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Ultra-Low-Voltage Electrowetting A. A. Kornyshev,† A. R. Kucernak,*,† M. Marinescu,† C. W. Monroe,†,‡ A. E. S. Sleightholme,†,‡ and M. Urbakh§ Department of Chemistry, Faculty of Natural Sciences, Imperial College London, SW7 2AZ London, United Kingdom, Chemical Engineering, College of Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109-2136, and School of Chemistry, Faculty of Exact Sciences, Tel AViV UniVersity, Tel AViV 69978, Israel ReceiVed: February 3, 2010; ReVised Manuscript ReceiVed: June 22, 2010
Electrovariable optical components based on liquid/liquid interfaces encompass microfluidic lenses and mirrors, fiber-optic switches, and display elements. These devices function on the basis of the electrowetting effect, where a liquid interface is deformed by an applied electric field. Existing electrovariable lenses use electrowetting on dielectrics; however, because of voltage losses across the dielectric layer typically a voltage variation of ∼60 V is required to achieve a full focal range. To date the lowest recorded voltage for shape deformation in display elements is 25 V, which is not ideal for portable devices. Herein we investigate the properties of an alternative system, based on the interface between two immiscible electrolytic solutions (ITIES). This system shows promise, reducing the voltage requirement for a substantial electrowetting response by 2 orders of magnitude. Presented are the first experimental demonstration of ITIES-based ultra-low-voltage electrowetting and a preliminary study of its dynamics. Introduction Today we witness a rapid development of electrovariable optics.1 A wide class of emerging devices is based on the electrical deformation of a liquid/liquid interface, such as voltage-driven variable-focus lenses that do not rely on mechanically driven adjustment of optical components. Liquid lenses for mobile telephone cameras and video equipment are already commercially available. Similar principles are being considered for the development of new electronic displays, fiberoptic switches, and various microfluidic devices. Common to all these technologies is the exploitation of the effect known as electrowetting, whereby the shape of a liquid/gas or liquid/liquid interface is dynamically modulated by application of an external electric field. The use of liquid self-assembly in variable optics is not a modern idea; its roots can be traced back to the astronomers of the 1870s, who produced telescopic mirrors from rotating pools of mercury.2 In the same era, physical chemists discovered that voltages, as well as mechanical forces, affect the shape of liquid/ liquid interfaces. The principles of electrocapillary phenomena, formulated by Lippmann and Gibbs,3 suggest that any ideally polarizable liquid/liquid interface under an applied voltage tends to change shape to maximize its capacitance. Electrocapillarity causes droplets to form spheroidal interfaces, suggesting a means to form viable liquid lenses.4,5 Some 60 years later, Frumkin et al. reported a change in contact angle in response to a bias when an oil droplet was placed on a mercury (Hg) electrode in different electrolytic solutions.6,7 Frumkin’s pioneering studies of electrocapillarity on liquid electrodes had a strong impact on electrochemistry,8 but the practical relevance of his electrowetting work was overlooked. One of the greatest problems * To whom correspondence should be addressed. E-mail: A.Kucernak@ imperial.ac.uk. The authors are listed in alphabetical order due to the different nature of their contribution. † Imperial College London. ‡ University of Michigan. § Tel Aviv University.
