Electrostatic Origins of the Positive and Negative Charging Difference

Nov 13, 2017 - The positive and negative charging difference in the contact charge electrophoresis of a water droplet suspended in oil was investigate...
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Electrostatic Origins of the Positive and Negative Charging Difference in the Contact Charge Electrophoresis of a Water Droplet Seok Hwan Yang and Do Jin Im* Department of Chemical Engineering, Pukyong National University, 365, Sinseon-ro, Nam-gu, Busan 608-739, Korea S Supporting Information *

ABSTRACT: The positive and negative charging difference in the contact charge electrophoresis of a water droplet suspended in oil was investigated to find out the origin of this charging difference. Through numerous experiments and numerical analysis, the charging difference has been found to be mainly originated from electrostatic sources. Two electrostatic sources were found in the present experimental setup, and by excluding those two sources the charging difference was successfully diminished. The present findings well explain previous experimental results and also provide design guidelines for consistent droplet movement control in contact charge electrophoresis-based digital microfluidic systems. Finally, further discussions on the obtained results, its implications, and future work are discussed.

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INTRODUCTION The contact charge electrophoresis (CCEP) represents repeating electrophoretic bouncing motion of a conductive object (liquid droplet or solid sphere) between two electrified bare electrodes.1−5 When the object is a droplet, it was also referred to as the electrophoresis of a charged droplet (ECD).3 In CCEP, the conductive object is first charged by contact with an electrode surface in the presence of an electric field and then actuated by that field via electrophoresis. Interestingly, even though static electric field is applied in CCEP, each time the object contacts an electrode, its charge changes sign and its velocity changes direction, resulting in repeating bouncing motion between electrodes.1 Because of academic interests and unique features as a novel digital microfluidic technology, a growing number of research has been actively reported recently.6−13 Especially, recent advancement in the ECDbased digital microfluidic technology is noticeable,6,8,14 and the application to electroporation showed promising results.11,12 On the other hand, basic research on CCEP phenomenon is also actively studied by various groups.7,9,10,13,15−19 The history of CCEP related research is summarized in a recent review.20 In the early period (from 1990 to the early 2000s), the research focus was industrial applications such as heat transfer enhancement21 and dehydration of water from crude oil.22 From 2006, research in the microfluidics field started reporting the electric control of droplets in microchannel23 and digital microfluidic approach.24 From 2010, active research on various topics including industrial25−27 and biological28−30 applications and microfluidics technology1,31−34 as well as fundamental research3,4,35−38 has been reported. Through this variety of investigations into CCEP, potential features for microfluidic technology have been proposed, and fundamental understandings on CCEP such as contact charging © 2017 American Chemical Society

mechanism and the corresponding charge amount were much increased. However, there are still some open questions that need to be elucidated. Especially, nobody could clearly explain the origin of the charging amount difference between positive and negative electrodes, although there were lots of works reporting this difference3,32,39−41 even in experiments using a solid sphere.13,42 In previous work, this positive and negative charging difference was believed to be linked to the electrochemical reaction rate difference in each separate charging (positive and negative) because the charge transfer through pure water should involve electrolysis of water and the electrochemical reaction of each charging is different (oxygen formation on positive charging and hydrogen formation on negative charging).3,40 However, the positive and negative charging difference observed in solid sphere experiments13,42 implies that the difference can be originated from physical origins rather than from electrochemical origins. From a technological point of view, this positive and negative charging difference can be problematic in ECD-based digital microfluidic systems. Because the basic operational principle of an ECD-based chip is based on the consecutive charging of a droplet that keeps changing the polarity of charge in each charging step, for accurate and precise control, it is necessary to minimize the positive and negative charging differences. Therefore, in the present work, we investigated the positive and negative charging difference in CCEP of a water droplet suspended in oil to find out the origin of this charging difference for fundamental understandings and for more Received: September 18, 2017 Revised: November 9, 2017 Published: November 13, 2017 13740

