Electrowetting of Aqueous Solutions of Ionic Liquid in Solid−Liquid

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Electrowetting of Aqueous Solutions of Ionic Liquid in Solid-Liquid-Liquid Systems Mani Paneru, Craig Priest, Rossen Sedev,* and John Ralston Ian Wark Research Institute, ARC Special Research Centre for Particle and Material Interfaces, UniVersity of South Australia, Mawson Lakes, South Australia 5095, Australia ReceiVed: December 23, 2009; ReVised Manuscript ReceiVed: April 1, 2010

The influence of water on the electrowetting performance of the ionic liquid (IL) [BMIM][BF4] in contact with an ambient phase of hexadecane was studied. Electrowetting experiments on a fluoropolymer surface with IL concentrations between 0 and 99% enabled investigation of various electrowetting parameters, including the maximum range of accessible contact angles, the saturation contact angle (and voltage), and spreading dynamics. Experiments carried out using dc and ac potentials were successfully described with the Young-Lippmann equation prior to contact angle saturation. The saturation contact angles differed for ac and dc electrowetting, indicating distinct saturation mechanisms. The dilution of [BMIM][BF4] by water, within the range of concentrations studied, had no direct impact on the electrowetting behavior, aside from indirect effects due to altered viscosity and interfacial tension. Introduction The spreading of liquid can be induced by application of an electric potential between an insulated electrode and a liquid.1 The effect is precise, rapid, and reversible, making electrowetting a desirable method for manipulating small droplets on a surface. Numerous fluidic devices have been developed based on the electrowetting phenomenon, including liquid pixel displays,2 microlenses,3,4 digital microfluidic devices,5,6 and others.7-10 The ability to implement electrowetting in fluidic devices has maintained considerable interest in its fundamental behavior. Despite much research, electrowetting behavior varies significantly due to small differences in the experimental design. Moreover, the proliferation of unconventional liquids, such as ionic liquids, in fluidic processes has led to a new wave of electrowetting studies. There remains a strong research focus on the characterization of electrowetting for increasingly diverse applications. In this article, the influence of water on the electrowetting behavior of an ionic liquid in a solid-liquid-liquid system is addressed, and it is the first time a study of this type involving an ionic liquid has been reported. The characteristic response of an electrically conducting liquid droplet, resting on an insulated counter electrode, is welldescribed by the Young-Lippmann equation1

cos θ ) cos θ0 +

ε0εrV2 2dγ

(1)

where θ and θ0 are the contact angles under applied, V, and zero potential, respectively, εr is the dielectric constant of the insulating material, ε0 is the permittivity of vacuum, γ is the interfacial tension of the liquid droplet in contact with the ambient phase (vapor or liquid), and d is the dielectric thickness. According to eq 1, the observed contact angle decreases with increasing applied potential, with a dependence that is more sensitive to the potential for thinner insulating layers and reduced interfacial tensions. For relatively low potentials, the Young* To whom correspondence should be addressed. E-mail: rossen.sedev@ unisa.edu.au.