with Frumkin’s system - in terms of practicality - is that it involves a liquid mercury substrate, whereas similar voltages on most solid electrodes cause undesirable side effects, such as corrosion and electrolyte decomposition. In the late 1970s and early 1980s, interest in electrowetting was rekindled after it became clear that contact angles of sessile droplets can be controlled by changing the surface charge of polymer substrates.9,10 This electrowetting-on-dielectric (EWOD) configuration made electrowetting on solids feasible for practical implementations. Sondaghuethorst and Fokkink showed that gold surfaces with an insulating alkane-thiol coating can deliver a stable electrowetting effect.11,12 The thiol-coated system, although suffering from slow response times and large hysteresis, was also used by Whitesides et al.13 who, to the best of our knowledge, introduced the term “liquid lens” to electrowetting research. In an attempt to overcome substrate corrosion and liquid decomposition, Berge14 integrated a fluorinated organic insulating layer between the electrode and the droplet. This allowed a stable contact angle variation of 30° to 100° but required a voltage range of about 100 V. Berge demonstrated that a liquid lens could be produced using this technique,15 with switching times around 10 ms and operating voltages in the 300 V range. A number of companies have developed EWOD lenses (Varioptic) and displays (Liquavista). Recent progress in EWODbased devices has been covered in a number of reviews.4,5,16 EWOD technologies with 20-300 V bias voltages necessitate additional electronics and extra power. This could complicate portable implementations, as it increases the device weight and shortens the battery life. Reducing the operating voltage while keeping the same contact angle variation is desirable.17 In EWOD devices the reduction of the operating voltage is limited by the presence of the insulating film, whose thickness cannot be decreased significantly without weakening its blocking effect on Faradaic processes. In this article we explore an alternative system, based on the interface between two immiscible electrolytic solutions (ITIES), for which theoretical models of electrowetting have been
10.1021/jp101051e 2010 American Chemical Society Published on Web 08/17/2010
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Kornyshev et al. film. In an ITIES system, Figure 1b, the body of the droplet is electroneutral; the potential drops are localized within nanometer distances from each interface, where large electric fields of up to 107 V/cm are created. Just as the dielectric in EWOD devices decomposes at high voltages, ITIES systems also break down above a certain voltage range. The breakdown occurs when the localized voltage drop at the liquid/liquid interface becomes comparable to the free energy of transfer for ions. A typical ITIES system can sustain about 0.5 V across the interface, performing many functions below the breakdown voltage. Experimental Section
Figure 1. Cartoons of electrowetting systems based on (a) the EWOD configuration and (b) the interface between two immiscible electrolytic solutions (ITIES). Each figure shows two electrodes and an aqueous electrolyte surrounding the immiscible organic droplet. Adjacent to each figure, the potential distribution along the corresponding red lines is sketched. In part (a) a thin insulating film coats the electrodes, the organic phase contains no ions, and a large potential drop occurs over macroscopic distances across the droplet. In part (b) the oil phase contains organic anions and cations, excess charge builds up at the interface to balance the charges in the adjacent phase. The applied voltage is screened over a nanometer range, which causes electric fields up to 107 V cm-1 at the interface.
developed.18,19 Apart from revealing a significant electrowetting response for voltages less than 1 V, as we discuss below, the system also appears to provide new options for the study of fundamental physical chemical phenomena, such as wetting and friction. The unique feature of the ITIES system is that applied voltages induce the formation of back-to-back electrochemical double layers at the liquid/liquid interface. The corresponding localization of the potential drop results in a large electric field at the interface. Electrochemical research into ITIES goes back to Nernst. Although modern investigations began in the mid20th century,20 interest was recently boosted by the possibility to self-assemble molecular, supramolecular, and nanoarchitectures at the interface.21–26 The best studied ITIES systems are water/nitrobenzene and water/1,2-dichloroethane with small inorganic ions such as alkali halides within the aqueous phase and large organic ions in the oil phase. A cartoon comparing the potential and charge distributions in EWOD and ITIES electrowetting configurations is shown in Figure 1. In an EWOD device, Figure 1a, the potential drop is distributed across both the macroscopic droplet and the dielectric