DOI: 10.1021/acs.langmuir.7b03281 Langmuir 2017, 33, 13740−13748

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Langmuir

780 pixels, 50 frames/s) recorded by the digital camera was converted (using the software Batch Video To Image Extractor and ImageJ) to 8bit sequential images, and these images were further processed by the LabVIEW image-processing tool kit making binary images to extract droplet size and horizontal velocities as a function of droplet position as shown at the bottom of Figure 1. For precise and accurate charge measurements, special efforts were devoted to experiments and image analysis. Because any tiny dust can disturb the droplet CCEP movement, all of the parts (cuvette, copper electrodes pair, electrode holder) were carefully cleaned with dry air before use, and fresh silicone oil and cuvette were used in every experiment. Because any impurity on the copper electrode surface can induce insufficient charging,13 it was first treated with a grinding machine and further cleaned with isopropyl alcohol (IPA) and clean water, respectively. To minimize deformation effects that can confuse the analysis, small droplets (100−600 nL) were used for the experiments. To minimize measurement error by image analysis, some processes were revised in addition to previous efforts for accurate charge estimation.40 If the cuvette is tilted, the resulting droplet velocity profile becomes asymmetric, which can make measurement error as shown in the previous work.3 Therefore, we checked every droplet velocity profile and used data having a symmetric velocity profile only as shown in Figure 1. Because of the inaccuracy of a micropipette for dispensing small droplets (about 10%), every droplet volume (radius) was measured by image analysis. In the previous work, a droplet radius was measured from the 2D area of droplet shaped binary image. Because the binary image shape is not a perfect circle but ellipse, the droplet radius was estimated as an effective radius of a perfect circle having the same 2D area as that of the 2D ellipse binary image. However, strictly speaking, the droplet radius should be estimated as the effective radius of a 3D sphere having the same volume as that of a 3D ellipsoid having a projection image of the 2D ellipse binary image. Therefore, the droplet radius was estimated on the basis of this rigorous geometric principle in the present work. Typical horizontal droplet velocity measurement results are shown in Figure 1S. We can observe a very consistent flip-flop type velocity profile having high symmetry. In all experiments, more than 40 bouncing events (20 for each positive and negative) were measured. Because of the highly consistent bouncing motion, the standard deviation of horizontal velocity at the center of a cuvette was less than 1% of the average value. Despite this consistent motion, if we directly measure the droplet charge using an electrometer, it showed much greater standard deviation than in the previous work.40 Therefore, we did not use an electrometer in the present work to measure the droplet charge. The obtained horizontal droplet velocity data were converted into droplet charge (Q) data by balancing the Coulomb force (QE0) and the Stokes drag force (6πμoilaU) at the center of a cuvette as shown below:

accurate droplet movement control. Numerous experiments were performed in the previously developed experimental setup40 using a plastic and a glass square cuvette with a rectangular copper electrodes pair. For precise charge measurement, the image analysis method was revised, and various types of Faraday cages were tried. In addition, two power supplies setting was used to exclude grounding effects induced by an optical table. Humidity effects are also considered. To explain the origin of the charging amount difference quantitatively, a 3D numerical model was developed and analyzed. Finally, further discussions on the obtained results, its implications, and future work are discussed.

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EXPERIMENTAL SECTION

The experimental setup for the CCEP of a water droplet is illustrated in Figure 1. Basic experimental design concept and charge measure-

Q= Figure 1. Schematic view of the experimental system for the charge measurement of a water droplet under CCEP and the time lapse images of a 114 nL bouncing water droplet under 3.9 kV/cm. The time step between droplets in the photo is 0.14 s.

6πμoil aU E0

where E0 is the applied electric field strength (3.3 kV/cm for 3 kV departing 9 mm), μoil is the oil viscosity (0.096 kg/m/s for silicone oil), a is the droplet radius (415 μm for a 300 nL droplet), and U is the measured droplet velocity (1−4 cm/s) at the center of a cuvette. More detailed and thorough charge estimation principle and methods can be found in refs 3, 32, and 40. One prominent physical origin for the positive and negative charging difference is an electrostatic one. Therefore, a numerical model for the simulation of electrostatics was developed and analyzed to explain the origin of the charging difference not only qualitatively but also quantitatively. According to the previous work, the electrical double layer inside an aqueous droplet and the charge leakage from a charged water droplet into silicone oil can be ignored.32,43 Therefore, the electric potential (ϕ) distribution of the system can be described by the following Laplace equation:

