Asymmetrical Electrowetting on Dielectrics Induced by Charge

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Asymmetrical electrowetting on dielectric induced by charge transfer through an oil/water interface yuanyuan Guo, Yong Deng, Bojian Xu, Alex Henzen, Rob Hayes, Biao Tang, and Guofu Zhou Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01718 • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018

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Asymmetrical electrowetting on dielectric induced by charge transfer through an oil/water interface Yuanyuan Guoab, Yong Dengcd, Bojian Xucd, Alex Henzenab, Rob Hayesab, Biao Tangab*, and Guofu Zhouabcd* a. Guangdong Provincial Key Laboratory of Optical Information Materials and Technology & Institute of Electronic Paper Displays, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, P. R. China. b. National Center for International Research on Green Optoelectronics, South China Normal University, Guangzhou 510006, P. R. China. c. Shenzhen Guohua Optoelectronics Tech. Co. Ltd., Shenzhen 518110, P. R. China d. Academy of Shenzhen Guohua Optoelectronics, Shenzhen 518110, P. R. China

ABSTRACT Electrowetting on dielectrics is a fascinating as well as a precise way in microfluid manipulation. As one of the controversial conclusions, charge trapping on the dielectric surface might be one of the causes which induce water contact angle saturation and form one of the significant issues that bear on the applications of 1 / 32

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electrowetting on dielectrics. Recently, it was demonstrated that the contact angle saturation can be significantly reduced by employing an oil lubrication layer on the hydrophobic surface. In this work, we have investigated the influence of an oil layer’s effects on electrowetting behavior by dissolving a nonpolar oil-soluble dye in the oil phase. We monitored the contact angle of water drops with varying pH on an oil-lubricated hydrophobic insulator. Interestingly, we found asymmetry in the electrowetting curve. Several analysis methods were proceeded to try to explain this asymmetric electrowetting phenomenon. Foremost, the electrochemical properties of dye

were

investigated

by

cyclic

voltammetry

which

demonstrates

that

oxidation-reduction reactions of the dye can indeed happen on the electrode and one irreversible peak was found which indicated that the dye molecule might decompose at a higher voltage. Secondly, thin layer cyclic voltammetry confirmed ions can transgress the oil/water interface. Also, the conductivity of the oil phase increases with the dissolved dye concentration, which indicates that charges can be transported in the oil phase. Finally, to further understand the transfer mechanism, the transient current of dye-doped oil was measured, which indicates that the formation of inverse micelles in the oil phase at high voltage could be one of the charge carriers. We demonstrated the oil-property dependent asymmetry phenomenon of electrowetting, and its association with charge transfer through the oil/water interface for the first time.

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KEYWORDS: oil/water interface, charge transfer, asymmetrical electrowetting, cyclic voltammetry

1. INTRODUCTION

Electrowetting-on-dielectric (EWOD) is one of the most efficient ways to manipulate micron-sized drops by changing the interfacial tension, which has a comprehensive scientific and technological application in chemistry, bioengineering, sensors, and displays

1-5

. A liquid drop can be deformed, transport, detached, separated or merged

in a digital microfluidic chip which is controlled by electrowetting process while chemical reactions, molecular detection and analysis or video recording can occur 6-7. The voltage-dependent wettability manipulation of polar liquids based on EWOD can be described by the Young - Lipmann equation:

where θ is the contact angle under specific applied voltage (V), θ0 is the initial static contact angle,

is the electric permittivity of vacuum,

between the droplet and air or surrounding oil, d and

is the interfacial tension

are the thickness and relative

permittivity of the dielectric layer, respectively.

The charge trapping mechanism has investigated to explain the contact angle saturation during electrowetting

8-11

. Even though different perspectives were

considered, to some extent there was agreement that the charge trapped at the

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three-phase contact line would cause this saturation. Under ideal circumstances, there should be little, or no charge accumulation at the dielectric surface when no external voltage is applied, while, in the presence of an external voltage, the charges will accumulate at the liquid/solid interfaces and a contact angle θ decreases would be observed and exhibit a significant deviation from the Young - Lipmann equation

12

.

Although a lot of effort is being spent on understanding the wettability of EWOD, a consistent conclusion is still not achieved. The limitation of EWOD on the digital microfluidic system is mainly because of the contact angle saturation and hysteresis which result in instability and less control accuracy 13. Some work has been done to reduce contact angle saturation such as multilayer dielectrics; oil lubricates dielectric layers, ionic liquids, and AC driving to reduce the contact angle hysteresis and lower contact saturation angle

14-21

. The oil lubrication of dielectric layers can sufficiently

minimize the contact angle saturation and can be described by a modified Young’s – Lippmann equation 22. The infused oil is typically an ideal insulator material, such as silicon oil. Unlike polymer dielectric materials (AF, parylene C), there is no preference for the charge to bond to the oil/water interface so that saturation could be delayed to even higher voltage. On the other hand, if there are molecules in the infused oil layer (which cannot dissolve in water) that can be charged during electrowetting process, then electrochemical reaction may occur at the oil/water interface. Charges will accumulate at the oil/water interface and then transfer in the oil phase and accelerate the saturation, resulting in decreased or no electrowetting response. Depending on the property of the added molecule, there would be an 4 / 32