An electrochemical cell consisting of a 4 cm × 2 cm × 2 cm glass cuvette was used, as depicted schematically in Figure 2. Gold working and counter electrodes were mounted on a mobile PTFE stand. An aqueous LiCl solution was poured into the cell to cover the working, counter, and reference electrodes, and a micropipet was used to place a 0.1 µL droplet of organic electrolyte onto the working electrode. The working electrodes were planar, made of gold-coated quartz substrates. The 1 cm ×1 cm quartz substrates were cleaned by repeated bathing in boiling piranha solution (3:1, concentrated H2SO4:30% H2O2) for 30 min. The substrates were then dipped into 1 mol dm-3 NaOH, rinsed with deionized H2O, and dried at 200 °C for 30 min. The conductive electrode surface was produced by sputtering a 200 nm gold layer on top of a 20 nm titanium interlayer to facilitate adhesion of gold to the quartz. Contact angles were measured from optical images captured by a charge-coupled device (CCD) camera through a video zoom microscope (Edmund Optics Infinity K2/S Long Distance Video Lens). The contact angles were determined by fitting the digital images with the Surftens program (OEG Gesellschaft fu¨r Optik, Germany) using five points along the droplet/surrounding solution boundary. Each droplet/electrode pair was cycled three times, and contact angle values were measured. The measurement error for one electrode was (5°. In all measurements the aqueous electrolytes were 0.1 or 0.5 M solutions of LiCl (Aldrich, 99.0%) in ultrapure water (18 MΩ cm, Millipore Elix 5); the organic electrolyte was 0.01 or 0.1 M tetrabutylammonium tetraphenylborate (TBATPB, Fluka, >99%) in nitrobenzene (Fluka, 99%, purified in a column containing acidic alumina particles). Each experiment was performed using at least three separate substrates in order to provide an estimate of the variance associated with the electrode surface. In general, contact angles measured after ten pulses were found to have a standard deviation of (10°, presumably owing to random variability in the quality of the sputtered gold surfaces. Measurements were performed at room temperature (22 ( 2 °C) using an Autolab general purpose electrochemical system (Ecochemie, Netherlands) to control the voltage versus an Ag/ AgCl reference electrode. The counter electrode was a gold wire, very large in surface area compared to the droplet. Before each set of measurements, the droplet/electrode system was prepared by pulsing five times positively and five times negatively at amplitudes of 2, 2.5, or 3 V (according to the pulse-voltage procedure discussed below) around a baseline bias voltage of 0 V vs Ag/AgCl. This preconditioning step was followed by a series of experiments in which the bias voltage was lowered progressively from 0 to -650 or -750 mV, by -50 mV increments; at each of these negative bias voltages, ten voltage pulses of negative amplitudes were applied to bring the droplet to a final shape. (Up to fifteen pulses were applied
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Figure 2. Schematic diagram of experimental geometry. The glass cuvette cell is 4 cm × 2 cm × 2 cm in size. The electrical connection to the Au coated quartz slide is not shown.
Figure 3. Unassisted ultra-low-voltage driven contact angle variation of a 0.1 µL droplet of nitrobenzene containing 0.01 M TBATPB electrolyte, surrounded by 0.5 M LiClaq, atop a sputtered gold-on-quartz electrode substrate. The contact angle variation, which displays strong hysteresis, is measured during cyclic voltammetry at 10 mV s-1.
in some experiments, but only the first ten were observed to induce a significant change in the droplet shape.) Before lowering the bias voltage to the next step, the voltage set point was returned to 0 V vs Ag/AgCl, and 10 positive amplitude pulses were applied. Initial contact angles were obtained within (5°. This demonstrates the repeatability of the initial contact angle after the electrowetting effect has been induced. To avoid electrochemical degradation reactions, the pulse duration was made shorter than the characteristic time scale of capacitive relaxation. In every experiment presented here, the aqueous electrolyte resistances and double layer capacitances combined to give time constants larger than 50 µs. The procedure for the dynamic experiment discussed in Figure 4 differed from the above in that the amplitude of the voltage pulse applied was 2 V and not 3 V. Results The low-voltage electrowetting response obtained during cyclic voltammetry of the ITIES-based electrowetting system is shown in Figure 3. As the potential was brought to more negative values, the contact angle Rc increased significantly. Comparative studies verified that there is no appreciable difference in the scale of the electrowetting effect when the solvent is 1,2-dichloroethane. Each cyclic experiment exhibited a pronounced hysteresis in the contact angle response, arising from the pinning of the three-