Lippmann equation is generally in close agreement with experimental data, as reported widely.11-15 However, at higher potentials (where the contact angle is significantly reduced from θ0), the contact angle becomes insensitive to continued increases in the potential, which is termed contact angle saturation.14,16-20 Various descriptions of contact angle saturation have been presented, including charge injection,14 air ionization,20 finite conductivity,19 and dielectric breakdown.16,17 In addition, we have put forward an alternative explanation in which the liquid-solid interfacial tension is driven to zero by the applied potential.18,21 The latter is in quite good agreement with results presented in the literature.22-24 Nonetheless, a universal description remains contentious, perhaps, in part, due to dissimilar experimental apparatus and liquids that make direct comparisons difficult. Electrowetting behavior remains an intriguing area of research, courtesy of a wide variety of existing and potential applications. Studies have revealed the influence of the insulator25 and liquid22,24,26 properties, solid structure (e.g., roughness),27-29 and type of potential applied (i.e., ac or dc).21,30-32 Furthermore, digital processing of droplets using electrowetting circuits is now a routine tool for microfluidics.5,6,33,34 New challenges are now emerging. The unique properties of ionic liquids (ILs) are being exploited in a diverse range of chemical and physical processes. As reaction solvents, they are exciting due to their negligible vapor pressure,35 low flammability36 and high thermal stability,37 while, broadly, their physical and chemical properties can be tuned by varying the nature of the cations and anions.38,39 These unique characteristics make ILs well-suited to applications, including organic synthesis,40 catalytic reactions,41 and electrowetting.26,42,43 The latter, electrowetting of ILs, has been demonstrated by our group43 and others11,13,26,42 for liquid-vapor systems, although unanswered questions remain regarding the electrowetting performance of ILs. One uncertainty is the effect, if any, of water contamination or dilution on the performance of a water-miscible IL. It is wellknown that ILs are readily contaminated by water.44,45 Avoiding or removing adverse water contamination is a difficult task, which has the potential to greatly limit the applicability of ILs in electrowetting applications. Water addition affects the physi-

10.1021/jp912115n  2010 American Chemical Society Published on Web 04/15/2010

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Figure 1. (a) Experimental configuration of the electrowetting of the ionic liquid [BMIM][BF4] on a Teflon-coated electrode immersed in hexadecane. (b) Molecular structure of the ionic liquid [BMIM][BF4].

cal properties of ILs, including surface tension, viscosity, density, and electrical conductivity.39,46 For the specific case of 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF4], the physical properties change considerably with aqueous dilution, as reported by Liu et al.46 Previous studies using IL aqueous solutions focused on the electrowetting behavior in solid-liquid-vapor systems.13,26,42,43 In this article, we present a systematic study of electrowetting behavior for aqueous IL solutions in solid-liquid-liquid systems, that is, fluoropolymer-[BMIM][BF4]-hexadecane. [BMIM][BF4] was chosen for its model electrowetting behavior, based on previous studies,13,26 and its miscibility with water over the whole concentration range. Experimental Section Indium tin oxide (ITO)-coated glass slides (unpolished float glass slides, 30 nm ITO coating, Delta Technologies Ltd., Stillwater, MN) were cleaned with isopropanol, dried in a stream of filtered nitrogen, and dip-coated with AF1600 fluoropolymer (DuPont Fluoroproducts, Wilmington, DE). The coating solution consisted of 6% AF1600 dissolved in Fluorinert FC-75 solvent (perfluoro-2-butyl tetrahydrofuran, Derbyshire, U.K.). The slides were immersed in the coating solution at 200 µm/s and withdrawn at the same speed after a 15 s pause. The slides were then dried for 30 min in a laminar flow cabinet and then heated for 30 h at 100 °C. The AF1600 film thickness was determined by measuring capacitance. The capacitance under a 10 µL droplet of IL was measured using an HP Impedance Analyzer (HP4192A), and the base area of the droplet was determined from the optically measured diameter. The dielectric thickness was then calculated from the capacitance, base area of the droplet, vacuum permittivity, and dielectric constant (εr ) 1.93). The thickness obtained from the electrical method was confirmed using a stylus profilometer (Zeiss Handy Surf) at a step made in the AF1600 film. Both methods were in good agreement, giving film thicknesses of 2.3 ( 0.2 µm. The roughness of the surface was measured using atomic force microscopy (Digital Instruments Nanoscope III in tapping mode). The rms roughness of the fluoropolymer surface was 0.35 nm over a 500 nm × 500 nm scan area. The coated slides were loaded into a tailor-made cell, which was filled with hexadecane (99%, Sigma-Aldrich), before measurements using the configuration shown in Figure 1a. Electrowetting experiments were carried out by placing a 2 µL droplet of IL on the fluoropolymer surface using a micropipet. A platinum needle electrode was used to contact the droplet for application of a potential difference between the droplet and the insulated ITO electrode. For dc experiments, the ITO electrode was grounded. Potentials were increased/decreased in increments of 10 V using a power supply and amplifier (Trek