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asymmetrical contact angle with respect to voltage polarity. The contact angle measured in oil lubricating system is called the “apparent contact angle”, and two circumstances will happen: the oil clock the water drop or not cloak which depending on the spreading coefficient，where Sow = γwa - γwo - γoa, Sow > 0 the oil will cloak the water droplet or if the Sow < 0, implies otherwise 23. The interfacial science on immiscible two-phase liquids, normally oil/water, is attracting increasing attention in electrowetting systems. Electrowetting at two immiscible liquid interfaces with ion transfer (ITIES) has been discussed over a decade. Two theoretical assumptions were used: two immiscible pure- dielectric liquid system wherein the organic phase ions or free charges cannot transfer across the liquid-liquid interface, or two impermeable electrolytes solutions were used, and ions or free charges can freely transfer across the interface over a specific electrode potential 24. Until now, the above theoretical simulations have not been well validated by experiments. In the ITIES system with one aqueous electrolyte solution and one oil organic electrolytes, when the applied potential is smaller than the smallest ionic energy of transfer in the two electrolytes, a double layer was formed at the liquid/liquid interface. With the change of potential, the contact angle of an organic electrolyte drop in an aqueous environment would change. In this case, electrowetting could happen at a very low applied voltage 25. This research also indicated that if for the liquid/liquid/solid system, the operation voltage exceeds the polarization potential window, ion transfer across the liquid/liquid interface would happen which will result 5 / 32


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in change of the double-layer capacitance, and disturbed of the electrowetting curve. One can expect that if there is ion transfer across the liquid/liquid interface, the double- layer capacitance would reduce and the contact analog modulation would decrease as well. And in many cases, dielectric organic phases and electrolyte solutions might appear at the same time in electrowetting applications. Recently, Melittin adsorbtions at the liquid-liquid interface was reported by Manuel et al. and an asymmetrical contact angle of melittin drop on the metallic electrode was found during cyclic voltammetry which might be because of the ion transfer between water and 1,2-Dichloroethane (DCE) interface 26. Asymmetrical electrowetting under DC or low-frequency AC (0.1Hz) was reported due to the polarization of dielectric materials with two liquid configuration

27

. In general, due to the property of insulator coating

material, depending on the pH of electrolytes, the surface tends to adsorb ions or cations from t electrolytes solution or through chemical interactions (protonation or deprotonation) of some chemical groups. For instance, Fluoropolymer AF 1600 was reported to tends to adsorb hydroxide ions rather than hydrogen. They concluded that the initial charges located at the interface (point of zero charges) were not biasinduced, but depending on pH and strongly on material-electrolyte interaction that leads to the asymmetrical electrowetting. However, the effect of the surfactant (polypropylene glycol P425) which would slightly dissolve in dodecane on the oil/water interface was not taken into account. This could also be one of the reasons causing asymmetrical electrowetting. Liu et al. also reported that the asymmetry in the electrowetting curve between positive and negative voltage could be explained by the 6 / 32


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different bond strengths of ions within the surface 28. In their simulation, the charged molecules are pulled from the drop by large local fields and when the peak electric force near the edge of the drop exceeds the molecular binding force saturation occurs. The Coulomb interaction increased with decreasing molecular chain length that thus promotes saturation. Since the anions and cations have opposite charge and different size, these will be bound with different strengths, and as a result, asymmetricak electrowetting might happen with voltage polarity. Experimentally, He et al. found large asymmetric electrowetting on a PFOTS modified silicon oil lubricated PTFE surface. At positive voltage, the contact angle decreased rapidly with voltage, and they explained the cause might be the structural rearrangement and self-assembly of a PFOTS monolayer at the interface. Unfortunately, no further discussion was provided 22

. Endres et al. also found asymmetrical electrowetting in ionic liquids. The

electrowetting contact angle change happened at negative voltage but not at positive voltage

29

. They also related this phenomenon to the molecular property of the long

chain cation aggregation and rearrangement during cathodic polarization. These rearranged molecules could then change their orientation and diffuse to the three-phase contact line on the negatively charged surface. The positive cation then reduces the local field strength and therefore no electrowetting is observed. Dielectric failure induced asymmetrical electrowetting was also reported that related to the ion size and type in the liquid phase, while in this research surfactants also appeared in the water phase, and the effect of surfactants on oil phase was not taken into account 18

. The charge/ion transfer in the oil/water interface might occur during electrowetting

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attenuation. For example, in the application of electrofluidic displays, a nonpolar organic dye solution is used to get full-color display

30

. Meanwhile, high rupture

voltage or flow back were found that related to the oil/water interface properties, primarily related to the organic molecular structure 31. In most literature, studies were done with pure oil phases such as alkanes or silicon oil, but in application aspect, where biomolecules, surfactants, proteins, dyes or some reactants from the water phases will appear at the oil/water interface or diffuse into the oil phase will result in unexpected variations. In this work, a nonpolar oil-soluble azo red dye, insoluble in water, was dissolved in decane as the oil phase. Asymmetrical electrowetting of a water drop on a doped oil lubricated fluoropolymer dielectric layer was observed under positive and negative voltage. Serial experiments were preceded to analyze, and an explanation of the charge generation and transport process between oil/water/dielectric interfaces that influence on electrowetting was presented.

2. EXPERIMENTAL Substrate preparation: ITO (100Ω/□) glass was adequately cleaned with acetone, ethanol and UP water (18.2MΩ/cm2) to remove organic and inorganic impurities and then was dried at 120 degrees in an oven for 2 hours. Amorphous fluoropolymer AF 1600, 4.2% (Chemours, εr 1.934) solution was spin coated on ITO glass at 1500rpm, 800nm AF coating was achieved after curing at 180°in a clean oven for 2 hours.

