments were conducted at a fixed voltage Our next study was conducted to see the dependency of the AC frequency (f ) and root- of VRMS = 42.4 V. mean-square voltage (VRMS) on contact angle modification. Figure3 presents the measured 2 contact angles in terms of cos(θ) varied with the square of the AC voltage input (i.e., VRMS ). Since an initial contact angle is larger than 90◦ on a hydrophobic surface, the negative value Our next study2 was conducted to see the dependency of the AC frequency (f) and of cos(θ) is shown at VRMS = 0. As increasing VRMS, larger electric energy is stored across theroot-mean-square ion gel dielectric layer, resulting voltage in the ( contactVRMS) angle on decreased contact and angle the value modificati of cos(θ) on. Figure 3 presents the 2 increased. One interesting observation is the linear relationship between cos(θ) and VRMS . 2 Themeasured slope in such contact a graph of angles cos(θ) versus in termsVRMS represents of cos( theθ) capacitancevaried with effect ofthe the square of the AC voltage input gel(i.e., layer V used,RMS2 as). indicatedSince an by the initial Young-Lippmann contact Equationangle is (1). larger The sharpest than slope 90° is on a hydrophobic surface, the negative value of cos(θ) is shown at VRMS2 = 0. As increasing VRMS, larger electric energy is stored across the ion gel dielectric layer, resulting in the contact angle decreased and the value of cos(θ) increased. One interesting observation is the linear relationship between cos(θ) and VRMS2. The slope in such a graph of cos(θ) versus VRMS2 represents the capacitance effect of the gel layer used, as indicated by the Young-Lippmann Equation (1). The sharpest slope is presented for the DC case. Then, the slope becomes shallower as increasing f from 1 to 100 kHz. This decreasing trend in the slope results from the capacitance

Micromachines 2021, 12, x FOR PEER REVIEW 6 of 11

Micromachines 2021, 12, 320 6 of 11

effect of the gel layer with f (i.e., poor contact angle modulation). Another feature attract- presenteding our forinterest the DC is case. the Then, slopes theslope very becomes close at shallower the frequency as increasing rangef from from 1 to 1 to 10 kHz, indicating 100 kHz. This decreasing trend in the slope results from the capacitance effect of the gel layerthat with thef gel(i.e., layer poor contact capacitance angle modulation). is almost Another frequency feature attracting independent. our interest This is frequency-independ- theent slopes characteristic very close at the of frequency the ion rangegel for from the 1 to capaci 10 kHz, indicatingtance is thatalso the supported gel layer by the experimental capacitanceresults in is almostFigure frequency 2, where independent. the contact This frequency-independent angles on the gel characteristic layer have of a nominal variation at the ion gel for the capacitance is also supported by the experimental results in Figure2, wherethe frequency the contact angles range on thefrom gel layer 1 to have 15 akHz. nominal Above variation 20 at kHz, the frequency a resistive range response of the ion gel fromdominates, 1 to 15 kHz. and Above the 20 slopes kHz, a resistiveof cos(θ response) drastically of the ion decrease gel dominates, with and VRMS the 2. These high-frequency 2 slopes of cos(θ) drastically decrease with VRMS . These high-frequency AC electrowetting performancesAC electrowetting result from theperformances unique characteristics result for from the ion the gel’s unique capacitance, characteristics which for the ion gel’s featurescapacitance, very differently which from features a conventional very dielectricdifferently of SiO from2. a conventional dielectric of SiO2.

