2086 Myung Mo Ahn1 Do Jin Im2 Byeong Sun Yoo1 In Seok Kang1 1 Department
of Chemical Engineering, Pohang University of Science and Technology, Pohang, South Korea 2 Department of Chemical Engineering, Pukyong National University, Nam-Gu. Busan, South Korea
Received March 18, 2015 Revised May 6, 2015 Accepted May 6, 2015
Electrophoresis 2015, 36, 2086–2093
Research Article
Characterization of electrode alignment for optimal droplet charging and actuation in droplet-based microfluidic system The actuation method using electric force as a driving force is utilized widely in dropletbased microfluidic systems. In this work, the effects of charging electrode alignment on direct charging of a droplet on electrified electrodes and a subsequent electrophoretic control of the droplet are investigated. The charging characteristics of a droplet according to different electrode alignments are quantitatively examined through experiments and systematic numerical simulations with varying distances and angles between the two electrodes. The droplet charge acquired from the electrified electrode is directly proportional to the distance and barely affected by the angle between the two electrodes. This implies that the primary consideration of electrode alignment in microfluidic devices is the distance between electrodes and the insignificant effect of angle provides a great degree of freedom in designing such devices. Not only the droplet charge acquired from the electrode but also the force exerted on the droplet is analyzed. Finally, the implications and design guidance for microfluidic systems are discussed with an electrophoresis of a charged droplet method-based digital microfluidic device. Keywords: Droplet actuation / Droplet charging / Electrode alignment DOI 10.1002/elps.201500141
Additional supporting information may be found in the online version of this article at the publisher’s web-site
1 Introduction
Correspondence: Professor Do Jin Im, Department of Chemical Engineering, Pukyong National University, 45 Yongso-ro, Nam-Gu. Busan 608737, South Korea E-mail: [email protected]
electric field and have been used in various chemical and biological applications [10–12]. Electrowetting on dielectric (EWOD) is another significant actuation method for the control of conducting droplets electrically [13, 14]. Because EWOD requires relatively low electric potential and has precise control of individual droplets, it is used in a wide range of applications in chemical and biological fields [15, 16] as well as in the display and liquid lens industry [17,18]. Recently, the direct charging of a droplet in a dielectric medium with an electrified bare electrode has been proposed as a novel actuation method of droplets [19–24]. Due to its essential merits, such as the intuitive and straightforward actuation and minimal contact with solid substrates, the direct charging method has been used in a growing number of studies [25–30]. In microfluidic systems using electric force as the driving force of the actuation method, the electric field distribution is a critical influence on the motion of a droplet; thus, electrodes are a significant part of the microfluidic devices because the electrified electrodes generate the electric field distribution. Therefore, the effects of the electrode geometry and alignment have been investigated in well-known droplet actuation
Abbreviations: ECD, electrophoresis of a charged droplet; EWOD, electrowetting on dielectric
Colour Online: See the article online to view Figs. 1–6 in colour.
Electric force is widely used as a driving force in the actuation method for droplet-based microfluidic systems. Besides its intrinsic advantages such as fast response times and precise individual control of droplets, the actuation method using electric force is easily integrated with various microfluidic systems [1–6]. As a result of these advantages, many microfluidic devices, especially chip-based microfluidic platforms, have adopted electric force as an actuation method. DEP, which is the motion of uncharged polarizable particles in a nonuniform electric field, is a representative actuation method in microfluidic systems using electric force [7–9]. Due to the interaction of the electric field with the induced dipole moment, the dielectrophoretic forces can actuate droplets along the gradient of the
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methods, such as EWOD and DEP [7, 31–37]. Furthermore, the droplet motions have been examined experimentally and numerically depending on the electrode geometry. Droplets have exhibited different behaviors with different electrode geometries even in the same microfluidic system, and these results could be used to optimize the devices for better droplet actuation [33, 34]. In our previous work, the effects of the electrode geometry on direct charging of a droplet were investigated [38]. The experimental and numerical studies were performed through varying the cross-sectional area and length of charging electrodes. From those results, it was found that for a given droplet size, a long and sharp electrode is favorable for the charging and actuation of a droplet; that is, a pin electrode is more efficient for droplet charging and actuation compared with a planar one. Although these findings can provide important information for the design of electrodes for microfluidic devices, the electrode alignment including the distance and angle between electrodes should be also considered to offer a complete design guideline. However, there has not yet been a systematic study on the effect of electrode alignment on the direct charging and subsequent electrophoretic control of a droplet. In this study, the effects of charging electrode alignment on charging characteristics of a droplet are quantitatively examined through experiments and numerical simulations. Two key factors of charging electrode alignment are examined: the distance and the angle between electrodes. Although there are numerous alignment setups for microfluidic devices and it is difficult to analyze all individual electrode alignments, most alignments can be covered through varying the distance and angle between electrodes. The droplet charges are estimated from the experiments with varied distances and angles of charging electrodes and these are compared with the charges from numerical simulations. A droplet-based microfluidic application is also demonstrated and analyzed using the findings of the electrode alignment effects. Finally, the implications for basic understanding of the droplet contact charging phenomena on the electrode according to the geometry and alignment of electrodes, and for the improvement of efficiency of microfluidic devices, are discussed.