phase contact line by surface roughness and inhomogeneities on the gold electrode. The observed hysteresis loops were frequently not closed; droplets rarely returned to their initial state after being cycled from 0 V down to another voltage and back. In wetting of solid surfaces pinning effects are well-known to cause differences between advancing and receding contact angles and open hysteresis loops are not uncommon.5,41 Some degree of pinning accompanies electrowetting dynamics on any realistic solid substrate.27 Hysteresis can be avoided on clean liquid metal electrodes, such as Hg, but such surfaces are impractical. Polymeric dielectric coatings seemingly eliminate pinning in current EWOD devices, but at the cost of larger operating voltages. To overcome the pinning that causes the hysteresis in Figure 3, we have implemented a method whereby short voltage pulses are applied in addition to the steady-state bias voltage. Mechanical vibrations were previously successfully used to overcome hysteresis in contact angle measurements on surfaces.28 Electrical pulses, however, are more suitable for practical implementation due to their easier control and reproducibility. They activate the droplet by inducing extra stresses that help to overcome the pinning of the three-phase boundary. Since the duration of each pulse is very short, the energy cost of pulsing is minor. A video displaying the droplet dynamics under pulsing can be seen in the Supporting Information. The superposition of short voltage pulses onto the baseline potential yielded reproducible contact angles for any bias within the stability window. This approach improved the reproducibility of the contact angle and the response time without requiring atomically smooth electrodes. In principle, one can use a sinusoidal signal30 instead; in the context of low voltage ITIES electrowetting, however, pulsing appears to be the better option, as its effect depends on polarity, allowing for additional control. The step-by-step motion of the droplet under pulsing is shown in Figure 4a. This study of the time response is invaluable for understanding the details of the dynamics of electrowetting on an apparently smooth, but atomically rough or inhomogeneous surface. The study begins at open circuit, measured independently as 0.005 V vs Ag/AgCl and a contact angle of 70°; a constant bias voltage of 0 V is then applied, followed by a -0.650 V bias. Ten 50 µs short -2.0 V pulses, each two seconds apart, are applied in addition to the bias. Each pulse induces a change in the contact angle, but the change decreases with the pulse number. After ten pulses, the contact angle has saturated to an angle of 118°. Disconnecting the potentiostat produces an
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Kornyshev et al. were obtained from such images. They are shown in Figure 6 as a function of applied bias voltage at various electrolyte concentrations in the aqueous and nonaqueous phases. Each data point in Figure 6 represents an average over several experiments on identically prepared gold electrodes. The variability of the gold surface quality led to an average error of (10° in the final contact angle between different electrodes. Nevertheless, the main trend was reproducible: the electrolyte pair with the lowest concentration (0.1 M aqueous/0.01 M organic) exhibited negligible contact angle variation in the window between -0.50 and -0.75 V; the next more concentrated pair (0.5 M aqueous/0.01 M organic) exhibited a statistically significant 20° variation in the same voltage range. The most concentrated electrolyte pair (0.5 M aqueous/0.1 M organic) exhibited a 50° change in contact angle. Discussion
Figure 4. Contact angle pinning can be eliminated by a pulsed-voltage control technique. (a) Electrowetting dynamics for a 0.1 µL droplet of 0.1 M TBATPB in nitrobenzene surrounded by 0.5 M LiClaq on a sputtered-gold substrate. Pinning is eliminated by potential pulses to +/-2.0 V from the constant bias of -0.65 or 0.00 V vs Ag/AgCl. The lower part of the diagram shows the applied potential profile. During the periods 0-4.5 s and 24-32 s, the electrode was disconnected and so the potential is uncontrolled. (b) Absolute difference between the cosine of the angle in the ith pinned state and the “final” value of cosine, which is essentially established after the tenth pulse.