FigureFigure 3. High-frequency 3. High-frequency AC electrowetting AC electrowetting performance varied perf withormance AC frequency varied (f ) and with voltage AC frequency (f) and volt- (V ). A device was fabricated with an 8.3 µm gel layer covered with a 150 nm Teflon layer. While ageRMS (VRMS). A device was fabricated with an 8.3 μm gel layer covered with a 150 nm Teflon layer. varying f and VRMS, the droplet’s contact angle was measured. For the AC frequencies from 1 to RMS 10While kHz, the varying variations f inand the slopeV are, the not droplet’s significant. Beyond contact this angle frequency, was the measured. decreasing trend For in the AC frequencies from the1 to slop 10 is kHz, slow. the variations in the slope are not significant. Beyond this frequency, the decreasing trend in the slop is slow. We have further studied the thickness effect of the gel layer on high-frequency AC electrowetting, while varying VRMS at a fixed f = 5 kHz. Figure4 shows an initial contact angle ofWe the droplethave further without the studied voltage application.the thickness With the effect increase of inthe a bias gel voltage, layer on high-frequency AC theelectrowetting, contact angle correspondingly while varying decreases. VRMS As at described a fixed previously, f = 5 kHz. the Figure capacitance 4 shows an initial contact of the gel layer is independent on the layer thickness because it is solely contributed by theangle thickness of the of thedroplet EDLC, notwithout determined the byvoltage the thickness application. of the gel layerWith itself the [ 19increase]. in a bias voltage, Consequently,the contact an angle almost correspondingly negligible variation in thedecreases. contact angles As isdescribed observed in Figurepreviously,4, the capacitance of althoughthe gel the layer gel layer is independent thickness was largely on variedthe layer from 5.8 thickness to 10.7 µm. Thesebecause experimental it is solely contributed by the results were further compared to the ones in the DC case. A decreasing trend in the contact anglethickness is much steeperof the for EDLC, the DC situationnot determined than the AC. by For example,the thickness the saturation of the voltage gel layer itself [19]. Conse- isquently, observed at anVsat almost= 63.6 V fornegligible the AC (Figure variation4), while at inV satthe= 28.3contact V for the angles DC (Figure is 4observed) in Figure 4, alt- much earlier than the AC case. In addition, it is worth to note that we have not observed anyhough bubbles the caused gel layer by electrolysis thickness for both was DC largely and AC varied cases by from using the5.8 ion to gel 10.7 as aμm. These experimental dielectricresults layerwere at afurther given range compared of VRMS. to the ones in the DC case. A decreasing trend in the contact angle is much steeper for the DC situation than the AC. For example, the saturation voltage is observed at Vsat = 63.6 V for the AC (Figure 4), while at Vsat = 28.3 V for the DC (Figure 4) much earlier than the AC case. In addition, it is worth to note that we have not observed any bubbles caused by electrolysis for both DC and AC cases by using the ion gel as a dielectric layer at a given range of VRMS.

MicromachinesMicromachines 20212021, 12, 12, x, 320FOR PEER REVIEW 7 of 11 7 of 11 Micromachines 2021, 12, x FOR PEER REVIEW 7 of 11

Figure 4. Thickness-independent contact angle modulation. Although the gel layer thickness was largely varied from 5.8 to 10.7 μm, the variation in contact angles is nominal for different layer thicknesses.FigureFigure 4. 4.Thickness-independent Thickness-independentFor the DC situation, contact such contacta angle decreasi modulation. angleng trend modulati Althoughin the contacton. the Although gel angles layer is thicknessthe much gel steeper. layer was thickness was Anlargelylargely applied varied varied AC from frequency from 5.8 to5.8 10.7was to 10.7µfixem,d theμ atm, variation5 kHzthe variation for in all contact experiments. in anglescontact is nominalangles is for nominal different for layer different layer thicknesses.thicknesses. For For the DCthe situation, DC situation, such a decreasingsuch a decreasi trend inng the trend contact in angles the contact is much steeper.angles Anis much steeper. 4.2.appliedAn Low-Frequency applied AC frequency AC frequency AC was Electrowetting ﬁxed was at 5 kHz fixe for dwith at all 5 experiments.the kHz Ion for Gel all experiments. 4.2. Low-FrequencyWhen a low-frequency AC Electrowetting AC signal with the(typically Ion Gel less than a few hundred Hz) is applied, a 4.2.dropletWhen Low-Frequency experiences a low-frequency itsAC oscillation Electrowetting AC signal on (typically a hydrop with less thehobic-coated than Ion a Gel few hundred dielectric Hz) layer is applied, [24–26]. a This oscillationdropletWhen experiences phenomenon a low-frequency its oscillation results from onAC a hydrophobic-coated signalthe time (typically-harmonic less dielectricelectric than input layera few [which24 hundred–26]. alternates This Hz) is applied, theoscillationa dropletMaxwell phenomenon experiences stress exerted results its on oscillation the from three-phase the time-harmonic on a hydropcontact electriclinehobic-coated of the input droplet. which dielectric To alternates understand layer [24–26]. This thethe capacitance Maxwell stress effects exerted of the on theion three-phase gel on low-frequency contact line ofAC the electrowetting droplet. To understand performance, oscillation phenomenon results from the time-harmonic electric input which alternates thethe same capacitance device, effects consisting of the of ion an gel 8.3 on μm low-frequency gel layer covered AC electrowetting by a 150 nm hydrophobic performance, layer onthethe an same MaxwellITO-coated device, stress consistingglass exertedsubstrate, of an on was 8.3 theµ preparedm three-phase gel layer. Figure covered contact 5 presents by a line 150 the nmof droplet’s the hydrophobic droplet. oscillation To understand layer on an ITO-coated glass substrate, was prepared. Figure5 presents the droplet’s patternsthe capacitance relying on effectsf (see Video of the S1) ionwhen gel V RMSon =lo 26.5w-frequency V. Each imag ACe presents electrowetting a superposi- performance, oscillation patterns relying on f (see Video S1) when V = 26.5 V. Each image presents a tionthe of same over device,5 images consisting taken by a high-speedof an 8.3 μ cameram gel layerRMS during covered a half period by a 150of the nm oscillation. hydrophobic layer superposition of over 5 images taken by a high-speed camera during a half period of the Itoscillation.on is observed an ITO-coated It that is observed the glass oscillation that substrate, the amplitud oscillation wase amplitudeisprepared maximized is. Figure maximized at corresponding 5 presents at corresponding resonancethe droplet’s fre- oscillation quencies of f1 = 25 Hz, f2 = 75 Hz, and f3 = 120 Hz. Droplet’s oscillation shape is also featured resonancepatterns frequencies relying on of f 1(see= 25 Video Hz, f 2 = S1) 75Hz, when and fV3RMS= 120 = 26.5 Hz. Droplet’s V. Each oscillation image presents shape a superposi- withistion also even of featured over node 5 with points images even (which nodetaken pointsare by indicated a (which high-speed arewith indicated the camera arrows with during in the Figure arrows a 5)ha in atlf Figure eachperiod resonance5) atof the oscillation. frequency.eachIt is resonanceobserved frequency. that the oscillation amplitude is maximized at corresponding resonance frequencies of f1 = 25 Hz, f2 = 75 Hz, and f3 = 120 Hz. Droplet’s oscillation shape is also featured with even node points (which are indicated with the arrows in Figure 5) at each resonance frequency.

