Electrophoresis of a Charged Droplet in a Dielectric Liquid for Droplet

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Electrophoresis of a Charged Droplet in a Dielectric Liquid for Droplet Actuation Do Jin Im,* Jihoon Noh, Dustin Moon, and In Seok Kang Department of Chemical Engineering, Pohang University of Science and Technology, San31 Hyoja-dong, Nam-Gu, Pohang, Gyeongbuk, 790-784, South Korea

bS Supporting Information ABSTRACT: Electrophoretic motion of a charged droplet in a dielectric fluid under an electric field has been investigated experimentally for use as a microdroplet actuation method. The effects of the droplet size, electric field strength, and electrolyte concentration and ion species on the charging of an aqueous droplet have been examined. The amount of electrical charging has been measured by two different methods: indirect measurement using the image analysis of droplet motion and direct measurement using the electrometer. Quantitative comparison of the droplet charge measured experimentally and the theoretical value of a perfectly conductive sphere shows that an aqueous droplet is less charged than the corresponding perfectly conductive sphere. The limiting effect on electrical charging is more significant for an electrolyte droplet, and the effect is positively correlated to the electrolyte concentration rather than the ion species. This implies that the low electrical conductivity of water is not a major cause of the limiting effect. The scaling law of the charging amount for a deionized water droplet nearly follows that of the perfect conductor, whereas for an electrolyte droplet, the scaling law exponent is slightly higher. Some advantages and potentials of the current droplet actuation method are also discussed in comparison with the conventional ones.

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he droplet-based microfluidic system provides separate environments in which target samples and reagents are delivered precisely in discrete volumes without cross contamination or dilution due to diffusion. A high surface area to volume ratio facilitates a faster reaction time, resulting in high-throughput chemical reactions,1,2 and independent control of each droplet makes parallel processing and experimentation possible, allowing large data sets to be acquired efficiently.3 Mixing of reagents in droplets has been proved to be achievable within milliseconds, making multistep chemical reactions via droplet microfluidics possible.4 Accurate control by confinement of reagents in droplets enables single-cell5,6 and subcellular organelle6,7 analyses, which are essential for understanding and controlling how biological systems function. Due to these advantages unique to droplet-based systems, there are increasing numbers of chemical and biological applications,8,9 and it is becoming a rapidly growing technology in the microfluidics field.3,10 Electrowetting and dielectrophoresis are the two major manipulation methods for moving droplets in a microfluidic device.11 The electrowetting, or electrowetting on a dielectric (EWOD), method is also referred to as digital microfluidics (DMF) because droplets are manipulated on the arrays of electrodes,12
14 and this characteristic is a good match for array-based biological and chemical applications.15,16 However, electrowetting requires a droplet to be in contact with a surface, r 2011 American Chemical Society

which causes contact line pinning and biofouling due to surface contamination.11 In dielectrophoresis (DEP), a droplet does not have to contact a surface.17
19 The DEP force depends only on the dielectric property of the droplet, which provides the capacity for processing droplets composed of various media and promises great versatility across a wide spectrum of applications.18,20
22 DEP also has been used to manipulate various neutral matter such as cells,23 DNA,24 viruses,25 and single molecules26 for decades. However, the droplet actuation method using DEP is not as popular as EWOD because it needs a higher voltage27 and the resulting droplet velocity is slower than that of the EWOD method.18 Recently, we have investigated electric charging of a water droplet in a dielectric fluid on the electrode surface and the following electrophoretic motion of the charged droplet.28 Even though we are not able to explain all the experimental observations and the charging mechanism, we have found numerous possibilities of electrophoresis of a charged droplet (ECD) as a new droplet manipulation method.29,30 First, ECD is free from surface contamination. Although there is minimal contact with the electrode when charging occurs, the contact area is very small Received: January 28, 2011 Accepted: May 31, 2011 Published: May 31, 2011 5168