Contact angle measurement: Azo red dye (synthesized in our group) was dissolved in decane to formulate a 0.1M solution. 1μL of oil was deposited on the AF coated 8 / 32


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substrate first, then 10μL 0.01M NaCl (99.5% igma ldrich) was carefully deposited on top of the oil drop to force the oil to spread under the water drop. A pico-ammeter (Keithley 6487) was used as a DC power source during contact angle measurement. A 0.5mm diameter Pt wire (φ = 0.5mm) was used as the electrode. The apparent contact angle was monitored and calculated with time directly by deltaphysics Contact Angle System (OCA 15Pro) during DC voltage variation. Three pH buffer solutions were made from a commercial package of potassium hydrogen phthalate (pH 4), mixed phosphate (pH 6.86) and sodium tetraborate (pH 9.18). For each condition, the apparent contact angle was measured twice, and four values were measured for each voltage step, and the average was taken for the voltage-contact angle curve. The accuracy of the contact angle measurement is ±1°. Oil conductivity measurement: A testing cell with 22 μm gap (area = 2cm2) between two ITO glass electrodes was made, and the 0.1 M colored oil was filled in the cavity by capillary force. A DC voltage was applied between the ITO electrodes, and the current was recorded by a Pico ammeter (Keithley 6487) at different voltages for 25s to compare the conductivity of oil. The same cell was subsequently used to measure transient current. The filled cell was first shorted for 120s, and then held at 50V for 30s. Immediately after removing 50V, -50V was applied, and the transient current was recorded. The current through the oil/water interface was measured by filling 5μl decane or dye-doped oil solution onto the water in a quartz glass tube (φ = 0.8mm) between two ITO electrodes. The tube was fully filled with liquid to make sure that 9 / 32


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liquid contact was made with both electrodes. A DC voltage was swept between ± 50V at ramp speed 1v/s.

Cyclic voltammetry measurement: For cyclic voltammetry test, 50 mM oil solution was prepared by dissolving azo red dye in dimethylformamide (DMF) (TCI, 99.5%). The electro-chemical properties of the dye were detected by a CHI 660E electrochemical workstation in a sealed conventional three-electrode cell. A GC working electrode, a Pt wire (φ = 0.5mm) counter electrode, and a Ag/Ag+ reference electrode (10mM AgNO3 in acetonitrile) were used for cyclic voltammetry of the dye. A supporting electrolyte (Tetrabutyl ammonium perchlorate) was purchased from TCI (98%) and recrystallized twice in ethanol and at last recrystallized in ethyl acetate, then dried in a vacuum oven for 48h

32

. All the solutions were degassed with dry Ar

(99.99%) for 30min before testing. A graphite electrode (φ = 3mm) and a pyrolytic graphite electrode were polished sequentially with 0.3 μm, 0.05 μm suspensions of aluminum oxide powder into a mirror surface and then ultrasonically cleaned with 95% ethanol and double distilled water before each measurement. The polished electrode potential has been checked against the ferrocene/ ferrocenium couple (Fc+/Fc) before each experiment. Thin-Layer Cyclic Voltammetry was performed on a pyrolytic graphite electrode (φ = 2mm) with a Pt wire (φ = 0.5mm) counter electrode, and a Ag/AgCl reference electrode (Cell B). Before each measurement, the pyrolytic graphite electrode was dried with a hot air gun for one minute and immersed in dichloroethane

(DCE)

for

prewetting.

10 / 32


10

mM

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Bis(triphenylphosphoranylidene)ammonium

tetrakis(4-chlorophenyl)borate

(BTPPATPBCl) (synthesized and purified according to reference

33

) was added to

DCE dye solution to improve the redox cyclic voltammograms of dye in DCE for thin layer cyclic voltammetry. 10 mM potassium perchlorate and lithium chloride were added to the water phase. 1 μL dye solution (dissolved in DCE) was deposited on the pyrolytic graphite electrode surface; a thin liquid layer was formed on the electrode surface because of the hydrophobic property of the electrode material. Then the electrode was immediately vertically put into the aqueous phase to proceed with the experiment.

3. RESULT AND DISCUSSION

3.1 The asymmetry phenomenon of electrowetting on dielectric-material system dependence

The contact angle measurement was executed by varying the applied voltage between the Pt electrode and the substrate electrode in the range of -50V to 50 V at ramp speed 1V/1s. Commercial pH buffer (pH = 4, 6.68, 9.18) was dissolved in water to achieve different pH values, and the buffer solution helped to stabilize the pH during the electrowetting process. The spreading coefficient in our system was: for decane lubricated case, Sow = (45-48-25) mN/m < 0, and for the dye-doped oil lubricated case, Sow = (45-35-25) mN/m < 0, so the oil was not cloaked by the water drop in both cases. During measurement, the leakage current was also monitored to make sure there was no dielectric breakdown. Fig. 1 a and b showed the contact angle of water on pure 11 / 32


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decane lubricated AF surface during electrowetting actuation. The decane layer can be treated as another series capacitor since it has a dielectric constant about two which is similar to the AF layer. The contact angle modulation at positive voltage is about 2 degrees larger than the negative voltage for all pH value. The slight deviation might be because of the aggregation of a double layer of pH buffer molecules at different polarized voltages 29. The pH of 0.01NaCl was 8.2 in our experiment, and the contact angle variation was almost the same as at pH 9.18 under negative voltage. The initial static contact angle was 95.6°, 91°and 86°for pH 9.18, 6.86 and 4 respectively. The initial static contact angle decreased with a decreasing pH buffer solution, likely due to the reduced water/oil interfacial tension. Contact angle hysteresis of water on the pure decane lubricated surface was minimal both at negative and positive voltage. Low contact angle hysteresis was also reported in on liquid infused PTFE film 34. In the case of contact angle on dye-doped oil lubricated AF surface, the initial contact angle was reduced to 90°, also because of the decreased water/oil interfacial tension. The contact angle modulation of negative voltage was not changed compared with the case where pure decane was used. But the contact angle of water on dye-doped oil was utterly different at pH 4, and 9.18 at positive potential compared with pure decane (Fig. 1 c and d). The contact angle modulation at positive voltage was 6° smaller at pH 4, 2°smaller at pH 6 and 12°smaller at pH 9.18 than the modulation at the corresponding negative voltage. There was almost no change of contact angle at the condition of pH 9.18 which indicated that electric field was reduced rapidly across the dye-doped oil layer while increasing the voltage. The tertiary amino group at the 12 / 32


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side chain of the dye molecule was used to improve the solubility in decane. On the other hand, the dye molecule shows surfactant property due to the branched carbon chain at the end of the dye molecule. The oil/water interfacial tension dropped from 48 mN/m to 25 mN/m by adding dye. Lee et al. provided evidence that a nonionic surfactant could increase the conductivity of the oil phase, and they also found that the nonionic surface with amine headgroup was positively charged at the oil/water interface

35

. Similarly, the tertiary amino in our dye might easily become positively

charged at the electrode surface under positive voltage. The conductivity would increase with adding of dye in decane since the dye molecule can be protonated which would cause more considerable contact angle hysteresis than the case with pure decane. At base (high pH) condition, the protonated dye molecule could interact with a hydroxyl group from water at the interface

36

. Charge transfer would also happen

during interface reaction. The interface reaction reduces the electric field across the oil layer, and there is less contact angle change. At pH 4, there was a sufficient amount of hydrogen ion in the water phase which makes the interface reaction difficult and causes large contact angle hysteresis between voltage increasing and decrease process.