(a) (b) (c)

FigureFigure 5. Frequency-dependent oscillation oscillation patterns patterns of aof water a water drop. drop. Low-frequency Low-frequency AC electrowet- AC electro- wettingting performance performance was was studied studied with with a 15 aµ L15 water μL water drop drop placed placed on an on ITO-coated an ITO-coated glass substrateglass sub- stratecovered covered with 8.3withµm 8.3 ion μm gel ion and gel 150 and nm 150 hydrophobic nm hydrophobic layers, respectively.layers, respectively. An AC inputAn AC voltage input voltage keeps constant at VRMS = 26.5 V, while the applied AC frequency (f) was varied. At a low- keeps constant at(Va)RMS = 26.5 V, while the applied AC(b frequency) (f ) was varied. At a low-frequency(c) frequencyrange, the dropletrange, the mainly droplet undergoes mainly its underg oscillationoes whoseits oscillation shape relies whose on f .shape The oscillation relies on amplitudef. The oscillation amplitude is noticeably enlarged at corresponding resonance frequencies of (a) f1 = 25 Hz, (b) isFigure noticeably 5. Frequency-dependent enlarged at corresponding oscillation resonance frequencies patterns ofof (a) waterf 1 = 25 drop. Hz, (b) Low-frequencyf 2 = 75 Hz, and AC electro- f2 = 75 Hz, and (c) f3 = 120 Hz, and shows a decreasing trend with the resonance frequency. The (c) f 3 = 120 Hz, and shows a decreasing trend with the resonance frequency. The arrows indicate the arrowswetting indicate performance the oscillation was nodestudied points. with a 15 μL water drop placed on an ITO-coated glass sub- oscillationstrate covered node points. with 8.3 μm ion gel and 150 nm hydrophobic layers, respectively. An AC input voltageTo quantify keeps constant the low-frequency at VRMS = 26.5 AC V, electrow while theetting applied performance AC frequency with the(f) wasion gel,varied. the At a low- oscillationfrequency amplitude range, the of droplet the droplet mainly was underg characterizedoes its oscillation by normalizing whose theshape droplet relies size on ra-f. The oscilla- diallytion amplitudeelongated byis noticeably the Maxwell enlarged stress toat thecorresponding original base resonance diameter frequenciesof 3.25 mm. ofFigure (a) f1 6= 25 Hz, (b) presentsf2 = 75 Hz, the and normalized (c) f3 = 120 oscillation Hz, and magnitudeshows a decreasing at each resonance trend with mode. the resonance On the gel frequency. layer, The arrows indicate the oscillation node points.