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Analytical Chemistry and the contact time is less than 1 ms.28 Once a droplet acquires charge, we can handle the charged droplet without further contact with the solid surface. Second, electrophoresis is simpler and more straightforward in principle than EWOD and DEP. Because we should understand complicated variation of the surface tension and induced dipole to predict the motion of the droplet in EWOD and DEP, respectively, there is a lot of theoretical31
33 and numerical34,35 research to explain the underlying physics. However, the electrophoretic motion of a charged droplet can be easily predicted if the amount of charge of the droplet is known under the given electric field. Third, the coalescence of droplets can be easily controlled. It is difficult to merge droplets in a microchannel, especially when the droplets are stabilized with surfactant, which has limited the widespread use of droplet technology in robust microreactors in microfluidic devices.36,37 An electric field can be used to make charged droplets merge in a microchannel36
38 quite easily, and furthermore, under a sufficiently high electric field, we can also cause droplets not to merge.39,40 In this paper, we present a quantitative analysis on the charging behavior of an aqueous droplet including electrolytes of various compositions and concentrations in a microfluidic biochemical application context. In the previous study, we found that the electric field strength and the droplet size are the key factors to electric charging of a water droplet. However, we were not able to give reasonable explanations of the discrepancies between the perfect conductor theory and experimental observations even in a qualitative way. Therefore, in the present work, special efforts have been made to obtain quantitative information on how much charge a droplet acquires upon contact with the electrode. A precise experimental setup has been devised, and the charge measurement has been performed in two different ways for comparison and cross-checking. In the biological application point of view, we should consider the electric charging of an electrolyte droplet rather than just a deionized water droplet, which was the main subject of the previous work.28,41,42 Because the electric charging of a droplet is supposed to be an electrochemical process, systematic investigation on the difference in the charging characteristics of an electrolyte droplet and a deionized water droplet can help to understand the charging mechanism of an aqueous droplet. This new droplet actuation method is expected to be an alternative or complementary one to accelerate the evolution of droplet microfluidics.

’ EXPERIMENTAL SECTION A schematic of the experimental system is shown in Figure 1. A transparent rectangular 12  12  45 mm3 (external dimensions) plastic test cell is placed between the cold light source and the microscope. The electrodes are designed to apply a uniform electric field and to minimize interactions with the walls of the test cell. Two rectangular 8  60 mm2 copper electrodes are aligned parallel to each other using acrylic spacers (Figure 1b), and the distance between the electrodes is designed to be 7.9 mm for the electrode assembly not to touch the test cell wall. A direct current (dc) power supply (Trek Inc.) and an electrometer (Keithley model 6514) are connected to these electrodes. A highspeed camera (Photron Fastcam 1024 PCI model 100K) is mounted on the microscope for data acquisition, and LabVIEW is used for image processing. A cooled light-emitting diode (LED) light source is used to avoid the thermal effect, and all experiments are performed at room temperature.

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Figure 1. (a) Schematic view of the experimental setup. (b) Schematic of the test cell and the electrode assembly.

Silicone oil (Shin-Etsu KF-96 100 cSt) is used as the suspending medium without any surfactant. Deionized water or electrolytes commonly used in biological systems such as phosphatebuffered saline (PBS), NaCl, and KCl solutions at various concentrations are used as a discrete aqueous phase. A micropipet (Eppendorf) is used to dispense a single aqueous droplet in the test cell. The experimental procedure is as follows. The silicone oil is prepared in the test cell. The electrode assembly is put into this test cell and connected to a dc power supply and an electrometer. A single aqueous droplet is dispensed through the hole of the upper acrylic spacer, and the power supply is turned on. Owing to the initial positive charge, the dispensed droplet moves toward the ground electrode. The polarity of the initial charge is checked by changing the polarity of the electrodes. Due to the high viscosity of silicone oil (100 cSt kinematic viscosity) and low density difference between silicone oil (971 kg/m3 density) and water, the droplet shows horizontal bouncing motion between electrodes as in Figure 2a. The horizontal velocity of a moving droplet is extracted from the high-speed camera image (bottom of Figure 2a). To precisely estimate the charge acquired in each contact of a droplet to the electrodes, we measure each droplet size and take velocity data only at the center of the test cell for the calculation. Using the Hadamard
Rybczynski solution for the drag force of a droplet,43 we estimate the charge, and this estimated charge is compared with the directly measured data using an electrometer. The drag force acted on a drop (the Hadamard
Rybczynski drag) is smaller than that on a solid particle (the Stokes drag) due to the interfacial slip. However, it is known that, for a fluid system which contains small quantities of contaminant, the drag force of a drop can be changed from the Hadamard
Rybczynski drag to the Stokes drag depending on the size of the drop and contaminant concentration.43 Because the estimated value by the image analysis can be influenced by the drag force calculation, we have checked the drag force of a freely falling droplet in silicone oil by measuring the droplet terminal velocity. A transparent rectangular 60  40  45 mm3 (external dimensions) acrylic reservoir is used for the silicone oil container. The same sized deionized water droplets used in the current study are dispensed into silicone oil (100 cSt) and the free-falling movement of the droplet is captured and analyzed using the same image processing procedure. The gravitational, buoyancy, and drag forces are calculated by measuring the droplet size, the densities of water and silicone oil, and the droplet terminal velocity. Using the force balance, we 5169