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Figure 1. The contact angle of different pH water drop in (a, b) decane lubricated surface; (c, d) dye-doped lubricated surface. The maximum standard deviation of contact angle measurement was 0.45°.

From the above discussion, the asymmetry phenomenon of electrowetting on oil-lubricated dielectric layer was observed while dye was added into the oil phase. The interaction of protonation of dye with the hydroxyl of water at the base/acidic condition would be the possible cause to reduce the potential across the oil layer resulting in small changes contacted at a positive voltage. The detail of how the charge is generated and transfers across the oil/water interface will be further discussed below. 14 / 32


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3.2 Charge generation and charge transfer at the oil/water interface

During electrowetting actuation, an electrochemical reaction of organic dye molecules might happen on the substrate electrode. So, firstly, the electrochemical properties of the dye molecules were examined by cyclic voltammetry to assess the electrochemical stability of the dye under different potential. Since there was not an efficient electrolyte that can dissolve in decane as a supporting electrolyte, DMF was chosen as a protic solvent. The cyclic voltammetry of dye on GC electrode was done from -2.2 V to 0.7 V at scan rate of 0.02 mV/s, 0.05 mV/s, 0.1mV/s and 0.15mV/s (Fig. 2). The two reduction peaks at E = -1.3V and E = -1.8V were contributed to the azobenzene moiety 37. Two electrons transferred at two-step reaction, and the reduction at -1.8V was irreversible which means the dye molecule could be decomposed on the electrode at higher potential

38

. Once the dye molecule decomposed, the polar product will

increase the polarity of the oil solution and enhance the charge transfer through oil phase and enhance the charge transfer at the oil/water interface. The other redox couple at -0.8V was also reversible and was attributed to the tertiary amines unit

37

.

Tertiary amines can be oxidized by chemical one-electron electrochemically, and electrolytic dealkylation can lead from tertiary to secondary and primary amines 39-40. In an organic solvent, for example, acetonitrile (where water would always be present as an impurity), aliphatic amines are smoothly dealkylated electrochemically at the electrode surface to give an amine of lower order and an aldehyde

41

. D.

Zarzeczan´ska et al. reported nitrogen atom could be protonated and form a hydrogen 15 / 32


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bond with water in acetonitrile. The presence of two ethyl group at the 1-amino substituent, which prevents the formation of an intramolecular hydrogen bond involving the carbonyl group of the anthraquinone moiety in the neutral form of the derivative, while making it possible for the protonated species. A simulation calculation also proved that the tertiary amine (AQNEt2) was more easily protonated due to steric reason and the hydrogen bonding with water would significantly stabilize pronated molecule 42. The azo dye used in our research was also a highly conjugated structure, as was the case with anthraquinone. The pronation of the tertiary amine unit could happen at a lower potential probably because of the energetic electron withdraw effect from the next azobenzene. The steric-hindrance effect will prevent the pronated dyes attach to the electrode surface 43, and the positively charged dye would move to the oil/water interface where the binding of a hydroxyl group would be promoted. The reduction and oxidation peak current was linearly related to scanning rate square root (R2 ≥ 0.995) which also confirmed that the reaction was reversible at the electrode surface and a diffusion mechanism controlled the electrochemical reaction. The standard ferrocene/ferrocenium (Fe0/Fc+) redox system was used as an internal standard to calculate ΔE. The Esce of Ferro in DMF was 0.47, and the ΔE of the peak at E = -1.3V and E = -1.8V was 0.079mV and 0.071mV respectively. The number of the electron was 0.8 that also indicated a reversible one-electron transfer reaction at each step.

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Figure 2. Cyclic voltammograms of 50mm dye in DMF with different scan rate; the insert indicates the relationship of peak current with the square root of scan rate and dye structure.

ITIES (interface between two immiscible electrolyte solutions) are used as a novel electrochemical method to detect the ion or charge transfer between water and organic solvent

44

. It is suitable for studying interface properties of electrowetting directly

because applied potentials can be varied during electrowetting actuation and is subject to electrochemical control; the water and oil interfacial properties can be independently changed by adjusting the solution composition. As one of the interfacial electrochemistry methods, thin layer cyclic voltammetry (TLCV) was used to study the charges transfer process at the oil/water interface in our work. The thin layer cyclic voltammetry method does not need complicated experimental setup and can proceed in a conventional three-electrode system 44-45. As shown in Fig. 4 below, after adding dye into the organic solvent, oxidation and reduction peaks appeared compared with only the supporting electrolyte in the organic solvent which indicated 17 / 32


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that there was electron transfer between organic layer and water phase. Because DCE was volatile and it was not easy to form an organic layer on the electrode between, so 1μl of the solution was used. The IR drop across the organic layer was larger and cause reduction of the peak current. The cyclic voltammetry confirmed that there was charge transfer across the liquid/liquid interface and according to Karyakin et al. the charge should be transferred from water phase to organic phase to compensate the charge of the oxidized mediator

46

. The kinetics and mechanism of the electron

transfer were not within the scope of this paper.

Figure 3. Thin layer cyclic voltammogram of oil on EPG electrode: only support electrolyte in DCE (black line) and dye-doped oil together with support electrolyte in DCE; the insert showed the setup of thin layer cyclic voltammetry.