Paneru et al. model 610D high-voltage amplifier/controller, Medina, NY), unless otherwise specified. A signal generator (Kenwood, CR Oscillator, model AG-203) was used to apply ac potentials (700 Hz, sine wave or square wave). For each electrowetting curve, the voltage was increased from zero until potentials slightly beyond contact angle saturation. A fresh solid surface was used for every electrowetting measurement. Contact angles were determined from the profile of the droplet resting on the surface (ImageJ software), captured using a digital camera (CV-M10BX, JAI, Japan). To follow the rate of spreading, a high speed camera (Olympus model Encore MAC-2000) was used at a recording speed of 1000 f/s. The base area and the contact angle of the droplet were evaluated with a time resolution of 1 ms from the droplet profile (ImageJ software). The ionic liquid (IL) used in this study was 1-butyl-3methylimidazolium tetrafluoroborate, [BMIM][BF4], which has the molecular structure shown in Figure 1b. Throughout this article, this ionic liquid will be referred to as IL. The IL was purified by filtration through ultra-high-purity charcoal powder (Sigma-Aldrich) and a 0.2 µm Teflon filter, extraction with ethyl acetate, and vacuum (0.1 mbar) for 24 h.47 After purification, the IL was diluted with water to ionic liquid concentrations, [IL], between 0 and 99% by weight. Water concentrations less than 1% were not studied here due to potential uncertainty in the actual water content (the dry IL is readily contaminated by water45). Thus, we leave open the questions relating to electrowetting behavior at very low water concentrations (below 1%). For every [IL], the viscosity, density, and interfacial tension were determined. All the measurements were repeated three times per sample to give average values, which are reported in this study. The densities of the IL solutions were determined using gravimetric analysis, that is, weighing a known volume of aqueous [BMIM][BF4] solution in a 10 mL pycnometer (Hirschmann Laborgeräte, Germany). The weights were measured to 1/10 of a milligram using a Sartorius R200D balance (Sartorius AG, Germany). Viscosity was measured using a glass semimicro capillary viscometer (size 200, Cannon Instrument, U.S.A.). The liquid samples were allowed to flow freely through the two markings, and the efflux times were measured. The kinematic viscosity of the sample was calculated by multiplying the efflux time in seconds by the viscometer constant (0.09022 mm2/s2). The dynamic viscosity was then obtained as the product of the kinematic viscosity and the sample density. The interfacial tensions of the aqueous [BMIM][BF4] solutions were measured under hexadecane using the pendant drop method (OCA20, DataPhysics, Germany). For each solution, a droplet was formed at the end of a stainless steel needle (1.65 mm external diameter, Terumo Corp., Philippines) and, prior to detachment, the profile of the droplet was determined. From the profile of the droplet, the interfacial tension was calculated using the Young-Laplace equation (OCA20 software analysis). The interfacial measurements were repeated three times per sample to give average values, which are reported in this study. All experiments were carried out at room temperature (25 °C) in a class 1000 clean room (45% humidity). Results To fully characterize our experimental system, we measured the fundamental liquid properties of solution density, viscosity, and liquid-liquid interfacial tension. Figure 2 shows the density and dynamic viscosity as a function of the [BMIM][BF4] concentration in high-purity water. The values of the density

Electrowetting of Aqueous Solutions of IL

Figure 2. Density, F, and viscosity, η, as functions of the concentration of the ionic liquid [BMIM][BF4] in the aqueous mixture, [IL].