To quantify the low-frequency AC electrowetting performance with the ion gel, the oscillation amplitude of the droplet was characterized by normalizing the droplet size radially elongated by the Maxwell stress to the original base diameter of 3.25 mm. Figure 6 presents the normalized oscillation magnitude at each resonance mode. On the gel layer,

Micromachines 2021, 12, 320 8 of 11 Micromachines 2021, 12, x FOR PEER REVIEW 8 of 11

To quantify the low-frequency AC electrowetting performance with the ion gel, the oscillationthe droplet amplitude was radially of the droplet elongated was characterized 73.8% more by th normalizingan that of theits dropletoriginal size base diameter at radiallythe first elongated mode of by f the1 = 25 Maxwell Hz. At stress the to second the original and base third diameter modes of (i.e., 3.25 mm.f2 = 75 Figure Hz6 and f3 = 120 Hz), presentssmaller the oscillation normalized amplitudes oscillation magnitude were observed at each resonance as 60.2% mode. and On 40.6%, the gel layer,respectively. the For com- droplet was radially elongated 73.8% more than that of its original base diameter at the ﬁrst parative study, the experiment was repeated for a conventional dielectric of SiO2. The mode of f 1 = 25 Hz. At the second and third modes (i.e., f 2 = 75 Hz and f 3 = 120 Hz), smaller oscillationlayer thickness amplitudes was were varied observed from as 50 60.2% to 300 and nm 40.6%, to provide respectively. several For comparative different capacitance val- study,ues of the the experiment SiO2 layer. was repeatedIt is worth for a to conventional note that dielectric the resonance of SiO2. Thefrequencies layer thickness for droplet oscilla- wastion varied are not from affected 50 to 300 by nm the to providecapacitance several of different a dielectric capacitance layer valuesused [41] of the. Thus, SiO2 the oscillation layer. It is worth to note that the resonance frequencies for droplet oscillation are not affectedperformance by the capacitance on the SiO of a2 dielectriclayers was layer characterized used [41]. Thus, theat the oscillation same performanceresonance frequencies as onthe the ones SiO2 oflayers the wasion characterizedgel. Figure at6 shows the same comparative resonance frequencies experimental as the ones results. of the A high-capaci- iontance gel. characteristic Figure6 shows comparative of the ion experimental gel enhances results. the A oscillation high-capacitance performance characteristic much larger than ofthe the SiO ion2gel layers enhances for all the three oscillation resonance performance modes. much At larger the thanfirst theresonance SiO2 layers mode, for the gel layer all three resonance modes. At the ﬁrst resonance mode, the gel layer enabled 27.0% larger enabled 27.0% larger oscillation performance than that of a 50 nm thick layer of SiO2. oscillation performance than that of a 50 nm thick layer of SiO2. When it is compared to When it is compared to a 300 nm SiO2 layer, the oscillation performance was enlarged as a 300 nm SiO2 layer, the oscillation performance was enlarged as much as 57.6%. Such enhancementmuch as 57.6%. in the droplet’sSuch enhancement oscillation is similarly in the observeddroplet’s at higheroscillation resonance is similarly modes observed at byhigher using resonance the ion gel layer. modes by using the ion gel layer.