For thin layer cyclic voltammetry, the organic phase has limited to DCE, but in electrowetting systems, decane was used as the organic phase. So a simple setup was used to measure the current across water/dye-doped oil interface, the installation as described in section 2. The voltage was scanned between 50V and -50V, where an 18 / 32


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apparent peak was seen when the voltage first went to positive (Fig. 4). No current peak was observed if only decane were filled with water neither whether voltage first goes to positive or negative. In the case of decane, the absolute value of the current was almost the same at both negative and a positive voltage. But with dissolved dye in decane, the current increased dramatically, and the absolute value of the current was not the same (comparing negative and positive voltage branch), which is caused by the positively charged dye molecules and the charge transfer across the oil/water interface and diffusion in the oil phase.

Figure 4. The current through the oil/water interface: blue and green lines indicate a current of decane/water interface a with different voltage sweep direction; red and black line shows the current through dye-doped oil and water with different voltage sweep direction.

The peak appeared while the voltage sweep direction was from positive to negative, and could be explained by the assumption that the dye was protonated and transferred to the oil/water interface. The counter ion from water was attracted to the interface to 19 / 32


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maintain the electric neutrality in the liquid phase. As soon as the voltage changed to the negative voltage, the excess amount of counter ions were released rapidly from the interface and went to the opposite electrode surface, where a current peak appeared.

3.3 Charge diffusion in the oil phase during asymmetric electrowetting

In the dye-doped oil phase, the current increased with increasing voltage (Fig. 5). As soon as the applied voltage between two electrodes exceeds 15V, the current increases dramatically which also indicated that the dye-doped oil solution was not stable at higher DC potential, and the dielectric constant of oil increased with voltage. Using a simple calculation learns that, every charged molecule was about 1μm away from other charged molecules. Typically, the dye molecule size was in the range of nanometer or even smaller which indicated that there might be micelles having formed in the oil phase.

Figure 5. The current of 0.1M dye-doped oil under different voltage in a 22μm gap cell 20 / 32


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The mechanism of charge transfer by inverse micelles formation with surfactant in the nonpolar solvent was well studied during the investigation of the electrophoresis paper principle

47-48

. They explained that the surfactant would form inverse micelles

in a nonpolar solvent and a water drop in an organic solvent would be encapsulated in the core of the inverse micelles which thus became charge carriers.

Figure 6. The transient current of 0.1 M dye-doped oil under -50V in a 22 gap cell, the insert shows the schematic of an inverse micelle in the oil phase.

The transient current was used to explore the kinetics of the transfer principle. The inverse micelles formed by OLOA were around 7nm with uniform size distribution, and quite stable in the nonpolar organic solution. The dye molecule used in our experiment also has polar and nonpolar groups which mean that it can perform as a surfactant in decane 49. The average size of inverse micelles of the surfactant “ OT” in dodecane was reported to be around 7 nm, and the size of the charge carrier was larger than the inverse micelles

50

. The inverse micelles size in dye-doped oil cannot

be measured by dynamic light scattering because the solution samples did not scatter 21 / 32


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strongly enough. So, according to the literature, the estimated size of inverse micelles in decane should be in the range of 10 - 20 nm. The testing cell was put under 50 V for 60 s to make sure that all charge carriers were transported to the electrode with opposite polarity. Then the transient current was measured after the voltage reversed to -50 V. Since the dye molecule does not behave like a regular surfactant with has positive and negative ions, a higher voltage was needed to protonated dye molecule to form inverse micelles. Current peaks appearing after the voltage was reversed at the different time might be because of the different size of micelles formed by dye molecules (Fig. 6). The diffusion speed of large size inverse micelles would be slower than the smaller size ones, and the diffusion rate of the inverse micelles will decide the speed of charge transfer. As mentioned before, charges might be generated at the dielectric layer surface because of electrochemical reactions or because of the protonation of the dye. The inverse micelles would form as soon as charged molecules are present in the oil phase, and these inverse micelles can transfer charge towards the oil/water interface. Even though the transient current measurement provides the evidence that the pronated dye can form inverse micelles in the oil phase, there is also a possibility that the water impurities dissolving in oil also can form inverse micelles which also would increase the conductivity of oil. More studies are still needed to understand the mechanism of inverse micelle formation and charge transfer.

4. CONCLUSION

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Asymmetrical electrowetting was found on a dye-doped oil lubricated dielectric surface with varying pH water phase. The reason for small contact angle changes with increasing positive voltage at high pH value was explained by the association of protonated dye in the organic solvent with negative hydroxyl group from the water. The diazo bond might undergo irreversible electrochemical reaction as shown in the cyclic voltammogram, which also proved that the tertiary amino donor group would be protonated readily under an applied voltage. A couple of reduction and oxidation peaks from thin layer cyclic voltammetry confirmed that there was ion/charge transfer from the water phase to the organic phase. Furthermore, the peak of the current through the water/oil interface was observed when the voltage first went to positive which could confirm the interaction of counter ions from water with protonated dye molecules. Even though the kinetics of how the charge is transferred at the oil/water interface are not fully understood, the transient current measurement indicated that there might be inverse micelles that formed in the oil phase assisting the charge transfer. It should be noticed that this study has researched only the influence of organic molecules during EWOD, the impact of oil film instability was not within the scope of this paper. We used pure decane as a reference (where the viscosity of pure decane and 0.1M dye-doped oil was almost the same) and assume that the oil film instability would happen in both cases similarly. In this way, we can exclude the possibility that the asymmetrical electrowetting was induced by oil instability. Notwithstanding its limitation, this experiment research result will hopefully serve as

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useful information to better understand the influence of organic molecules in the oil phase or the water phase on EWOD applications in different areas.