ment principle is the same as previous work:40 measuring the velocity of a bouncing water droplet inside a square plastic cuvette (12 × 12 × 45 mm3) containing silicone oil (Shin-Etsu KF-96 100 cSt) and rectangular copper electrodes pair (9 × 60 mm2 with 0.5 mm thickness and 9.0 mm apart). A water droplet (100−600 nL) was dispensed through the center hole of the electrode holder using a micropipette after turning on the two high voltage power supplies (Keithley model 248) connected to the copper electrodes pair. The horizontal bouncing motion of the water droplet due to CCEP was recorded by a digital camera (Canon EOS 100D) with macro lens (Canon EF-S 60 mm f/2.8 USM Macro Lens) and back lit setting (using a homemade light emitting diode light source). The movie file (1280 ×

∇2 ϕ = 0 13741

DOI: 10.1021/acs.langmuir.7b03281 Langmuir 2017, 33, 13740−13748

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To include all of the electrostatic sources of the experimental system, a full 3D model was developed as shown in Figure 2. The

Article

RESULTS AND DISCUSSION

The CCEP of a water droplet has been successfully explained by classical electrostatics of a perfect conductor.3,18,20,32,40,42 This perfect conductor theory predicts that when a conductive sphere is brought in contact with an electrified electrode, the sphere is charged as a result of redistribution of electric field around the sphere, and the corresponding charge amount Qtheory is given as20 Q theory =

where a is the radius of sphere (415 μm for a 300 nL droplet) and E0 is the applied electric field strength (3.3 kV/cm for 3 kV departing 9 mm). Typical Qtheory value has the order of 10−11 C for the present experiments. Because this theoretical charge Qtheory can serve as a reference charge amount, all of the droplet charge values are presented as a dimensionless charge Q/Qtheory in the present work. According to the perfect conductor theory, there is no difference between positive and negative charging amount. However, in real experiments, there is almost always a positive and negative charging difference even in solid sphere experiments.13,42 One possible explanation can be found in the previous work of droplet charging on a pin-shaped electrode where electric field concentration effects significantly change droplet charge amount.32 However, because planar electrodes are used in the present work, it may not be possible to apply the same principle. Therefore, numerical modeling as well as experiments were performed to check this point. This electric field concentration effects were also observed in the present planar electrodes system as shown in Figure 3. When a positive voltage is applied by a single power supply paring with ground electrode (+ signed electric field cases), the positive charge (Q+) is greater than the negative charge (Q−) due to the concentration of electric field on the positive electrode. Because the experimental system is placed on an optical table, which plays a role of a ground electrode, electric potential contours are denser on the positive electrode as shown at the top right of Figure 3. In contrast, when a negative voltage is applied (− signed electric field cases), the negative charge (Q−) becomes greater due to the same reason as shown in the middle right of Figure 3. If an evenly distributed electric field is applied by using two power supplies (no signed electric field cases), the positive and negative charging differences become negligible due to an even distribution of electric field on both electrodes. Although this qualitative description of electric field concentration effects seems to well explain the observed charging differences, quantitative comparisons between experiments and numerical modeling show discrepancies as shown in the center of Figure 3. Furthermore, in some other experiments, even though we use two power supplies setting, there occurred fairly large charging differences as shown in Figure 2S. Therefore, we need to find an additional origin for the charging differences. One possible reason for this discrepancy between theory and experiment is the surrounding electrostatic effect. To screen out the surrounding electrostatic effect, we first used a conventional enclosure type Faraday cage, but there was no difference in experimental results. Therefore, we devised a new type of Faraday cage as shown in Figure 4. Contrary to common expectations, the use of Faraday cage influenced the

Figure 2. (a) A 3D numerical model for the charge calculation of a droplet on an electrified electrode. The distance between electrodes is set to be the same as in the experimental setup (9 mm), and the volume of a droplet is 300 nL. To include environmental electrostatic effects, all key experimental structures (cuvette, copper electrodes, acrylic holders, and a metallic stage) were included in the model. (b) Mesh structure around a 300 nL droplet. The mesh consisted of 342 958 elements, which was determined by changing mesh size and by comparing the droplet charge values. (c) Electric potential distribution around the droplet under 3.3 kV/cm (3 kV/9 mm with +1.5 and −1.5 kV on each electrode).