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Figure 2. (a) High-speed sequential images of a translating 78 nL droplet under 2.5 kV/cm and corresponding horizontal translational velocity profile. The droplet moves from left to right. (b) Drag force coefficient of a freely falling water droplet in 100 cSt silicone oil. a is the droplet radius, F the density difference, g the gravitational acceleration, μ the viscosity of silicone oil, and U the terminal velocity.

estimate the drag force coefficient of a freely falling water droplet in silicone oil as in Figure 2b. Because of the high viscosity of silicone oil, the Hadamard
Rybczynski drag force coefficient of a water droplet is 4.02, which is almost the same as that of a bubble. All the experimental data show that the water droplets of the current study undergo the Hadamard
Rybczynski drag not the Stokes drag when they moves in silicone oil. For the comprehensive analysis of the charging process of an aqueous droplet, we have focused on four parameters. The electrolyte ion species and concentration, size of a droplet, and electric field strength are considered. Because we are interested in biochemical applications, we use PBS, physiological saline (154 mM NaCl solution), and KCl solution with the same mole concentration to see the effects of different ion species. To examine the concentration effect, we use three different concentrations of KCl solutions of 1.54, 15.4, and 154 mM. We investigate the scaling law for the amount of charging of an aqueous droplet by varying the droplet volume from 100 to 1000 nL (from 288 to 620 μm in radius) under different electric field strengths (from 1.9 to 5.1 kV/cm).

’ RESULTS AND DISCUSSION For a perfectly conducting solid sphere in contact with an electrified planar electrode, the amount of charge acquired from the electrode is44 Qtheory ¼

π2 ð4πa2 ÞεE 6

where a is the radius of the sphere, ε the permittivity of the surrounding medium, and E the applied electric field strength. Because charges can exist only at the surface in a conductor and there is only one length scale in the problem, the acquired charge is proportional to the surface area of the solid sphere. Even though an aqueous droplet in a dielectric medium might be considered as a conductor, there are still some points to be considered before adopting the perfect conductor theory. First, an aqueous droplet has a deformable surface. We observed the deformation of a water droplet upon contact with an electrode and found that this deformation can affect the charging.28 Second, water has a finite conductivity. Because the charging occurs in less than 1 ms, the relatively low conductivity may limit the charging amount to a value that is smaller than the theoretical maximum. Third, unlike a metallic conductor, charge transfer by

Figure 3. Comparison between the measured charge and the perfect conductor theory for an aqueous droplet under 5.1 kV/cm. The relative standard deviation of each measurement is as follows: deionized water by image analysis (0.2
0.6%), deionized water by an electrometer (18.5
30.4%), KCl 154 mM by image analysis (0.8
4.9%), KCl 154 mM by an electrometer (13.4
31.9%).

free electron movement may not be possible in water. It seems to have a different charge transfer mechanism such as electrochemical reaction of ions. Therefore, we have investigated the charging characteristics of an aqueous droplet in comparison with the perfect conductor theory not only qualitatively but also quantitatively to get the information needed for droplet microfluidic device design. We have estimated the charge of an aqueous droplet by highspeed image analysis as in the previous study, and we have also directly measured the charge using the high-resolution electrometer. The two measurements have been performed simultaneously and compared with the perfect conductor theory as shown in Figure 3. The charge amount is on the order of several tens of picocoulombs, and the typical value of the surface charge density is 5.5  10
5 C/m2. Compared with the typical surface charge density of a fully ionized surface in water (0.3 C/m2),45 the surface charge density of a charged droplet is not so high. This implies that all the charges can be located only at the droplet surface and there is still some room available for more charge. 5170

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Figure 4. Dimensionless charge of deionized water and electrolyte droplets of various sizes under different electric fields. Each column represents a different electric field, and within each column, the droplet radius increases from left to right. (a) Effects of the electrolyte concentration. (b) Effects of the electrolyte ion species.