immediate relaxation to 88°. The potential is then set to 0 V, and ten 50 µs 2.0 V pulses are applied; this causes the contact angle to return asymptotically to the initial value of 70°. The dependence of the cosine of the pinned contact angle, related to the surface energy of the pinned state, on the pulse number is close to exponential, as shown in Figure 4b. Droplet motion in response to potential pulsessjumps between pinned statessoccurred over times shorter than the video frame duration of 40 ms. The total time period over which the ten pulses were delivered could be reduced by 2 orders of magnitude without influencing the droplet response. Long intervals between pulses were chosen, however, in order to facilitate contact angle measurement and elucidate the effect of each pulse. Figure 5 provides images of the truncated-sphere shape after pulsing for a set of bias voltages. The values of contact angles
Recent theoretical investigations18,19 have revealed a number of advantages to the inducement of electrowetting in ITIES systems. The theory of ITIES-based electrowetting is simpler than that of EWOD because the electric fields are screened by double layers at every interface, and the bulk of each liquid phase is electroneutral, as seen in Figure 1b. At the scale of the double-layer thickness the interface is flat and no geometryrelated electrostatic problem needs to be solved. The main competing factors in the system are easier to rationalize and optimize. The ITIES has a larger capacitance than that of the corresponding interface between a conductive and a nonconductive liquid. When either system is electrically polarized, the shape of the droplet tends to change to maximize the area of those interfaces that have the largest specific double layer capacitance. In terms of the liquid/liquid interface, the presence of organic electrolyte in the droplet amplifies the electrowetting response. Conversely, in the competition between the electrode/oil and the electrode/water interfaces, the presence of electrolyte in the droplet weakens the response. The capacitance of the electrode/ oil interface becomes larger with added electrolyte in the oil phase, which inhibits the replacement of the electrode/oil interface by the electrode/water interface. A third effect should not be overlooked: the presence of electrolyte in the oil droplet, the phase with lower dielectric constant, reduces the surface tension of the liquid/liquid interface31,32 due to an effect similar to that described by the Onsager-Samaras theory.33 This effect amplifies the electrowetting response, as the liquid/liquid interface becomes less stiff. The experimental data in Figure 6 shows that the strength of the response increases with added electrolyte in either liquid. A reproducible ultra-low-voltage electrowetting response is obtained. The plateau in the contact angle seen at potentials close to zero may be associated with a roughening of the gold surface in this potential range.34 The reduction of operation voltage brought about by the ITIES systems allows a substantial change of the shape of the droplet within the range of ideal polarizability of the electrode, i.e., before the onset of redox processes at the electrode/electrolyte interface. An advantage of the ITIES system is that it inhibits the electric-field divergence at the three-phase contact line.35 Large fields at the contact line are responsible for distortions of the truncated-spherical shape when an electrolyte-free droplet is surrounded by a conducting liquid. For ITIES systems these fields are screened, and, as seen in Figure 5, the spheroidal shape is maintained over the whole range of operation voltages,
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Figure 5. Droplet profiles (after ten pulses to -3.0 V) from the displayed applied bias voltages vs an Ag/AgCl reference electrode for a 0.1 µL nitrobenzene/0.1 M TBATPB electrolyte droplet surrounded by a solution of 0.5 M LiClaq. The electrode surface is emphasized by a solid bar. The dotted line represents a circular arc fitted to the droplet; the droplet geometry is a truncated sphere.
Figure 6. Pulse-assisted electrowetting curves. Contact angles measured after ten pulses from each bias voltage point (as given), for a 0.1 µL droplet composed of nitrobenzene TBATPB electrolyte surrounded by LiClaq. The composition of the electrolytes is: (circles) 0.01 M TBATPB in nitrobenzene (droplet) surrounded by 0.5 M LiClaq; (squares) 0.01 M TBATPB in nitrobenzene (droplet) surrounded by 0.1 M LiClaq; and (triangles) 0.1 M TBATPB in nitrobenzene (droplet) surrounded by 0.5 M LiClaq.