Laplace equation was solved using the electrostatic module of a commercial numerical software COMSOL Multiphysics using appropriate boundary conditions: specific voltage at the electrodes, ground at the bottom metallic stage surface and Faraday cage (when it was used), and ∂ϕ/∂n = 0 at the other boundaries. The dielectric constants used in the simulation are presented in Table 1S. The droplet charge (Q) was numerically obtained from the integration of the surface charge density (σs) over the surface of the droplet (S) in contact with an electrified electrode as follows:

Q=

π2 (4πa 2)εE0 6

∫s σs dS , σs = n·(ε E)

where n is an outward unit normal vector, ε is the permittivity of the surrounding medium (2.44 × 10−11 F/m for silicone oil), and E is the electric field around the droplet surface, which can be obtained by numerical simulation. Any electrostatic source can change this electric field E, and, as a result, there will be changes in the numerically calculated droplet charge (Q). 13742

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Figure 3. Dimensionless positive and negative charges of a 100 nL droplet under various electric field distributions by three different power supply settings and the corresponding numerical results. The “+” sign of electric field represents a single power supply setting with positive voltage and ground, the “−” sign represents that with negative voltage and ground, and no sign represents two power supplies setting with positive and negative voltages of equal amount (for example, 2.8 means +1.4 and −1.4). The positive charge (Q+) represents charging amount on a positive electrode, the negative charge (Q−) represents charging amount on a negative electrode, and Qav is an average of the two. Also shown is the contour of electric potential distributions for 3 kV voltage difference (corresponding electric field of 3.3 kV/cm) with three different power supply settings.

Figure 4. Photos of basic setup and the revised setup using a newly devised Faraday cage and the corresponding results. The two setups were placed on a metallic stage. The middle numerical models show electric potential distribution change due to the use of Faraday cage for 3.3 kV/cm case, and the lower graphs show experimental results. (a) Basic experimental setup. (b) The experimental setup using a newly devised Faraday cage. Material of cuvette holder was replaced by steel instead of acryl. Using a pair of copper electrodes and aluminum foil, a small Faraday cage was made to screen out surrounding electrostatic effects. The Faraday cage (copper electrodes pair + aluminum foil + bottom steel cuvette holder) was connected to ground, but there was no direct contact between the structures (copper electrodes pair + aluminum foil) and cuvette.

experimental results in an unexpected way. In one case, the positive and negative charging difference disappeared, but, in the other case, the differences were enlarged by the use of a Faraday cage. Moreover, the numerical modeling predicted no significant effects of the Faraday cage on the droplet charge amount as shown in Figures 4b, 6b, and 6e. The inconsistent Faraday cage results imply another origin of the droplet charging difference, and one possible source is the surface charges on cuvette and acrylic structures, which cannot be screened out by the Faraday cage. To investigate the effects of these surface charges on the droplet charging difference, systematic numerical modeling was performed as presented in Figure 5. According to the numerical analysis, the surface charges on the front and back surfaces of cuvette (transparent observing surface) have dominant effects as compared to other surfaces. By assigning surface charge densities on the front and back surfaces of the cuvette (the corresponding electric potential ranged from 1−100 V), the effects of surface charge were simulated as shown in Figure 6. Except for a slight increase in Qav for high electric field cases due to droplet deformation, the numerical model well predicted the droplet charging differences with fairly reasonable surface charge density values (several μC/ m2)44 at the surfaces. Furthermore, we could predict all other experimental results by adjusting the polarity and the magnitude of surface charges regardless of the use of Faraday cage. That is, the reason why we had the inconsistent Faraday cage results was because the Faraday cage has nearly no effect

on the charging difference, but the surface charges developed on cuvette have significant effects. This explains why there was no difference in the results when we used a conventional enclosure type Faraday cage. Because we do not have to change the cuvette in the comparison experiments with and without the Faraday cage, we should change the cuvette for comparison if we use the newly devised Faraday cage. This also explains why we have totally different results for the same experimental setup (the two power supplies setting without Faraday cage) as shown in Figures 3, 2S, and 3S. In Figure 6a, it looks like the surface charge developed on the cuvette surface is proportional to the applied electric field strength. However, this was not always the case, and it rather showed time-dependent behaviors. To show this time-dependent behavior, additional experiments were performed as shown in Figure 7. Initially, the cuvette surface was wiped with IPA right before the experiment, which resulted in a small positive and negative charging difference. After 10, 30, 60, and 120 min, an additional droplet was dispensed, and the resulting droplet charge was measured at each time. For the last experiment, the surface of cuvette was wiped again with IPA right before 13743