2 Materials and methods A schematic view of the experimental setup is shown in Fig. 1A. A transparent acrylic cube (internal dimensions: 5 × 5 × 5 cm3 ) was used as a test cell; it was placed between a light source and a microscope and filled with silicone oil (ShinEtsu KF-96 1000 cSt) as the dielectric suspending medium. In order to avoid thermal side effects, a cooled light-emitting diode (LED) was used as a light source. As mentioned in introduction, a pin electrode is advantageous in charging and actuation of a droplet; thus, pin electrodes were used in this study [38]. Two copper pin electrodes (radius: 0.325 mm; length: 20 mm) were fixed on the stages to change the distance and angles between electrodes; they were connected C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 1. (A) Schematic view of the experimental setup. Timelapse images of the bouncing motion of a 500 nL water droplet under 3 kV/cm when the angle between the electrodes is 0°. (B) Numerical results for the electric potential distribution around a droplet and the electrodes. The potential difference between the equi-potential lines is 100 V. (C) Definitions of the distance and the angle between two electrodes. The distance (d) is the shortest length between the centers of the facing surfaces of the two electrodes and the angle () is the sum of the individual angle of the two electrodes (i.e. = 1 + 2 ). (D) Representative angles of two electrodes: 0° and 180° setups.
to a direct current power supply (Trek Models 610E) and the opposite sides of the electrodes were soaked in the middle of the test cell. After a single deionized water droplet was dispensed from a micro-pipette between the two pin electrodes in the test cell, a horizontal bouncing motion of the droplet was recorded using a high speed camera (Photron Fastcam 1024 PCI model 100K) mounted on the microscope as shown in the bottom of Fig. 1A. The recorded images were analyzed using a LabVIEW Vision Assistant Toolkit. In order to exclude deformation and wall effects, the radius and horizontal velocity of the droplet were only measured in the middle region between the two electrodes. From the measured data, the droplet charge was estimated using the force balance between the electrical and hydrodynamic drag forces (consult the reference [39] and supplementary www.electrophoresis-journal.com
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information for details). Because the electric field between two pin electrodes is not uniform unlike a uniform electric field between two planar electrodes, numerical simulations were performed to obtain the electric field distribution between the two pin electrodes and to find a theoretical charge amount. Three-dimensional numerical simulations were performed to obtain the electric field distribution between the two pin electrodes and to calculate the droplet charge amount as shown in Fig. 1B. The water droplet in the present experimental conditions can be assumed to be a perfect conductor regardless of the electrical double layer inside the droplet as a result of the previous research [40]. Silicone oil can be assumed to be a perfect dielectric due to the relatively long charge relaxation time (100 s) compared with the bouncing time (1 s) of the droplet. Therefore, the electric field distribution of the system can be calculated using the following Laplace equation: ∇2 = 0
(1)
where is the electric potential. The equation was solved using the electrostatic module of a commercial numerical analysis program (COMSOL Multiphysics, v4.2) using appropriate boundary conditions (i.e. positive voltage and negative voltage at the electrodes, ∂/∂n = 0 at the test cell walls). The droplet charge was obtained from the integration of the surface charge density over the surface of the droplet in contact with the electrified electrode. The force exerted on the droplet was calculated using the following equation: (2) F = n · TdS S
where n is the outward normal vector, S the droplet surface, and T the Maxwell stress tensor. These calculations were conducted with varying distances and angles between the two electrodes. Consult supplementary information for numerical details. For effective comparison, the charges of the droplets in the simulations and experiments were normalized with the theoretical value Qth for a perfectly conductive sphere in contact with an electrified infinite planar electrode given as follows [41]: Qth =
2 4a 2 ε E 0 6
(3)
where a is the sphere radius, ε the permittivity of the surrounding medium, and E0 applied electric field strength (applied voltage/distance). The normalized charge (dimensionless charge, Q/Qth ) explains how it differs from the above theory. The distance and angle were defined as shown in Fig. 1C. The distance was defined as the shortest length between the centers of the facing surfaces of the two electrodes, which was varied from 2 to 60 mm. The angle between the two electrodes was defined as the sum of the individual angle of the two electrodes; the individual angle of one electrode was defined as the acute angle between the electrode C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
and the horizontal line, which is the same straight line used to measure the distance. The angle was varied from 0°, which means that the two pin electrodes are placed in a line facing each other (left side of Fig. 1D), to 180°, which means that two pin electrodes are parallel to each other (right side of Fig. 1D).