beneficial for lenses and mirrors. The removal of large fields at the three-phase contact line also reduces the danger of corrosion and liquid decomposition. The data is rationalized below according to the phenomenological theory in Marinescu et al.,36 where quantitative treatment is included. Changes in the shape of a truncated-spherical droplet can be identified with the dynamics of the three-phase contact line; the contact angle and the diameter of the droplet are directly related to the diameter of the three-phase contact circle. Since the displacement of the contact line after each pulse is much larger than the characteristic dimension of the roughness of the studied gold surface, the resistance to the motion of the contact line can be described in a first approximation by a positionindependent mean friction force. Under applied voltage, there are two forces acting on the contact line: a driving force, which tends to bring the droplet to the equilibrium shape for that voltage, and a friction force, which opposes this motion. The friction includes a viscous-like dissipation term proportional to the sliding velocity and a velocity-independent static term resulting from energetic and morphological inhomogene-
ities on the surface.37–39 If the driving force is larger than the static friction, the contact line moves. The driving force decreases as the droplet approaches its equilibrium shape. Free energy calculations40 for a given bias voltage yield a paraboliclike dependence on contact line diameter. The driving force can be approximated as decreasing linearly with displacement. Under a pulse of the same sign as the bias voltage, the driving force is enhanced. The pulse-related extra potential is initially unscreened and drops across the whole system. The emerging electric fields are small, and should first have little impact on the droplet geometry. However, depending on the amplitude of the pulse and on its duration, there can be a sizable quantity of charge built up at each interface. Because of the intrinsic nonlinearity of the phenomenon, even a minor screening of the pulse voltage leads to an extra field localized at the interface, which increases the driving force enough to change the position of the contact line. During a short pulse, the position of the contact line barely changes, but it acquires a finite velocity. After the pulse is turned off, the contact line moves toward its equilibrium position due to inertia and then stops balanced by the static friction. Each subsequent pulse activates the contact line to contract further toward a final position, determined by the balance between driving and friction forces. The sliding distance is shorter with each subsequent voltage pulse due to the weaker driving force. Since the friction term contains both static and kinematic components, the viscosity and mass of the moving liquid also influence the final geometry. Important parameters of the dynamics of pulse-assisted electrowetting are: the physical properties of the droplet, the nature and magnitude of the friction force, the free-energy profile as a function of droplet diameter, and the duration and height of the pulse. For typical parameter values, the general trend is an exponential-like decrease of the sliding distance after each pulse,36 as seen in Figure 4b. The initial preconditioning of the system allows the pulsing procedure to mitigate pinning. The droplet motion before polarizing the system with the stationary voltage and after returning to 0 V has the same history. In both cases the droplet is “equilibrated” by a series of pulses. Because of the friction forces inherent to the nonideal electrode surface, all stationary positions of the contact line within an interval around the equilibrium position are metastable. The width of the interval is determined by the static term of the friction force. The same force prevents the droplet from sliding back to its initial geometry after the pulse is turned off. This allows for a
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directional change in the contact angle of the droplet with pulsing. The electrowetting response is thus stronger than without pulsing. Due to the short pulse duration, this amplification is reached without the risk of triggering electrochemical reactions. Conclusions By varying the applied bias voltage between 0 and -0.7 V we obtained reproducible electrowetting responses in an ITIES system with sputtered-gold electrodes. Contact angle variation was between 65 and 125°. Insulating layers typical of EWOD systems were not required, as the operating voltage was well below the level at which Faradaic processes occur. Pulsedvoltage control techniques led to shape-change dynamics on the order of tens of milliseconds and enhanced the reproducibility of contact angles, while decreasing the variability of measured angles owing to friction. These observations pave the way toward a new type of electrowetting-based devices that use conductive substrates. The perspective of a dramatic voltage reduction is especially relevant for portable electrowetting-based technologies. The durability of their performance over thousands of cycles has to be tested before claiming their