AUTHOR INFORMATION

Yuanyuan Guo received her Master degree in Chemical Engineering from Twente University in 2010. She worked as a research engineer at electro-wetting display technology area in Electronic Paper Display Institute of South China Normal University for 3years, mainly responsibly is working on the influence of color oil property on electrowetting behavior. Currently, she is a Ph.D. in physical chemistry from Electronic Paper Display Institute of South China Normal University.

Dr. Young Deng received his Ph.D. in applied chemistry from the Dalian University of Technology, State key laboratory of fine chemicals in 2013. After that, he spends two years as a post-doc at the Zhejiang Sci-Tech University. He subsequently worked as a senior project engineer at Shenzhen Guohua Optoelectronics Tech. Co. Ltd. He focused primarily on designation and synthesis of reflective colored oil materials in electro-wetting displays. 24 / 32


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Dr. Bojian Xu obtained his Ph.D. in NanoElectronics at the University of Twente, Enschede, The Netherlands in 2017. He is now a postdoctoral researcher in Shenzhen Guohua Optoelectronic Tech. Co., Ltd, Guangzhou, China. His research interests are Electrowetting on Dielectric (EWOD) and organic electronics.

Prof. Alex Henzen obtained his MSc. In Organic Chemistry at the University of Leiden, The Netherlands. He is now a distinguished professor of South China Normal University. He is a well-known expert in the field of flat panel displays, co-inventor of electronic paper displays based on electrophoretic technology. He has 30 years of experience in liquid crystal display and electrophoretic display research and development. He received the IEC1906 award, the American green optoelectronic award and the Society for Information Displays Special Recognition Award.

Prof. Robert A. Hayes received his Ph.D. in Material Science from Brunel University, UK (1998), his M. App. Sci. in Mineral Processing Chemical Technology from the University of South Australia, Australia (1986), and his BS in Physical Chemistry from the University of Melbourne, Australia (1993). He is a full 25 / 32


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professor working at the Institute of Electronic Paper Displays at South China Normal University, China. Professor Hays has been working on electrowetting displays or more than 15 years. His research interests are interfacial chemistry and materials and processing of electrofluidic displays. Corresponding Author

Prof. Biao Tang received his Ph.D. in mechanical engineering from the South China University of Technology, Guangzhou in 2013. He was a visiting scholar at School of Engineering of Matter, Transport, and Energy of Arizona State University, USA. He is now an associate professor in South China Normal University and charge of the R&D activities on microfluidic displays (or named electrowetting displays).

His

research

interests

also

include

micro/nanostructure

or

electrowetting-based new applications.

Prof. Guofu Zhou received his Ph.D. degree in Materials Science from the Institute of Metal Research in China (1991) and in Physics from the University of Amsterdam in the Netherlands (1994). Prof. Zhou has served Philips Research Laboratories as a senior scientist, principal scientist and now senior consultant since 1995. During the last 20 years at the headquarters of Royal Dutch Philips Electronics, Prof. Zhou has initiated and completed many innovative projects in science & 26 / 32


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technology research and converted them into the industry with great success. He is now a professor at the South China Academy of Advanced Optoelectronics of South China Normal University. His research interests are electronic paper displays and functional materials.

Author contribution The manuscript was written through contributions of all authors. All authors have approved the final version of the manuscript.

Funding Sources The research for this paper was financially supported by the National Key R&D Program of China (Grant No. 2016YFB0401501), National Natural Science Foundation of China (Grant Nos. 51561135014, 61771204), the Program for Changjiang Scholars and Innovative Research Teams in Universities (Grant No. IRT_17R40), Guangdong Innovative Research Team Program (Grant No. 2013C102), Science

and

technology

project

of

Guangdong

Province

(Grant

Nos.

2016A030310438, 2016B090909001, 2017A010103002), Science and Technology Project of Shenzhen Municipal Science and Technology Innovation Committee (Grant No. GQYCZZ20150721150406), Guangdong Provincial Key Laboratory of Optical Information Materials and Technology (Grant No. 2017B030301007), MOE International Laboratory for Optical Information Technologies, 111 Project. ACKNOWLEDGMENT

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This research was supported by EFD team in SCNU. We thank our colleagues form their greatly assisted the investigation. REFERENCES 1. Banpurkar, A. G.; Nichols, K. P.; Mugele, F., Electrowetting-Based Microdrop Tensiometer. Langmuir 2008, 24 (19), 10549-10551. 2. Krupenkin, T.; Taylor, J. A., Reverse electrowetting as a new approach to high-power energy harvesting. Nature Communications 2011, 2, 448. 3. Levy, U.; Shamai, R., Tunable optofluidic devices. Microfluidics and Nanofluidics 2008, 4 (1), 97-105. 4. Hayes, R. A.; Feenstra, B., Video-speed electronic paper based on electrowetting. Nature 2003, 425 (6956), 383-385. 5. Jang, I.; Ko, H.; You, G.; Lee, H.; Paek, S.; Chae, H.; Lee, J. H.; Choi, S.; Kwon, O.-S.; Shin, K.; Oh, H. B., Application of paper EWOD (electrowetting-on-dielectrics) chip: Protein tryptic digestion and its detection using MALDI-TOF mass spectrometry. BioChip Journal 2017, 11 (2), 146-152. 6. Mugele, F.; Duits, M.; van den Ende, D., Electrowetting: A versatile tool for drop manipulation, generation, and characterization. Advances in Colloid and Interface Science 2010, 161 (1), 115-123. 7. Clement, C. E.; Thio, S. K.; Park, S.-Y., An optofluidic tunable Fresnel lens for spatial focal control based on electrowetting-on-dielectric (EWOD). Sensors and Actuators B: Chemical 2017, 240, 909-915. 8. Quinn, A.; Sedev, R.; Ralston, J., Contact angle saturation in electrowetting. The journal of physical chemistry B 2005, 109 (13), 6268-6275. 9. Mugele, F., Fundamental challenges in electrowetting: from equilibrium shapes to contact angle saturation and drop dynamics. Soft Matter 2009, 5 (18), 3377-3384. 10. Papathanasiou, A.; Papaioannou, A.; Boudouvis, A., Illuminating the connection between contact angle saturation and dielectric breakdown in electrowetting through leakage current measurements. Journal of Applied Physics 2008, 103 (3), 034901. 11. Verheijen, H.; Prins, M., Reversible electrowetting and trapping of charge: model and experiments. Langmuir 1999, 15 (20), 6616-6620. 12. Verheijen, H. J. J.; Prins, M. W. J., Reversible Electrowetting and Trapping of Charge: Model and Experiments. Langmuir 1999, 15 (20), 6616-6620. 13. Klarman, D.; Andelman, D.; Urbakh, M., A Model of Electrowetting, Reversed Electrowetting, and Contact Angle Saturation. Langmuir 2011, 27 (10), 6031-6041. 14. Li, Y.; Parkes, W.; Haworth, L. I.; Ross, A. W. S.; Stevenson, J. T. M.; Walton, A. J., Room-Temperature Fabrication of Anodic Tantalum Pentoxide for Low-Voltage Electrowetting on Dielectric (EWOD). Journal of Microelectromechanical Systems 2008, 17 (6), 1481-1488.