Figure 3. Correlation of the interfacial tension between aqueous solutions of [BMIM][BF4] and hexadecane, γLL, with the static advancing contact angle at zero potential, θ0, on Teflon-coated surfaces.

and the viscosity are plotted on separate y axes against the weight percent of [BMIM][BF4] in the aqueous solution. The density of the IL/water mixture is required for the IL-hexadecane interfacial tension measurements using the pendant drop technique. The viscosity is an important factor in understanding the spreading behavior during electrowetting. The solution density increases steadily from 1 g/cm3 for pure water to 1.18 g/cm3 for pure IL. The dependence is linear over the full scale of [IL], consistent with the findings of Liu et al.46 The solution viscosity, on the other hand, falls sharply upon addition of small amounts of water. For example, the viscosity of the pure [BMIM][BF4] (154 mPa · s46) decreases by about 90% upon addition of only 10% water. Such a sharp decline in the viscosity in the IL-rich region is likely to be caused by dissociation of the IL into free cations and anions.48 Figure 3 shows the measured IL-hexadecane interfacial tension (γLL) and the initial value of the contact angle (θ0), that is, when no voltage is applied, plotted against the IL concentration (on a log scale) in the mixture. The liquid-liquid interfacial tension was determined from both pendant drop and electrowetting measurements. The interfacial tension and the initial contact angle determined for each of the [BMIM][BF4] solutions were used for theoretical predictions of electrowetting according to the Young-Lippmann equation. A strong correlation between γLL and [IL] was observed, as one would expect from the behavior of conventional surfactants.22,24 In other words,

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Figure 4. Electrowetting behavior of [BMIM][BF4] in water solutions (the percentage of water is listed in the legend). The straight lines are the best fits for the Young-Lippmann equation for each mixture. The liquid-liquid interfacial tension, γLL, was extracted from the slope of the solid lines and is shown in Figure 3.

[BMIM][BF4] in aqueous solutions behaves as a surfactant. The response of γLL to water addition was consistent with the observations of Liu et al.46 In contrast to γLL, there exists a plateau in our θ0 measurements at intermediate concentrations, before further reducing with increasing [IL]. Figure 4 shows the cosine of the contact angle plotted against the square of the applied voltage (dc). Also shown are the fits (solid lines) for the Young-Lippmann equation, which are in excellent agreement with our experimental data until the onset of contact angle saturation. As demonstrated by Banpurkar et al.,7 the gradient of the curves can be used to extract interfacial tension values for various liquids. In the same way, liquid-liquid interfacial tension values have been extracted for each of the IL solutions studied here. The values of γLL extracted from the best fits to our electrowetting results are in very good agreement with the values obtained from the pendant drop method (Figure 3). Figure 5 compares dc and ac electrowetting curves. dc curves are presented in Figure 5a for the different IL solutions. We show positive branches only, as the positive and negative sides of the electrowetting curves were always very symmetrical. Note that the initial contact angle (zero potential) increases with water content; cf. Figure 3. A significant contact angle change (>100°) was observed upon application of voltage for all [BMIM][BF4] solutions studied. The experimental data correlate well with the values predicted by the Young-Lippmann equation prior to contact angle saturation. The voltage required to achieve saturation varied between 95 V (pure [BMIM][BF4]) and 150 V (pure water); however, we observed a common saturation plateau for all solutions at ∼50°. In ac experiments, Figure 5b, the electrowetting behavior is similar to our dc experiments for applied potentials below 100 V. At higher potentials, the ac electrowetting curve departs from the Young-Lippmann equation and the contact angle gradually reduces with increasing potential. Until the point of departure, the Young-Lippmann equation holds for ac and dc electrowetting for all [BMIM][BF4] solutions studied here. For ac electrowetting beyond the point of departure, contact angles of less than 20° could be achieved when the water content was relatively low, for example, [IL] > 80%. Figure 6a shows the spreading behavior for both [BMIM][BF4] (with 1% water) and high-purity water after a step change in potential from 0 to 200 V, that is, a value beyond contact