FigureFigure 6. 6.Normalized Normalized oscillation oscillation amplitude amplitude at the first at threethe first resonance three frequenciesresonance offrequenciesf 1 = 25 Hz, of f1 = 25 Hz, f2 f = 75 Hz, and f = 120 Hz. Oscillation amplitude of the droplet was evaluated on various dielectric =2 75 Hz, and f33 = 120 Hz. Oscillation amplitude of the droplet was evaluated on various dielectric layers such as 8.3 µm ion gel, and 50, 100 and 300 nm SiO2, while covering with a 150 nm Teflon layers such as 8.3 μm ion gel, and 50, 100 and 300 nm SiO2, while covering with a 150 nm Teflon layer. The use of the ion gel dielectric allows large electric energy stored across its dielectric capacitor, layer. The use of the ion gel dielectric allows large electric energy stored across its dielectric capac- thus enhancing the oscillation performance much larger than a conventional dielectric material of the itor, thus enhancing the oscillation performance much larger than a conventional dielectric mate- SiO2. A thinner SiO2 layer also results in higher oscillation amplitude for all resonance frequencies. rial of the SiO2. A thinner SiO2 layer also results in higher oscillation amplitude for all resonance All experiments were conducted at VRMS = 26.5 V. frequencies. All experiments were conducted at VRMS = 26.5 V. Figure6 also presents the thickness-dependent capacitance effect of the SiO 2 layer on low-frequencyFigure AC6 also electrowetting. presents the The thickness-de oscillation performancependent increases capacitance by using effect a thinner of the SiO2 layer on SiOlow-frequency2 layer. These experimental AC electrowetting. results well The follow oscillati with ouron generalperformance understanding increases such by using a thin- that the capacitance of a dielectric layer is inversely proportional to the layer thickness. ner SiO2 layer. These experimental results well follow with our general understanding Thus, a thinner SiO2 layer enhances oscillation performance. However, this understanding ofsuch thickness-dependent that the capacitance oscillation of performancea dielectric cannot layer be is appliedinversely for the proportional ion gel dielectric. to the layer thick- Sinceness. the Thus, capacitance a thinner of the SiO gel2 layerlayer is enhances solely contributed oscillation by the performance. nanometer-thick However, EDLC this under- formedstanding at the of electrifiedthickness-dependent interface, it does oscillation not rely on itsperformance layer thickness, cannot but the be thickness applied for the ion gel of the EDLC [18,19,21]. Due to this unique characteristic of the ion gel dielectric, the low-frequencydielectric. Since oscillation the capacitance performance of is independentthe gel layer on theis solely gel layer contributed thickness, which by the nanometer- isthick verified EDLC from formed the experimental at the electrified results in Figure interface,7. Although it does the not gel rely layer’s on thickness its layer thickness, but wasthe largelythickness varied of fromthe EDLC 5.8 to 10.7 [18,19,21].µm, the variation Due to inth oscillationis unique magnitude characteristic is almost of the ion gel die- negligiblelectric, the for alllow-frequency three resonance oscillation modes. performance is independent on the gel layer thickness, which is verified from the experimental results in Figure 7. Although the gel layer’s thickness was largely varied from 5.8 to 10.7 μm, the variation in oscillation magnitude is almost negligible for all three resonance modes.

MicromachinesMicromachines 20212021, 12, 12, x, 320FOR PEER REVIEW 9 of 11 9 of 11