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Figure 6. Comparison between experimental and numerical results for 100 nL droplet charges under different electric fields with and without the Faraday cage. Solid lines represent experimental results, and dotted lines represent numerical results. (a,d) Experimental results. (b,e) Numerical results without any surface charge. The effects of Faraday cage were considered. (c,f) Numerical results obtained by assigning different surface charge densities on cuvette surface. The effects of Faraday cage were considered.

Figure 5. Numerical results for the surface charge effects of experimental structures, the cuvette and acrylic plates. The two power supply setup was used with an applied electric field of 3.3 kV/ cm. The graphs show the dimensionless positive charge amount of a droplet (100 nL) when negative voltage was assigned to each surface (surface charge was simulated by assigning surface voltage). (a) Effects of cuvette surface charges on the positive droplet charging. Front and back surfaces have dominant effects as compared to left and right side surfaces. (b) Effects of acrylic plates on the positive droplet charging. Comparing with the front and back surfaces of the cuvette, acrylic plate surfaces have negligible effects on the droplet charging.

dispensing the last droplet and measuring the charge. As time goes by, the droplet charge difference becomes larger even under the same electric field, which implies the accumulation of surface charges on the cuvette surface. However, if the surface charge is removed by wiping with IPA, the droplet charge difference was dramatically reduced. This time-dependent behavior of surface charges on cuvette surface explains the fairly large droplet charge difference of the two power supplies experiments shown in Figure 2S. We usually waited for more than 10 min before starting the experiment to stabilize the electric field distribution of the system without knowing this surface charge accumulation behavior. We have found two sources for the positive and negative charging difference: a single power supply setting inducing uneven electric field distribution and surface charges accumulated on cuvette surface. If the two sources mainly contribute to the droplet charging difference, we can eliminate this difference by controlling the two sources. The first source can easily be excluded by using the two power supplies setting. To exclude the second source, we first wiped the cuvette surface with IPA right before dispensing a droplet in every experiment. However,

Figure 7. Time-dependent changes of the droplet charging differences due to the accumulation of surface charges on cuvette surface. The droplet volume is 100 nL. No Faraday cage was used for the experiments.

this was not always successful. Hence, we lapped aluminum foil around the cuvette surface except for the observation part to discharge the surface charges that accumulated during the experiments. As shown in Figure 8, the second approach was successful, and the positive and negative charging differences were almost eliminated. Therefore, we can conclude that the positive and negative charging difference in the CCEP of a 13744

DOI: 10.1021/acs.langmuir.7b03281 Langmuir 2017, 33, 13740−13748

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Figure 8. Positive and negative charging difference of various sizes of water droplets (100, 200, and 300 nL) under various electric field strengths. Both electric field and droplet size increase from left to right. The two power supplies setting was used with aluminum foil lapped on the cuvette surface.

Figure 9. Effects of humidity on the droplet (300 nL) charging difference. No Faraday cage was used in all cases. The two power sources setting was used.

decreased even without aluminum foil lapping as shown in Figure 10a. Although the dependency of the charging difference