competitiveness. Current experimental results justify such future tests. Independently of facilitating the electrowetting response, the pulsing technique developed for ITIES systems has its own value for investigating the dynamics of wetting on nonideal surfaces. The understanding of the system behaviour would greatly benefit from further detailed studies, including (i) systematic investigations of different electrode materials and characterization of the homogeneity and roughness of their surface using scanning probe techniques, (ii) comparative studies on ideally flat single crystalline electrodes, where hysteresis may be minimal and pulsing not needed, (iii) experimentation with different liquids and electrolytes, including ionic liquids, and (iv) rigorous analysis of the effect of pulse amplitude and sign. Acknowledgment. The authors thank Tomer Barnea, Nico Cousens, Gleb Oshanin, Vladimir Tsionsky, and Galina Tsirlina for valuable discussions, and Steve Atkins for technical assistance. This work benefited a lot from discussions with and advice from the late Sasha Kuznetsov. This work was supported by the interchange grant of the Leverhulme Trust (UK), F/07058/P, and Israel Science Foundation (Grant 1109/09). Supporting Information Available: This contains a movie of the droplet dynamics under pulsing. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (2) (3) (4)
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Kornyshev et al. (5) Mugele, F.; Baret, J. C. J. Phys.: Condens. Matter 2005, 17, R705– R774. (6) Frumkin, A. N.; Gorodetskaya, A. V.; Kabanov, B.; Nekrasov, M. Phys. Z. Sowjetunion 1932, 1, 255–284. (7) Frumkin, A. N. Actual. Sci. Industr. 1936, 373, 5–36. (8) Parsons, R. Electrochim. Acta 2001, 46, 1095–1100. (9) Holly, F. J. J. Colloid Interface Sci. 1977, 61, 435–437. (10) Ponter, A. B.; Yektafard, M. J. Colloid Interface Sci. 1984, 101, 282–284. (11) Sondaghuethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1992, 8, 2560–2566. (12) Sondaghuethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1994, 10, 4380–4387. (13) Gorman, C. B.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1995, 11, 2242–2246. (14) Berge, B. C. R. Acad. Sci. II 1993, 317, 157–163. (15) Berge, B.; Peseux, J. Eur. Phys. J. E 2000, 3, 159–163. (16) Shamai, R.; Andelman, D.; Berge, B.; Hayes, R. Soft Matter 2008, 4, 38–45. (17) Kuiper, S.; Hendriks, B. H. W. Appl. Phys. Lett. 2004, 85, 1128– 1130. (18) Monroe, C. W.; Daikhin, L. I.; Urbakh, M.; Kornyshev, A. A. Phys. ReV. Lett. 2006, 97, 136102. (19) Monroe, C. W.; Urbakh, M.; Kornyshev, A. A. J. Phys.: Condens. Matter 2007, 19, 375113. (20) Randles, J. E. B. T. Faraday Soc. 1956, 52, 1573. (21) Girault, H. H. J.; Schiffrin, D. J. Electroanal Chem. 1989, 15, 1– 141. (22) Galletto, P.; Girault, H. H.; Gomis-Bas, C.; Schiffrin, D. J.; Antoine, R.; Broyer, M.; Brevet, P. F. J. Phys.: Condens. Matter 2007, 19, 375108. (23) Su, B.; Fermin, D. J.; Abid, J. P.; Eugster, N.; Girault, H. H. J. Electroanal. Chem. 2005, 583, 241–247. (24) Su, B.; Eugster, N.; Girault, H. H. J. Am. Chem. Soc. 2005, 127, 10760–10766. (25) Samec, Z.; Eugster, N.; Fermin, D. J.; Girault, H. H. J. Electroanal. Chem. 2005, 577, 323–337. (26) Kornyshev, A. A.; Kuimova, M.; Kuznetsov, A. M.; Ulstrup, J.; Urbakh, M. J. Phys.: Condens. Matter 2007, 19, 375111. (27) Marmur, A. Soft Matter 2006, 2, 12–17. (28) Decker, E. L.; Garoff, S. Langmuir 1996, 12, 2100–2110. (29) Monroe, C.; Kornyshev, A. A.; Kucernak, A.; Sleightholme, A.; Urbakh, M. US Patent US 2008/0283414, 2008. (30) Li, F.; Mugele, F. Appl. Phys. Lett. 2008, 92, 244108. (31) Kakiuchi, T.; Senda, M. Bull. Chem. Soc. Jpn. 1983, 56, 1753– 1760. (32) Markin, V. S.; Volkov, A. G.; Volkova-Gugeshashvili, M. I. J. Phys. Chem. B 2005, 109, 16444–16454. (33) Onsager, L.; Samaras, N. N. T. J. Chem. Phys. 1934, 2, 528–535. (34) Kornyshev, A. A.; Vilfan, I. Electrochim. Acta 1995, 40, 109–127. (35) Monroe, C. W.; Daikhin, L. I.; Urbakh, M.; Kornyshev, A. A. J. Phys.: Condens. Matter 2006, 18, 2837–2869. (36) Marinescu, M.; Urbakh, M.; Barnea, T.; Kucernak, A. R.; Kornyshev, A. A. J. Phys. Chem. 2010, submitted. (37) Daniel, S.; Chaudhury, M. K.; de Gennes, P. G. Langmuir 2005, 21, 4240–4248. (38) Buguin, A.; Brochard, F.; de Gennes, P. G. Eur. Phys. J. E 2006, 19, 31–36. (39) Urbakh, M.; Klafter, J.; Gourdon, D.; Israelachvili, J. Nature 2004, 430, 525–528. (40) Monroe, C. W.; Urbakh, M.; Kornyshev, A. A. J. Electrochem. Soc. 2009, 156, P21–P28. (41) Some degree of pinning accompanies electrowetting dynamics on any realistic solid substrate.27 Both can be avoided on clean liquid metal electrodes, such as Hg, but such surfaces are impractical. Polymeric dielectric coatings seemingly eliminate pinning in current EWOD devices, but at the cost of larger operating voltages.
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