FigureFigure 7. 7.Thickness-independent Thickness-independent oscillation oscillation amplitude. amplitude. The droplet’s The oscillation droplet’s performance oscillation was performance was evaluatedevaluated for for various various thicknesses thicknesses of the gel of layer.the gel An layer. almost An negligible almost variation negligible is observed variation in the is observed in the oscillationoscillation magnitude magnitude of the of droplet the droplet for all threefor all resonances, three resonances, although the although gel layer’s the thickness gel layer’s is thickness is largely varied. All experiments were conducted at VRMS = 26.5 V. largely varied. All experiments were conducted at VRMS = 26.5 V. 5. Conclusions 5. ConclusionsAC electrowetting performance dominantly affected by several unique characteristics of the ionAC gel electrowetting dielectric for its performance capacitance was dominantly investigated. affected To understand by several the effects unique of characteris- the ion gel, experimental studies have been conducted in two separated AC frequency regions.tics of the At a ion high-frequency gel dielectric region for aboveits capacitance 1 kHz, we have was mainly investigated. observed To the understand contact the effects angleof the modification ion gel, experimental of the droplet. Thestudies high-capacitance have been feature conducted of the ionin two gel can separated enhance AC frequency theregions. contact At angle a reductionhigh-frequency as large as region∆θ = 26.4 above◦, showing 1 kHz, more we than have 2-fold mainly improvement observed the contact comparedangle modification to that of a 50 of nm the SiO droplet.2 layer whenThe high-cf = 1 kHz.apacitance At the frequency feature of range the fromion gel can enhance 1 to 15 kHz, the droplet’s contact angle keeps almost constantly remained as θ ≈ 90.9◦ onthe the contact gel layer. angle This reduction is because theas large ion gel as dominantly Δθ = 26.4°, behaves showing in a capacitivemore than manner 2-fold improvement upcompared to 15 kHz. to As that further of a increasing 50 nm SiO the2 AClayer frequency when f above = 1 kHz. 15 kHz, At suchthe frequency a capacitive range from 1 to response15 kHz, of the the droplet’s gel layer sharply contact decreases, angle k resultingeeps almost in the drasticconstantly increase remained in the contact as θ ≈ 90.9° on the angle.gel layer. At a low-frequency This is because region the below ion agel few dominantly hundred Hz, thebehaves droplet in mainly a capacitive undergoes manner up to 15 its oscillation because of the time-harmonic AC electric input that alternates the Maxwell kHz. As further increasing the AC frequency above 15 kHz, such a capacitive response of stress exerted on the three-phase contact line of the droplet. Oscillation performance wasthe maximizedgel layer sharply at corresponding decreases, resonance resulting frequencies in the ofdrasticf 1 = 25 increase Hz, f 2 = 75in Hz,the andcontact angle. At a flow-frequency3 = 120 Hz. Due to region the high-capacitance below a few characteristic, hundred Hz, the the ion droplet gel significantly mainly enlarges undergoes its oscilla- thetion oscillation because performance of the time-harmonic by 73.8% at the AC 1st resonanceelectric input mode, whichthat alternates shows 57.6% the larger Maxwell stress ex- improvementerted on the than three-phase that of SiO contact2. It was line also of interestingly the droplet. observed Oscillation that the performance oscillation was maxim- performance is independent on the gel layer thickness (i.e., capacitance independency), unlikeized at conventional corresponding dielectrics. resonance This observation frequencies is because of f1 = 25 the Hz, capacitance f2 = 75 Hz, of the and gel f3 = 120 Hz. Due layerto the is solelyhigh-capacitance contributed by characteristic, the thickness of the EDLC,ion gel not significantly determined by enlarges the gel layer the oscillation per- thicknessformance itself. by Our73.8% study at hereinthe 1st on resonance the unique featuresmode, ofwhich the ion shows gel dielectric 57.6% will larger be improvement potentially helpful for various AC electrowetting applications with the benefits of mixing than that of SiO2. It was also interestingly observed that the oscillation performance is enhancement, large contact angle modification, and frequency-independent control. independent on the gel layer thickness (i.e., capacitance independency), unlike conven- Supplementarytional dielectrics. Materials: ThisThe observation following are available is because online the at https://www.mdpi.com/2072-666 capacitance of the gel layer is solely con- X/12/3/320/s1tributed by, Videothe thickness S1: Frequency-dependent of the EDLC, oscillation not de patternstermined of a water by drop. the gel layer thickness itself. AuthorOur study Contributions: herein T.L.,on the experiments, unique datafeatures acquisition of the and ion analysis, gel dielectric validation, andwill writing— be potentially helpful originalfor various draft preparation; AC electrowetting S.-Y.P., project applications conceptualization, with administration, the benefits supervision, of mixi writing—ng enhancement, large review and editing, and funding acquisition. All authors have read and agreed to the published versioncontact of theangle manuscript. modification, and frequency-independent control. Funding: This research was supported by the NSF Career Award (ECCS-2046134), USA, and the Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Video S1: Ministry of Education (R-265-000-588-114 and R-265-000-597-112), Singapore. Frequency-dependent oscillation patterns of a water drop. Conflicts of Interest: The authors declare no conflict of interest. Author Contributions: T.L., experiments, data acquisition and analysis, validation, and writing— original draft preparation; S.-Y.P., project conceptualization, administration, supervision, writing— review and editing, and funding acquisition. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by the NSF Career Award (ECCS-2046134), USA, and the Ministry of Education (R-265-000-588-114 and R-265-000-597-112), Singapore. Conflicts of Interest: The authors declare no conflict of interest.