28 / 32


Page 28 of 39

Page 29 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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15. Schultz, A.; Chevalliot, S.; Kuiper, S.; Heikenfeld, J., Detailed analysis of defect reduction in electrowetting dielectrics through a two-layer ‘barrier’ approach. Thin Solid Films 2013, 534, 348-355. 16. Papageorgiou, D. P.; Tserepi, A.; Boudouvis, A. G.; Papathanasiou, A. G., Superior performance of multilayered fluoropolymer films in low voltage electrowetting. Journal of Colloid and Interface Science 2012, 368 (1), 592-598. 17. Berry, S.; Kedzierski, J.; Abedian, B., Low voltage electrowetting using thin fluoroploymer films. Journal of Colloid and Interface Science 2006, 303 (2), 517-524. 18. Raj, B.; Dhindsa, M.; Smith, N. R.; Laughlin, R.; Heikenfeld, J., Ion and Liquid Dependent Dielectric Failure in Electrowetting Systems. Langmuir 2009, 25 (20), 12387-12392. 19. Paneru, M.; Priest, C.; Sedev, R.; Ralston, J., Electrowetting of Aqueous Solutions of Ionic Liquid in Solid−Liquid−Liquid Systems. The Journal of Physical Chemistry C 2010, 114 (18), 8383-8388. 20. Luo, J. T.; Geraldi, N. R.; Guan, J. H.; McHale, G.; Wells, G. G.; Fu, Y. Q., Slippery Liquid-Infused Porous Surfaces and Droplet Transportation by Surface Acoustic Waves. Physical Review Applied 2017, 7 (1), 014017. 21. Brabcova, Z.; McHale, G.; Wells, G. G.; Brown, C. V.; Newton, M. I., Electric field induced reversible spreading of droplets into films on lubricant impregnated surfaces. Applied Physics Letters 2017, 110 (12), 121603. 22. He, X.; Qiang, W.; Du, C.; Shao, Q.; Zhang, X.; Deng, Y., Modification of lubricant infused porous surface for low-voltage reversible electrowetting. Journal of Materials Chemistry A 2017, 5 (36), 19159-19167. 23. Smith, J. D.; Dhiman, R.; Anand, S.; Reza-Garduno, E.; Cohen, R. E.; McKinley, G. H.; Varanasi, K. K., Droplet mobility on lubricant-impregnated surfaces. Soft Matter 2013, 9 (6), 1772-1780. 24. Monroe, C. W.; Daikhin, L. I.; Urbakh, M.; Kornyshev, A. A., Principles of electrowetting with two immiscible electrolytic solutions. Journal of Physics: Condensed Matter 2006, 18 (10), 2837. 25. Monroe, C. W.; Urbakh, M.; Kornyshev, A. A., Double-layer effects in electrowetting with two conductive liquids. Journal of The Electrochemical Society 2009, 156 (1), P21-P28. 26. Méndez, M. A.; Nazemi, Z.; Uyanik, I.; Lu, Y.; Girault, H. H., Melittin Adsorption and Lipid Monolayer Disruption at Liquid–Liquid Interfaces. Langmuir 2011, 27 (22), 13918-13924. 27. Bonfante, G.; Roux-Marchand, T.; Audry-Deschamps, M. C.; Renaud, L.; Kleimann, P.; Brioude, A.; Maillard, M., Polarization mechanisms of dielectric materials at a binary liquid interface: impacts on electrowetting actuation. Physical Chemistry Chemical Physics 2017, 19 (44), 30139-30146. 28. Liu, J.; Wang, M.; Chen, S.; Robbins, M. O., Uncovering Molecular Mechanisms of Electrowetting and Saturation with Simulations. Physical Review Letters 2012, 108 (21), 216101. 29. Liu, Z.; Cui, T.; Li, G.; Endres, F., Interfacial Nanostructure and Asymmetric Electrowetting of Ionic Liquids. Langmuir 2017, 33 (38), 9539-9547.