water droplet is mainly originated from these two electrostatic sources: uneven electric field distribution induced by the use of a single power supply and the surface charges developed on the cuvette surface during experiments. This conclusion clearly explains previous experimental results in which the positive droplet charging is almost always greater than the negative droplet charging. When a single power supply is used as a positive high voltage source, electric field is concentrated on the positive electrode as shown in Figure 3. In addition, negative charges will be developed on the polystyrene (PS) cuvette surface according to the triboelectric series, which further concentrates electric field on positive electrode. As a result, the positive droplet charging becomes greater in almost all cases. On the other hand, when two power supplies are used, the charging difference is mainly affected by the surface charges developed on the cuvette surface. Interestingly, in the present experimental setup, two-thirds of all cases showed a higher negative droplet charging as shown in Figures 6 and 7, which implies positive surface charges developed on the cuvette surface. In contrast, the other cases showed higher positive droplet charging, implying negative cuvette surface charges as shown at the middle of Figure 4 and Figure 3S. That is, the sign and magnitude of the cuvette surface charges varied in each experiment. Although the reason why positive cuvette surface charges are more frequently developed in the present experimental setup is not clear, this change in the sign of cuvette surface charges resulting in the positive and negative charging difference reversal in the two power sources setup also confirms that the charging difference is mainly originated from electrostatic sources. For further investigations into the cuvette surface charge development, additional experiments were performed. Surface charge development can be affected by environmental factors such as temperature and humidity and the material of cuvette. Because the variation in temperature of usual laboratory environment is relatively small (less than 5 °C), the effects of humidity and cuvette material are investigated. As humidity increased, the charging difference showed a decreasing trend as shown in Figure 9. Furthermore, when the relative humidity was over 50%, the droplet charging difference was significantly

Figure 10. Effects of cuvette materials on the droplet charging difference. No Faraday cage or aluminum foil lapping was used. All of the experiments were performed at the same day under the same environmental conditions (24.3 °C, 50% of relative humidity). (a) Experimental results using a PS cuvette. (b) Experimental results using a glass cuvette.

on humidity followed the common knowledge on triboelectricity, the cuvette surface charge development was not simply explained by triboelectricity only. For example, the accumulation of cuvette surface charge without any friction is shown in Figure 7, and the reversal of the polarity of surface charges is shown in Figures 3, 2S, 3S, and 4S. When the cuvette material was changed to glass, the charging difference became greater than that of a PS cuvette even under the same humidity conditions as shown in Figure 10b. Positive charging was greater than negative charging, which implies negative cuvette surface charge development. We can also observe increasing trends of the charging difference as applied electric field increases similar to those of Figures 6a and 10a. 13745

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considered in the usual contact electrification field, and it is another research topic to be investigated separately. Apart from the origin and mechanism of cuvette surface charge development, it is clear that the negative and positive water droplet charging difference can be largely affected by the abovementioned electrostatic origins, especially cuvette surface charges. Although the positive and negative charging difference of a water droplet is mainly originated from electrostatic sources, the hypothesis that the charge transfer in CCEP of a water droplet involves electrochemical reaction (electrolysis of water) still holds. It seems that, at least for pure water, the electrochemical reaction rate is fast enough in both positive and negative electrodes to charge up to theoretical maximum. However, when electrolyte ions participate in this electrochemical charging process, there are some changes. An electrolyte droplet was less charged than a pure water droplet, and this limiting effect was positively correlated to the electrolyte concentration.3 Furthermore, the positive and negative charging difference was also greater in an electrolyte droplet charging, which can be linked to the different limiting effects of positive and negative ions. However, in the previous experiments, because electrostatic effects as well as electrochemical effects on the charging difference were included, it is impossible to investigate the sole effects of electrolyte ions on the charging difference. In that sense, the present work can contribute to exclude electrostatics effects on the positive and negative charging difference. Last, it is worthwhile to discuss the potential feature of the CCEP system for monitoring electrostatic environment. It has been found that the positive and negative charging difference in CCEP of a water droplet is very sensitive to electrostatic sources. When two power supplies are used, the difference is only dependent on the surrounding surface charge distribution. Therefore, we can measure surface charge density by monitoring the difference in positively and negatively charged droplet velocities. As shown in Figure 6, because the surface charge density of several μC/m2 can induce a fairly large difference in the droplet velocities, it can be used to measure small surface charge density (magnitude and polarity).