Micromachines 2021, 12, 320 10 of 11

References 1. Wheeler, A.R. Putting Electrowetting to Work. Science 2008, 322, 539–540. [CrossRef] 2. Pollack, M.G.; Richard, B.F.; Shenderov, A.D. Electrowetting-based actuation of liquid droplets for microfluidic applications. Appl. Phys. Lett. 2000, 77, 1725–1726. [CrossRef] 3. Park, S.-Y.; Teitell, M.A.; Chiou, E.P.Y. Single-sided continuous optoelectrowetting (SCOEW) for droplet manipulation with light patterns. Lab Chip 2010, 10, 1655–1661. [CrossRef] 4. Mugele, F.; Baret, J.-C. Electrowetting: From basics to applications. J. Phys. Condens. Matter 2005, 17, 705–774. [CrossRef] 5. Moon, H.; Cho, S.K.; Garrell, R.L.; Kim, C.-J. Low voltage electrowetting-on-dielectric. J. Appl. Phys. 2002, 92, 4080–4087. [CrossRef] 6. Lippmann, M.G. Relations entre les phénomènes electriques et capillaires. Ann. Chim. Phys. 1875, 5, 494–549. 7. Moon, H.; Wheeler, A.R.; Garrell, R.L.; Loo, J.A.; Kim, C.-J. An integrated digital microfluidic chip for multiplexed proteomic sample preparation and analysis by MALDI-MS. Lab Chip 2006, 6, 1213–1219. [CrossRef] 8. Jiang, D.; Lee, S.; Bae, S.W.; Park, S.-Y. Smartphone integrated optoelectrowetting (SiOEW) for on-chip sample processing and microscopic detection of water quality. Lab Chip 2018, 18, 532–539. [CrossRef] 9. Chiang, M.-Y.; Hsu, Y.-W.; Hsieh, H.-Y.; Chen, S.-Y.; Fan, S.-K. Constructing 3D heterogeneous hydrogels from electrically manipulated prepolymer droplets and crosslinked microgels. Sci. Adv. 2016, 2.[CrossRef] 10. Clement, C.; Thio, S.K.; Park, S.-Y. An optofluidic tunable Fresnel lens for spatial focal control based on electrowetting-on-dielectric (EWOD). Sens. Actuators B 2017, 240, 909–915. [CrossRef] 11. Hayes, R.A.; Feenstra, B.J. Video-speed electronic paper based on electrowetting. Nature 2003, 425, 383–385. [CrossRef][PubMed] 12. Clement, C.E.; Park, S.-Y. High-performance beam steering using electrowetting-driven liquid prism fabricated by a simple dip-coating method. Appl. Phys. Lett. 2016, 108, 191601. [CrossRef] 13. Thio, S.K.; Jiang, D.; Park, S.-Y. Electrowetting-driven solar indoor lighting (e-SIL): An optofluidic approach towards sustainable buildings. Lab Chip 2018, 18, 1725–1735. [CrossRef][PubMed] 14. Krupenkin, T.; Taylor, J.A. Reverse electrowetting as a new approach to high-power energy harvesting. Nat. Commun. 2011, 2, 448. [CrossRef] 15. Narasimhan, V.; Jiang, D.; Park, S.-Y. Design and optical analyses of an arrayed microfluidic tunable prism panel for enhancing solar energy collection. Appl. Energy 2016, 162, 450–459. [CrossRef] 16. Park, S.-Y.; Nam, Y. Single-sided Digital Microfluidic (SDMF) Devices for Effective Coolant Delivery and Enhanced Two-Phase Cooling. Micromachines 2017, 8, 3. [CrossRef] 17. Mohseni, K.; Baird, E. Digitized Heat Transfer Using Electrowetting on Dielectric. Nanoscale Microscale Thermophys. Eng. 2007, 11, 90–108. [CrossRef] 18. Narasimhan, V.; Park, S.-Y. An ion gel as a low-cost, spin-coatable, high-capacitance dielectric for electrowetting-on-dielectric (EWOD). Langmuir 2015, 31, 8512–8518. [CrossRef] 19. Lee, K.H.; Zhang, S.; Lodge, T.P.; Frisbie, C.D. Electrical impedance of spin-coatable ion gel films. J. Phys. Chem. B 2011, 115, 3315–3321. [CrossRef][PubMed] 20. Cho, J.H.; Lee, J.; Xia, Y.; Kim, B.; He, Y.; Renn, M.J.; Lodge, T.P.; Frisbie, C.D. Printable ion-gel gate dielectrics for low-voltage polymer thin-film transistors on plastic. Nat. Mater. 2008, 7, 900–906. [CrossRef][PubMed] 21. Fedorov, M.V.; Kornyshev, A.A. Ionic liquids at electrified interfaces. Chem. Rev. 2014, 114, 2978–3036. [CrossRef] 22. Cho, J.H.; Lee, J.; He, Y.; Kim, B.; Lodge, T.P.; Frisbie, C.D. High-capacitance ion gel gate dielectrics with faster polarization response times for organic thin film transistors. Adv. Mater. 2008, 20, 686–690. [CrossRef] 23. Jones, T.B. More about the electromechanics of electrowetting. Mech. Res. Commun. 2009, 36, 2–9. [CrossRef] 24. Hong, J.S.; Ko, S.H.; Kang, K.H.; Kang, I.S. A numerical investigation on AC electrowetting of a droplet. Microfluid. Nanofluid. 2008, 5, 263–271. [CrossRef] 25. Oh, J.M.; Ko, S.H.; Kang, K.H. Shape Oscillation of a Drop in ac Electrowetting. Langmuir 2008, 24, 8379–8386. [CrossRef] [PubMed] 26. Baret, J.-C.; Decre, M.M.J. Self-Excited Drop Oscillations in Electrowetting. Langmuir 2007, 23, 5173–5179. [CrossRef][PubMed] 27. Ko, S.H.; Lee, H.; Kang, K.H. Hydrodynamic Flows in Electrowetting. Langmuir 2008, 24, 1094–1101. [CrossRef][PubMed] 28. Mugele, F.; Baret, J.-C.; Steinhauser, D. Microfluidic mixing through electrowetting-induced droplet oscillations. Appl. Phys. Lett. 2006, 88, 204106. [CrossRef] 29. Wang, K.-L.; Jones, T.B. Saturation effects in dynamic electrowetting. Appl. Phys. Lett. 2005, 86, 054104. [CrossRef] 30. Berge, B.; Peseux, J. Variable Focal Lens Controlled by an External Voltage: An Application of Electrowetting. Eur. Phys. J. E 2000, 3, 159–163. [CrossRef] 31. Galiński, M.; Lewandowski, A.; St˛epniak,I. Ionic liquids as electrolytes. Electrochim. Acta 2006, 51, 5567–5580. [CrossRef] 32. Balducci, A.; Bardi, U.; Caporali, S.; Mastragostino, M.; Soavia, F. Ionic liquids for hybrid supercapacitors. Electrochem. Commun. 2004, 6, 566–570. [CrossRef] 33. Zhang, W.; Yuan, C.; Guo, J.; Qiu, L.; Yan, F. Supramolecular ionic liquid gels for quasi-solid-state dye-sensitized solar cells. ACS Appl. Mater. Interfaces 2014, 6, 8723–8728. [CrossRef] 34. Kim, G.T.; Jeong, S.S.; Xue, M.Z.; Balducci, A.; Winter, M.; Passerini, S.; Alessandrini, F.; Appetecchi, G.B. Development of ionic liquid-based lithium battery prototypes. J. Power Sources 2012, 199, 239–246. [CrossRef] Micromachines 2021, 12, 320 11 of 11