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30. Zhou, K.; Heikenfeld, J.; Dean, K. A.; Howard, E. M.; Johnson, M. R., A full description of a simple and scalable fabrication process for electrowetting displays. J. Micromech. Microeng. 2009, 19 (6), 065029. 31. Giraldo, A.; Massard, R.; Mans, J.; Derckx, E.; Aubert, J.; Mennen, J., 10.3: Ultra low‐ power Electrowetting‐based Displays Using Dynamic Frame Rate Driving. SID Symp. Dig. Tech. Pap. 2011, 42 (1), 114-117. 32. Jeziorek, D.; Ossowski, T.; Liwo, A.; Dyl, D.; Nowacka, M.; Woznicki, W., Theoretical and electrochemical study of the mechanism of anthraquinone-mediated one-electron reduction of oxygen: the involvement of adducts of dioxygen species to anthraquinones. Journal of the Chemical Society, Perkin Transactions 2 1997, (2), 229-236. 33. Samec, Z.; Trojanek, A.; Krtil, P., Dynamics of phospholipid monolayers on polarised liquid-liquid interfaces. Faraday Discussions 2005, 129 (0), 301-313. 34. Hao, C.; Liu, Y.; Chen, X.; He, Y.; Li, Q.; Li, K.; Wang, Z., Electrowetting on liquid-infused film (EWOLF): complete reversibility and controlled droplet oscillation suppression for fast optical imaging. Scientific reports 2014, 4, 6846. 35. Lee, J.; Zhou, Z.-L.; Behrens, S. H., Interfaces Charged by a Nonionic Surfactant. The Journal of Physical Chemistry B 2018. 36. Gohain, B.; Sarma, S.; Dutta, R. K., Protonated dye-surfactant ion pair formation between neutral red and anionic surfactants in aqueous submicellar solutions. Journal of Molecular Liquids 2008, 142 (1), 130-135. 37. Chiu, K. Y.; Tran, T. T. H.; Wu, C.-G.; Chang, S.-H.; Yang, T.-F.; Su, Y. O., Electrochemical studies on triarylamines featuring an azobenzene substituent and new application for small-molecule organic photovoltaics. Journal of Electroanalytical Chemistry 2017, 787, 118-124. 38. Fan, L.; Zhou, Y.; Yang, W.; Chen, G.; Yang, F., Electrochemical degradation of aqueous solution of Amaranth azo dye on ACF under potentiostatic model. Dyes and Pigments 2008, 76 (2), 440-446. 39. Lewis, F. D.; Ho, T.-I., Selectivity of tertiary amine oxidations. Journal of the American Chemical Society 1980, 102 (5), 1751-1752. 40. Portis, L. C.; Bhat, V.; Mann, C. K., Electrochemical dealkylation of aliphatic tertiary and secondary amines. The Journal of Organic Chemistry 1970, 35 (7), 2175-2178. 41. Ross, S. D., The mechanism of anodic dealkylation of aliphatic amines in acetonitrile. Tetrahedron Letters 1973, 14 (15), 1237-1240. 42. Zarzeczaoska, D.; Niedziałkowski, P.; Wcisło, A.; Chomicz, L.; Rak, J.; Ossowski, T., Synthesis, redox properties, and basicity of substituted 1-aminoanthraquinones: spectroscopic, electrochemical, and computational studies in acetonitrile solutions. Structural Chemistry 2014, 25 (2), 625-634. 43. Adenier, A.; Chehimi, M. M.; Gallardo, I.; Pinson, J.; Vila, N., Electrochemical oxidation of aliphatic amines and their attachment to carbon and metal surfaces. Langmuir 2004, 20 (19), 8243-8253. 44. Shi, C.; Anson, F. C., A Simple Method for Examining the Electrochemistry of Metalloporphyrins and Other Hydrophobic Reactants in Thin Layers of Organic Solvents 30 / 32


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Interposed between Graphite Electrodes and Aqueous Solutions. Analytical Chemistry 1998, 70 (15), 3114-3118. 45. Lu, X. Q.; Zhang, L.; Sun, P.; Yao, D., Thin-layer cyclic voltammetric studies electron transfer across liquid/liquid interface. European Journal of Chemistry 2011, 2 (1), 120-124. 46. Karyakin, A. A.; Vagin, M. Y.; Ozkan, S. Z.; Karpachova, G. P., Thermodynamics of Ion Transfer Across the Liquid|Liquid Interface at a Solid Electrode Shielded with a Thin Layer of Organic Solvent. The Journal of Physical Chemistry B 2004, 108 (31), 11591-11595. 47. Strubbe, F.; Prasad, M.; Beunis, F., Characterizing Generated Charged Inverse Micelles with Transient Current Measurements. The Journal of Physical Chemistry B 2015, 119 (5), 1957-1965. 48. Prasad, M.; Beunis, F.; Neyts, K.; Strubbe, F., Switching of charged inverse micelles in non-polar liquids. Journal of Colloid and Interface Science 2015, 458, 39-44. 49. Tehrani-Bagha, A.; Holmberg, K., Solubilization of Hydrophobic Dyes in Surfactant Solutions. Materials 2013, 6 (2), 580. 50. Yezer, B. A.; Khair, A. S.; Sides, P. J.; Prieve, D. C., Use of electrochemical impedance spectroscopy to determine double-layer capacitance in doped nonpolar liquids. Journal of Colloid and Interface Science 2015, 449, 2-12.

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Table of Contents Graphic

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Contact angle of different pH water drop ondye-doped lubricated surface. 53x34mm (300 x 300 DPI)


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Contact angle of different pH water drop on (a,b) decane lubricated surface; (c,d) dye-doped lubricated surface 142x116mm (300 x 300 DPI)


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Cyclic voltammograms of 50mm dye in DMF with different scan rate; the insert indicates the relationship of peak current with the square root of scan rate and dye structure. 64x49mm (300 x 300 DPI)


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Thin layer cyclic voltammogram of oil on EPG electrode: only support electrolyte in DCE (black line) and dyedoped oil together with support electrolyte in DCE; the insert showed the setup of thin layer cyclic voltammetry. 64x48mm (300 x 300 DPI)


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The current through oil/water interface: blue and green lines indicate a current of decane/water interface with different voltage sweep direction; red and black line shows current through dye-doped oil and water with different voltage sweep direction. 65x49mm (300 x 300 DPI)


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Current of 0.1M dye-doped oil under different voltage in a 22µm gap cell 64x48mm (300 x 300 DPI)


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Transient current of 0.1 M dye-doped oil under -50V in a 22 gap cell, the insert show the schematic of inverse micelle carreries in the oil phase. 63x46mm (300 x 300 DPI)


Asymmetrical Electrowetting on Dielectrics Induced by Charge

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