However, this increasing trend was not proportional to the strength of the electric field but proportional to the field application time similar to that of Figure 7 (even though we reverse the order of electric field strength, we observed the same pattern, that is, time-dependent behavior and not field strength-dependent behavior). Before discussing some unexpected results from a viewpoint of triboelectricity (the reversal of PS cuvette surface charge polarity and the negative surface charge development on a glass cuvette surface), it is necessary to remind of some intricate characteristics of tribocharging (contact electrification). Contact electrification is a complicated process where both the magnitude and the polarity of the developed charge are sensitive to the material composition, the contact process, and the environmental conditions.45 Contact electrification is also known to be a highly irreproducible phenomenon.46 This arises in part because many factors influence the contact charge that develops on the surface, such as the surface characteristics, the experimental conditions, and nature of the contact.45 Furthermore, there are some sets of materials that form a cyclic triboelectric series (for example, glass, zinc, silk, filter paper, and cotton form a cyclic series).47 The existence of such a cyclic series suggests that a single physical property cannot explain all instances of contact electrification, and indicates that contact electrification probably involves more than one mechanism.48 On the basis of the above knowledge, the reversal of PS cuvette surface charge polarity can be explained by irreproducibility and sensitive nature of tribocharging. In addition, PS is known to have a relatively low triboelectric surface charge,45 which was also observed in Figure 10. In some experiments, PS surface developed nearly no surface charge at all.45 In contrast, for a glass cuvette, it was impossible to do experiments under low humidity condition due to the sticking of droplet to the cuvette surface caused by too strong surface charges. Even under high humidity condition, due to the development of cuvette surface charge, at the end of the experiment, droplets stuck to the cuvette surface, and further experiment was impossible. Therefore, the polarity reversal of PS surface charge can be partly explained by relatively low surface charge development on the PS surface and irreproducibility. Glass is known to develop positive surface charges more frequently in most contact electrification.48 However, when it makes contact with air or dry human hand, negative charges can be developed on the glass surface according to linear triboelectric series. In addition, when glass makes contact with cotton, negative surface charges on glass can be developed even though cotton has a higher negative charge development tendency according to linear triboelectric series. This reversal of glass surface charge polarity is due to the existence of a cycle triboelectric series.48 Therefore, the negative surface charge on the glass cuvette can be partly explained by this complicated nature of contact electrification. There are also additional factors of the present experimental setup that make the cuvette surface charge development more complicated. The application of a strong electric field can make a huge change in cuvette surface charge development as shown in Figures 7 and 10. The IPA surface treatment also makes the problem even complicated. Furthermore, we should consider the fact that inside and outside cuvette surfaces are exposed to totally different environments, and the observed or estimated surface charge density is the collective result developed on the two different cuvette surfaces. All of these factors are not

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CONCLUSION The positive and negative charging difference in the contact charge electrophoresis of a water droplet has been found to be mainly originated from electrostatic sources rather than electrochemical ones. Through numerous experiments and numerical analysis, two electrostatic sources were found in the present experimental setup: uneven electric field distribution by the use of a single power supply and surface charges developed on cuvette surface. The first source was excluded by using two power supplies, and the second source was reduced by lapping aluminum foil around the cuvette surface to discharge the surface charges that keep developing on the cuvette surface during experiment. By removing the two electrostatic sources, the positive and negative charging difference was successfully diminished. The present findings well explain previous experimental results in which the positively charged droplet velocity is almost always greater than the negatively charged one. This work also provides design guidelines for consistent droplet movement control in ECD-based digital microfluidic systems: electric shielding is needed to screen out environmental electrostatic effects as well as two power supplies setting. By excluding electrostatic effects on CCEP of 13746

DOI: 10.1021/acs.langmuir.7b03281 Langmuir 2017, 33, 13740−13748

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Langmuir

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electrolyte droplets, electrochemical effects of electrolyte ions on droplet charging will be investigated. The sensitive nature of the CCEP system to electrostatic sources can be used for monitoring the electrostatic environment such as surface charge measurements.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b03281. Table 1S, dielectric constants used in the numerical simulation; Figure 1S, horizontal droplet velocity profile and the estimated droplet charge data; Figure 2S, additional experimental results using the two power supply setting; and Figure 3S, experimental results showing higher positive droplet charging (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Seok Hwan Yang: 0000-0003-3068-3909 Do Jin Im: 0000-0001-8328-1079 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the grant NRF2015R1D1A3A01019112 and also by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (no. 20174010201460).

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REFERENCES

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DOI: 10.1021/acs.langmuir.7b03281 Langmuir 2017, 33, 13740−13748

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DOI: 10.1021/acs.langmuir.7b03281 Langmuir 2017, 33, 13740−13748