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Figure 5. Electrowetting behavior under (a) dc and (b) ac potential. The potential was increased in steps of 10 V. Solid lines show the best fits of the Young-Lippmann equation. Contact angle saturation (dashed line) occurs (a) at a constant contact angle of about 50° under dc potential and (b) at significantly lower contact angles (down to 15°) under ac potential (sine wave, 700 Hz).

angle saturation. The behavior differs remarkably in both the rate of spreading and the nature of the approach to the equilibrium position (base area, Abase). The rate of spreading is much slower for the pure IL, probably due to the high viscosity of the liquid, and the equilibrium base area is gradually approached over more than 100 ms. In contrast, the rate of spreading of pure water is very rapid and induces an overshoot of the equilibrium base area. The resultant peak, probably due to the inertia of the liquid, is seen in Figure 6a immediately prior to relaxation of the droplet to the equilibrium base area. All the other IL solutions gave spreading curves that fall between these two extreme cases. For comparison of the spreading rates, the time required for the droplet to first arrive at the equilibrium base area, τ, was determined and plotted against [BMIM][BF4] concentration; cf. Figure 6b. On the same graph, viscosity is shown and clearly correlates with the spreading time. Discussion The density of the aqueous [BMIM][BF4] solutions increased linearly with [IL] from 1.0 to 1.18 g/cm3. The addition of water to the IL has a dramatic effect on the viscosity of the solutions. The pure [BMIM][BF4] is highly viscous (154 mPa · s) and, even with moderate amounts of water added (e.g., 10%), the viscosity of the IL solution drops sharply (cf. Figure 2), consistent with that reported elsewhere.46,49 At low concentrations, the [BMIM][BF4] will exist in the form of free, solvated cations

Paneru et al.

Figure 6. Spreading of an aqueous [BMIM][BF4] droplet (volume ∼1 µL) on a Teflon-coated electrode in hexadecane after a step change in the applied voltage from 0 to 200 V. (a) Droplet base area, Abase, vs time, t, for a concentrated ionic liquid and pure water. (b) Time required to first reach the final base area, τ, and viscosity, η, as functions of the ionic liquid concentration, [IL], in the IL-water mixture.

and ions,45 whereas at higher concentrations, the IL anions and cations begin to interact with one another, either directly or mediated by water, increasing the degree of order in the liquids.45 Thus, small amounts of water disrupt the structure within the [BMIM][BF4], leading to a sharp decrease in viscosity with decreasing [IL]. Conversely, the ionic liquid-hexadecane interfacial tension was constant for [IL] > 80% and increased considerably as [IL] approached zero. This behavior reflects the relative affinity of IL ions and water molecules for the interface. IL ions are known to assemble at various interfaces due to their amphiphilicity, that is, a behavior similar to that of conventional surfactants.50 Adsorption at the interface lowers the liquid-liquid interfacial tension. Once a complete layer assembles at the interface, the interfacial tension becomes constant and insensitive to further increases in [IL]. Consequently, the interfacial tension is affected very little by the addition of water until [IL] < 80%, at which the interfacial tension begins to increase sharply toward that of pure water in hexadecane (52 mJ/m2); cf. Figure 3. It appears that there is a distinct change in the physical properties of the solutions at [IL] ≈ 80%. This behavior might reflect aggregation, for example, as micellar structures,51 and/or solvation of the BF4- anions that readily form hydrogen bonds with water.44 Therefore, in solid-liquid-liquid electrowetting systems, the ionic liquid [BMIM][BF4] can be effectively used as a water-soluble surfactant. Changes in the interfacial tension of the IL solutions were accompanied by a significant change in the wettability of the