Overall trends and averaged values of measured data by two different methods show similar behavior, but the relative standard deviation of the measured data using the electrometer (13
32%) is larger than that of the estimated data using image analysis (0.2
4.9%) even though the two measurements have been made simultaneously. In the case of image analysis, variations of data are so small that all the error bars can be drawn within the data symbol and the error bars practically do not need to be illustrated. All the experimentally measured values for both deionized water and KCl 154 mM droplets are smaller than the theoretical values for a perfect conductor. Even though the conductivity of the KCl 154 mM solution (1.8 S/m) is much higher than that of deionized water (5.5  10
6 S/m), the droplet of the KCl solution does not get more charge than the deionized water droplet. Moreover, the KCl solution droplet is even less charged than the deionized water droplet and shows more variation of charging in each contact. Therefore, the low conductivity of water is not the major factor that limits the charging amount of an aqueous droplet to a smaller value than the theoretical maximum for a perfect conductor. To investigate the charging behavior of an aqueous droplet more systematically, we have measured the charge of deionized water and electrolyte droplets as shown in Figure 4. All the charges are nondimensionalized by the theoretical charge of the corresponding perfect conductor. For electric fields lower than 1.9 kV/cm, the droplet movement is so slow and unstable that it is difficult to get consistent experimental data, and higher electric fields over 5.1 kV/cm make an aqueous droplet break into small droplets due to electrospraying. In all the observable cases, an aqueous droplet is less charged than the perfect conductor, about 40
70% of the theoretical maximum. Larger droplets under a higher electric field tend to get more dimensionless charge than smaller droplets under a lower electric field. An electrolyte droplet is less charged than a deionized water droplet in all cases regardless of its concentration and ion species. All 1 μL droplets under 5.1 kV/cm converge to 74% of the theoretical value, which is the maximum observable dimensionless charge from the current experiments. To see the electrolyte concentration effect, KCl 1.5, 15, and 154 mM solutions are used. As the electrolyte concentration is increased, the charging of an electrolyte droplet is further limited in proportion to the concentration as shown in Figure 4a. This means that higher electrical conductivity does not help the electrolyte droplet get more charge than the deionized water droplet. It seems that electrolyte ions disturb the charging of the

droplet. Under lower electric fields, the limiting effect on charging due to electrolyte ions becomes more prominent and we can see the concentration effect more clearly, but as the electric field is increased, the limiting effect slightly decreases, especially for larger droplets. Under 5.1 kV/cm, the concentration effect becomes blurred and all the dimensionless charges converge to a value. To see the effects of the electrolyte ion species, KCl, NaCl, and PBS solutions of the same concentration are used as in Figure 4b. Under a higher electric field, a larger droplet is charged more, and the slope of this trend becomes steeper as the electric field is increased. We can clearly see this tendency for KCl and NaCl droplets, but the limiting effect on charging is slightly higher in the case of the KCl droplet. The charging characteristic of the PBS droplet is somewhat different, especially for smaller droplets: the PBS droplet gets more charge than the others. The amount of charging is also dependent on the polarity of the charge. In most cases, the positively charged droplet is faster than the corresponding negatively charged droplet, which implies that the negative charging is more limited than the positive charging (Figure 5 and Supporting Information). The data in Figures 3 and 4 are actually the averaged values of the two. For a deionized water droplet, the difference between negatively and positively charged droplet velocities is relatively small (The difference is at most 5%). Furthermore, unlike other cases, under an electric field lower than 3.8 kV/cm, the negatively charged droplet is slightly faster than the positively charged droplet as in Figure 5a. Under 3.8 kV/cm, the two droplet velocities are nearly the same. For a KCl 154 mM droplet and other electrolyte droplets (see the Supporting Information for other electrolytes), the difference is relatively large, and positively charged droplets are faster than negatively charged droplets in most cases. However, under 1.9 kV/cm, the difference is insignificant. It looks like the threshold electric field is lowered by the electrolyte ions. We may find some possible explanations for these charging phenomena from electrochemical reactions. Electric charging of an aqueous droplet in a dielectric medium cannot be explained by the free electron movement as in a perfect conductor. Electrolysis of water is one probable charging mechanism, and there exists related research that reports small gas bubble generation within a water droplet.41 The reduction potential of electrochemical reactions for the current electrolyte ion concentrations tells us that only water will be electrolyzed to produce hydrogen and oxygen. This means that Kþ, Naþ, and Cl
ions do not participate in the electrochemical reactions at the steady state. However, in a real dynamic situation, reversible reduction 5171