35. Zhou, F.; Liang, Y.; Liu, W. Ionic liquid lubricants: Designed chemistry for engineering applications. Chem. Soc. Rev. 2009, 38, 2590–2599. [CrossRef] 36. Lee, K.H.; Kang, M.S.; Zhang, S.; Gu, Y.; Lodge, T.P.; Frisbie, C.D. “Cut and stick” rubbery ion gels as high capacitance gate dielectrics. Adv. Mater. 2012, 24, 4457–4462. [CrossRef][PubMed] 37. Zhang, S.; Lee, K.H.; Frisbie, D.; Lodge, T.P. Ionic conductivity, capacitance, and viscoelastic properties of block copolymer-based ion gels. Macromolecules 2011, 44, 940–949. [CrossRef] 38. Kornyshev, A.A. Double-Layer in Ionic Liquids: Paradigm Change? J. Phys. Chem. B 2007, 111, 5545–5557. [CrossRef] 39. Mugele, F.; Duits, M.; van den Ende, D. Electrowetting: A versatile tool for drop manipulation, generation, and characterization. Adv. Colloid Interface Sci. 2010, 161, 115–123. [CrossRef] 40. Jones, T.B.; Wang, K.L.; Yao, D.J. Frequency-dependent electromechanics of aqueous liquids: Electrowetting and dielectrophoresis. Langmuir 2004, 20, 2813–2818. [CrossRef] 41. Lu, Y.; Sur, A.; Pascente, C.; Ravi Annapragada, S.; Ruchhoeft, P.; Liu, D. Dynamics of droplet motion induced by Electrowetting. Int. J. Heat Mass Transfer. 2017, 106, 920–931. [CrossRef]