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fluoropolymer, AF1600, surfaces in hexadecane. The static advancing contact angle (θ0) increased from 145° to 157° with decreasing [IL] until water and [BMIM][BF4] were present in equimolar proportions. This initial change in contact angle is not linked to any change in the interfacial tension, suggesting that the contact angle is sensitive to a degree of order present. Further addition of water has little effect on the contact angle until [IL] < 20%, where the contact angle increases further with decreasing [IL]. The increase was as much as 10° (cf. Figure 3) and can be linked to the changing liquid-liquid and, consequently, solid-IL interfacial tensions. It is reasonable to suggest that a decrease in the liquid-liquid interfacial tension would be accompanied by a decrease in the solid-IL interfacial tension due to the surface activity of the IL, thereby promoting spreading of the liquid and the observed decrease in the contact angle. Note that the solid-hexadecane interfacial tension is a property dependent only on the surrounding liquid (hexadecane) and the solid substrate (fluoropolymer) and remains unchanged irrespective of the concentration of [BMIM][BF4] used. Electrowetting results for the full range of [IL], that is, up to 99%, are shown in Figure 4. Prior to contact angle saturation, the data closely follow the Young-Lippmann equation, which is represented by the linear fits to the data in Figure 4 (solid lines). From the slope of the linear fit for each [BMIM][BF4] solution prior to contact angle saturation, the liquid-liquid interfacial tension can be determined; cf. eq 1. Extracting the interfacial tension from the electrowetting data in this way, consistent with Mugele et al.,7 permits a direct comparison with our pendant drop measurements, presented in Figure 3. The agreement is very good over the full range of concentrations, suggesting that this method may be effective where conventional liquid-liquid interfacial tension methods are not practicable, for example, a limited sample volume. The onset of contact angle saturation in dc electrowetting can be clearly distinguished in Figures 4 and 5a as a departure from the fit of the Young-Lippmann equation. Contact angle saturation in all of our dc electrowetting experiments occurred at a constant contact angle of about 50°. The saturation contact angle was insensitive to dilution of the [BMIM][BF4] with water, despite substantial changes in the physical properties of the droplet phase and the initial contact angle (at zero voltage). In this context, we revisit the mechanism for contact angle saturation. A number of explanations for contact angle saturation have been proposed, including charge injection,14 dielectric breakdown,16,17 and vapor phase ionization at the contact line,20 among others. We have previously proposed a hypothesis for saturation in which the solid-liquid interfacial tension is driven to zero by the applied field.18,21 In this instance, the saturation contact angle is determined by the liquid-ambient fluid (γLL) and solid-ambient fluid (γSL) interfacial tensions alone:

θsat ) arccos

γSL γLL

(2)

This hypothesis has been examined with some success for solid-liquid-vapor systems23 and solid-liquid-liquid systems22,24 but has yet to be applied to ionic liquids in solid-liquid-liquid systems. The physical picture does not differ significantly. The term that is driven to zero would now be the solid-ionic liquid interfacial tension, γS-IL, while γS-HD is unchanged, irrespective of the IL solution used. The saturation contact angle is, therefore, expected to increase with increasing γLL. The solid-hexadecane interfacial tension can be estimated

from the interfacial tensions of the hexadecane (γHD) and the fluoropolymer (γS). According to Fowkes52

γS-HD ) γS + γHD - 2(γSγHD)1/2

(3)