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Figure 5. Dependence of the average droplet velocity on the polarity of charge. (a) For a deionized water droplet, the velocity difference between positively and negatively charged droplets ranges from
0.08 to þ0.25 cm/s. (b) For a KCl 154 mM droplet, the velocity difference between positively and negatively charged droplets ranges from
0.04 to þ1.01 cm/s.

Figure 6. Scaling law of charging for an aqueous droplet. (a) Deionized water droplet. The reference state was chosen as a0 = 276 mm, E0 = 1.9 kV/cm, and the resultant charge was Q0 = 4.3  10
12 C. For the linear plot, R2 = 0.979. (b) KCl 154 mM droplet. The reference state was chosen as a0 = 258 mm, E0 = 1.9 kV/cm, and the resultant charge was Q0 = 2.9  10
12 C. For the linear plot, R2 = 0.965.

of Kþ and Naþ or oxidation of Cl
can be possible on a short time scale and the charge transfer by electrolysis of water may be disturbed by these reactions. Under a lower electric field, this electrochemical disturbing effect seems to be comparable with the amount of charging, whereas a under higher electric field, the disturbing effect is relatively less significant. A more detailed quantitative explanation is beyond the scope of the present study, and further research is needed to understand the complex electrochemical charging mechanism. The experimental results are summarized in the form of a scaling law to see the effect of the droplet size and electric field strength on the amount of electrical charging. Even though a deionized water droplet gets less charge than a perfect conductor, the scaling law of charging for a water droplet nearly follows the perfect conductor theory as shown in Figure 6a. When charging occurs upon contact with an electrode surface, a small droplet under a low electric field remains spherical whereas a large droplet under a high electric field is deformed into an elongated shape as in Figure 6a. Because the elongated deformation increases the amount of charge28 due to the increase in surface area, the scaling law exponent for the radius becomes slightly higher than the theory for a perfect conductor. If we look at the radius exponent separately for each electric field, the exponent gradually increases from 1.9 to 2.1 as the field strength increases. If the 5.1 kV/cm case, where deformation is particularly significant, is excluded from the data set, the scaling law follows the perfect conductor theory. This implies that, for small droplets under

a moderate electric field, which deforms the droplet little, the scaling law of charging for a water droplet can be assumed to follow the perfect conductor theory. In the previous study, the scaling law of a deionized water droplet was a bit different from that of the present study. The major differences of the present work from the previous one are the measurement of the dispensed droplet size and the use of the center velocity for charge estimation. Because the inaccuracy of the micropipet is larger than 10% for the current droplet volume range, in the present work all the droplet volumes are measured by image processing. To minimize the induction effect from the electrode surface, only the velocity data at the center of two electrodes are used for estimation of the charge. The exponent of the scaling law of charging for an electrolyte droplet is slightly higher than that of a water droplet. In Figure 6b, the scaling law for a KCl 154 mM droplet is shown as one typical example (for different concentrations and ion species, consult the Supporting Information). The higher scaling law exponent of 2.3 is obtained for the radius. This higher value is due to the significant interrupting effect of the ions on charging in the cases of smaller droplets as shown in Figure 4b. The scaling law exponent for the electric field is slightly higher because as the electric field is increased an electrolyte droplet gets more dimensionless charge, which means a decrease in the limitation effect on charging. For dilute electrolytes (KCl 1.5 and 15 mM), the exponent is between those of water and KCl 154 mM droplets, whereas for other electrolytes (NaCl 154 mM and 5172

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Figure 7. (a) ECD actuation of 500 nL droplets of complex fluids and nonaqueous liquids under 3.2 kV/cm. The time step for each sequential image is 0.15 s. For the cell solution, the cell concentration is 2 500 000 cells/mL and the buffer is the mixture of DMEM (Dulbecco’s modified Eagle's medium), 10% FBS (fetal bovine serum), and 100 g/mL penicillin and streptomycin. (b) Coalescence of 500 nL milk and vinegar droplets under 3.2 kV/cm. A few seconds after the coalescence, the precipitation of casein is observed (the last figure).

PBS), the scaling law exponents are the same. Even though the detailed charging characteristics of PBS droplets are somewhat different from those of the other droplets, the overall scaling law is the same. This means that the scaling law of charging for the electrolyte is mainly dependent on the concentration not on the ion species. To demonstrate the validity of the ECD system with complex biochemical and biological species, the translation and coalescence of various fluids are tested as in Figure 7. Although the translational velocity (we can infer from sequential images) and the deformation of each droplet are a bit different, a consistent actuation with the same order of magnitude in velocity is observed even for nonaqueous liquids such as pure glycerol and polyethylene glycol. In Figure 7b, a simple biochemical precipitation reaction is demonstrated using milk and vinegar. Because the oppositely charged droplets attract each other, the coalescence of milk and vinegar droplets is observed even though the two droplets are not aligned with each other. Because a high voltage and electric current flow may be harmful conditions for the living cells in a droplet, the fibroblast cell vitality has been checked after the ECD actuation shown in Figure 7a. Using the standard cell counting protocol with trypan blue, we have counted live and dead cells in a droplet. For a control group which is in the usual culture condition, in 90% air/ 10% CO2 at 37 °C, the average dead rate was 5.2% (with a 12.7% relative standard deviation), and for an experimental group, it was 6.0% (with a 39.9% relative standard deviation). The P value of the t test is 0.49, which means that there is no significant difference between the two groups. More rigorous studies on the effect of cells under ECD actuation, including cell growth rate comparison, are ongoing to ensure the validity of the ECD method for living biological systems. Although an aqueous droplet gets less charge than a perfect conductor, we can easily estimate the charge of a droplet under a given electric field because its scaling law of charging nearly follows the perfect conductor theory. This is useful in predicting the droplet velocity. Considering the high viscosity of the dielectric medium (96.5 cP viscosity) used in the current study,

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the droplet translational velocity is relatively fast (2
50 mm/s). By using a low-viscosity medium such as dodecane (1.34 cP viscosity), a much faster droplet velocity can also be possible. Using a high voltage (1
4 kV) for the droplet actuation can be a drawback of ECD as in DEP, especially when bearing in mind portable microfluidic applications, but because the charge is proportional to the electric field, not to the voltage, by decreasing the system length scale, we can solve this problem as in DEP.19 As the system size decreases, the electrophoretic force remains constant with respect to the length scale as the system size decreases under a fixed voltage. The electrophoretic force is proportional to the electric field strength times the charge. Since the charge is proportional to the square of the length scale and the electric field, the electrophoretic force is proportional to the square of the length scale and the square of the electric field. Since the electric field is inversely proportional to the length scale under a fixed voltage, the electrophoretic force on the charged droplet remains constant. In the viewpoint of droplet actuation, at least theoretically, the ECD method has advantages in smallscale systems. In addition to the merits in actuation, easy control of the coalescence of droplets is another good point of the ECD method.30 We also have many choices for the droplet packaging in a real application system. The droplet can be submerged in a dielectric medium,29 floated on a heavy oil,17 or located between two immiscible oils.19 Each different packaging method has its own advantages, and by designing an optimal ECD electrode system for each packaging, we can exploit each benefit. There are many possible applications using ECD in chemistry and biology. The fast droplet velocity and easy control of the coalescence of droplets can be useful in combinatorial chemistry. The simple basic principle is helpful for designing a digital chip which can control multiple droplets at the same time. Because the charging process can occur in various liquid systems,39 handling of a variety of chemical reagents using the ECD method is also possible if the reagent is immiscible with the dielectric oil medium. Minimal contact with the electrode surface for charging makes it possible to minimize the biofouling due to solid surface contamination in biological applications. Any aqueous biological system such as DNA, protein, and cell solutions can be treated in the same ECD system because any aqueous droplet can be actuated without further system modification. The ECD method is believed to have great potential in current biochemical engineering challenges.