Using the surface tensions of hexadecane (27.6 mJ/m2) and the fluoropolymer (12.4 mJ/m2),21 we estimate the interfacial tension of the solid-hexadecane interface at 3.0 mJ/m2. From the ionic liquid-hexadecane interfacial tensions measured for the pure water and the pure [BMIM][BF4], we find that the saturation contact angle should vary between 82 and 87°. In practice, we find that the saturation contact angle is consistently 50° in our dc measurements, irrespective of the IL concentration. In ac electrowetting, the onset of saturation, when defined as the point of departure from the Young-Lippmann equation, is 80-95° and much closer to the zero-interfacial-tension prediction, that is, eq 2; cf. Figure 5. However, at higher potentials, the contact angle gradually reduces further, indicating that the “saturation contact angle” as defined by eq 2 is not an absolute limit. In fact, contact angles as low as 20° were observed in some cases, which were significantly below the minimum contact angle reached using dc electrowetting (50°). It becomes evident from the solid-liquid-liquid results presented here that the zerointerfacial-tension theory must be reconsidered. Differences between ac and dc electrowetting saturation have been observed previously for standard aqueous salt electrowetting systems (both solid-liquid-vapor30 and solid-liquid-liquid22), and thus, this behavior is nonspecific to IL electrowetting. Nonetheless, these results point to significant differences between ac and dc electrowetting, which may partly explain the diversity of mechanisms proposed in the literature. We now turn to the dynamics of spreading induced by electrowetting. Viscosity is a major factor that determines the rate of spreading of any liquid.53 This is also true in electrowetting. As shown in Figure 3, the viscosity of the [BMIM][BF4] solutions increases with IL concentration. Figure 6a shows the different spreading profiles for pure water and [BMIM][BF4] (with 1% water) while stepping the voltage from 0 to 200 V (dc). This potential corresponds to contact angle saturation, irrespective of which liquid is used, and therefore, the final base area is identical in each case (the saturation contact angle is 50°). The rate of spreading differs dramatically for the two liquids. The less viscous water spreads so rapidly that, in fact, the droplet’s base area expands beyond its equilibrium value due to inertia, before relaxing back to the equilibrium value. In contrast, the highly viscous [BMIM][BF4] spreads gradually until the equilibrium base area is reached. Intermediate concentrations of IL fall between these two extreme cases, with all solutions with [IL] < 20 wt % (i.e., low viscosity), overshooting the equilibrium position to some extent. Figure 6b plots the spreading time (i.e., the time required for the droplet to first arrive at the equilibrium base area) for each [IL] together with the viscosity of the mixtures. There is a clear correlation between viscosity of the liquid and the spreading time, as could be expected from previous results for non-IL systems.10 Although the rate of spreading is affected by the addition of water, this appears to be a purely viscous effect and is, therefore, not unique to IL-water mixtures. Electrowetting offers a unique opportunity to study spreading by manipulating the adhesion in the system without affecting its chemistry. We have probed the dynamics of the contact line, and details will be reported separately. A key goal of this work was to distinguish the influence, if any, of the presence of water in IL electrowetting (solid-liquid-

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liquid) systems. Many water-miscible ILs are particularly hygroscopic such that water contamination is difficult to avoid and/or remove.45 This is also true for [BMIM][BF4] used in this study. The [BMIM][BF4] used in this study contained as much as 1% water, as mentioned in the technical data sheet from the supplier (Merck, Germany). Any adverse influence of water on the performance of ILs in electrowetting would inevitably lead to difficulties in applications, for example, environmental controls and limited device lifetime. Neither ac nor dc experiments revealed any adverse effects from the addition of water to the [BMIM][BF4] over the full range of [IL]. In fact, the only differences between the EW behavior can be attributed to the changing physical properties of the solutions, that is, viscosity and interfacial tension, rather than a change in EW behavior. This is a promising result for the emerging applications of electrowetting that exploits the unique properties of ionic liquids. Unlike some applications that are water sensitive, for example, electrodeposition54 and chemical synthesis,40 electrowetting is a robust method for the manipulation of ionic liquids and is, therefore, suitable for application in a wide range of environments and devices. Conclusion The influence of water present in an ionic liquid, [BMIM][BF4], on the electrowetting behavior has been investigated for a solid-liquid-liquid system. The addition of water had a moderate effect on the static advancing contact angle (at zero potential) and the liquid-liquid interfacial tension. The addition of water also decreases the viscosity of the droplet and, consequently, increases the rate of spreading during step changes in the applied potential. Nonetheless, the nature of electrowetting itself was not altered by the addition of water (over the concentration range studied here) because the Young-Lippmann equation remains valid for all of our experiments prior to the onset of contact angle saturation. Contact angle saturation occurred at 50° (dc voltage) and 10-20° (ac voltage). Spreading/ retraction cycles exhibited near zero hysteresis, and ac and dc electrowetting curves were indistinguishable for low potentials (