’ CONCLUSIONS Careful experiments have been performed on the charging behavior of an aqueous droplet at the electrode surface to see the effects of the droplet size, electric field strength, and electrolyte concentration and ion species. From the experiments, it has been observed that an aqueous droplet acquires less charge than a perfectly conductive sphere, but it is also revealed that the low conductivity of water is not the major factor that limits the charging amount of the droplet. Contrary to the common expectation, the charging is more limited for an electrolyte droplet with a higher concentration. This implies that complex electrochemical reactions are involved in the charge transfer from the electrode to the droplet. Despite the limiting effect on charging of an aqueous droplet, the scaling law of charging is only slightly different from that for a perfect conductor. Although several important findings have been discovered from the present 5173

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’ ASSOCIATED CONTENT

bS

Supporting Information. Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: þ82-54-279-2952. Fax: þ82-54-279-5528. E-mail: [email protected].

’ ACKNOWLEDGMENT This work has been supported by Grant R01-2010-0027653 from the National Research Foundation (NRF) of Korea and by the BK21 program of the Ministry of Education, Science and Technology (MEST) of Korea. We thank Prof. Jihwa Lee of Seoul National University for helpful discussions on electrochemical reactions. ’ REFERENCES (1) deMello, A. J. Nature 2006, 442, 394–402. (2) Kiss, M. M.; Ortoleva-Donnelly, L.; Beer, N. R.; Warner, J.; Bailey, C. G.; Colston, B. W.; Rothberg, J. M.; Link, D. R.; Leamon, J. H. Anal. Chem. 2008, 80, 8975–8981. (3) Teh, S. Y.; Lin, R.; Hung, L. H.; Lee, A. P. Lab Chip 2008, 8, 198–220. (4) Anna, S. L.; Bontoux, N.; Stone, H. A. Appl. Phys. Lett. 2003, 82, 364–366. (5) Chiu, D. T.; Lorenz, R. M.; Jeffries, G. D. M. Anal. Chem. 2009, 81, 5111–5118. (6) He, M. Y.; Edgar, J. S.; Jeffries, G. D. M.; Lorenz, R. M.; Shelby, J. P.; Chiu, D. T. Anal. Chem. 2005, 77, 1539–1544. (7) Gadd, J. C.; Kuyper, C. L.; Fujimoto, B. S.; Allen, R. W.; Chiu, D. T. Anal. Chem. 2008, 80, 3450–3457. (8) Arora, A.; Simone, G.; Salieb-Beugelaar, G. B.; Kim, J. T.; Manz, A. Anal. Chem. 2010, 82, 4830–4847. (9) Salieb-Beugelaar, G. B.; Simone, G.; Arora, A.; Philippi, A.; Manz, A. Anal. Chem. 2010, 82, 4848–4864. (10) Huebner, A.; Sharma, S.; Srisa-Art, M.; Hollfelder, F.; Edel, J. B.; Demello, A. J. Lab Chip 2008, 8, 1244–1254. (11) Mukhopadhyay, R. Anal. Chem. 2006, 78, 1401–1404. (12) Washizu, M. IEEE Trans. Ind. Appl. 1998, 34, 732–737. (13) Pollack, M. G.; Fair, R. B.; Shenderov, A. D. Appl. Phys. Lett. 2000, 77, 1725–1726. (14) Lee, J.; Moon, H.; Fowler, J.; Schoellhammer, T.; Kim, C. J. Sens. Actuators, A 2002, 95, 259–268. (15) Abdelgawad, M.; Wheeler, A. R. Adv. Mater. 2009, 21, 920–925. (16) Jebrail, M. J.; Wheeler, A. R. Curr. Opin. Chem. Biol. 2010, 14, 574–581. (17) Velev, O. D.; Prevo, B. G.; Bhatt, K. H. Nature 2003, 426, 515–516. (18) Gascoyne, P. R. C.; Vykoukal, J. V.; Schwartz, J. A.; Anderson, T. J.; Vykoukal, D. M.; Current, K. W.; McConaghy, C.; Becker, F. F.; Andrews, C. Lab Chip 2004, 4, 299–309. (19) Hunt, T. P.; Issadore, D.; Westervelt, R. M. Lab Chip 2008, 8, 81–87. (20) Issadore, D.; Franke, T.; Brown, K. A.; Hunt, T. P.; Westervelt, R. M. J. Microelectromech. Syst. 2009, 18, 1220–1225.

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