3 Results and discussion The distance between the electrodes is a significant consideration for the alignment of electrodes in microfluidic applications because the distance is directly relevant to the scale of a microfluidic system. Figure 2A shows the dimensionless charges of a droplet on the electrode as a function of the distance between the two pin electrodes from numerical simulations. As the distance increases, the droplet charge increases even under the same applied electric field (applied voltage/distance). The droplet charge obtained from the electrode is dependent on the electric field strength around the charging electrode. If the electrodes are planar, the electric field is uniform and consistent even though the applied voltage is increased to maintain the electric field strength according to the increasing distance between electrodes. Therefore, under the same applied electric field, the droplet always obtains the same amount of charges (Qth ) from the electrode with different distances in the planar electrodes system. However, the electric field distribution is not uniform in the pin electrodes system. Hence, the electric field distribution must be analyzed first, to understand the droplet charging characteristics on the pin electrode. According to the analytic solution for the electric field distribution between two parallel disk electrodes (radii: 325 m) of infinitesimal thickness [42], the local electric field strength near the electrode is greater for longer distances as shown in Fig. 2B. The difference of electric field distribution around electrode between the pin and planar electrodes system is illustrated in Fig. 2C. As the distance increases, the electric field around the pin electrodes is more concentrated even under the same applied electric field (applied voltage/distance) whereas the electric field around the planar electrodes remains unchanged. Therefore, the droplet on the pin electrode experiences a stronger electric field with longer distances; as a result, the droplet acquires more charges from the pin electrode than from the planar electrode even under the same applied electric field (applied voltage/distance). The effect of the distance between two electrodes with constant applied voltage is also investigated for comparison as shown in Supporting Information Fig. 4. The other significant consideration for the electrode alignment in microfluidic devices is the angle between the electrodes, because it is important to the arrangement and structure of the components of microfluidic devices. As shown in the inset graph of Fig. 2A, the droplet charge is barely affected by the angle between the two electrodes. Under the same distance between electrodes and the same applied voltage, the amount of droplet charge on the electrode www.electrophoresis-journal.com
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Table 1. Numerical results of the average dimensionless charges with various angles between two electrodes from 0° to 180° (The gap is 20°)
Distance between electrodes (mm)
Dimensionless droplet charge (Q/Qth )
2 4 6 8 10
1.557 ± 0.025 2.633 ± 0.008 3.663 ± 0.022 4.668 ± 0.040 5.647 ± 0.049
The droplet volume was 100 nL, the electric field is 3 kV/cm, and the electrode radii were 325 m.
Figure 3. Numerical results for the effects of the angles and distances between electrodes. The dimensionless charges of a droplet on the electrode are plotted as a function of the ratio between the electrode radius and the droplet radius. The droplet volume is 100 nL, the electric field is 3 kV/cm, and the electrode radii are 325 m.
Figure 2. (A) The dependency of the dimensionless droplet charges on the distance between electrodes. The droplet volume is 100 nL; the electric field is 3 kV/cm; and the electrode radii are 325 m. The red line indicates a linear fit line (slope: 0.4663; intercept: 1.017). (B) The electric field strength distribution according to the distance between two electrodes from the analytic solution. Eavg denotes the average electric field (3 kV/cm) that is the same as applied electric field. In each distance case, the electric potential is set differently in order to maintain the average electric field, e.g. 3 kV/cm = 1200 V/4 mm. (C) Electric potential distribution around the electrodes. Black lines are equi-potential lines with potential difference of 100 V.
is almost identical even though the angle is varied from 0° to 180° as shown in Table 1 and Supporting Information Fig. 5. Therefore, the effect of the angle between the electrodes is insignificant for the droplet charging on the electrode. In other words, this result provides a great degree of C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
freedom in the design of electrodes for microfluidic devices utilizing droplet contact charging. For a more comprehensive understanding of these results, a more systematic analysis was conducted according to the system scale. In order to demonstrate the effects of the distance and angle between electrodes simultaneously, the dimensionless charge of a droplet is plotted with representative angles and distances with variations in the aspect ratio between the electrode radius and droplet radius in Fig. 3. In Figure 3, a greater aspect ratio indicates that the electrode is more planar and a lesser aspect ratio indicates that the electrode becomes a sharper pin. If the radius of the electrode is approximately 10 times greater than the droplet, the droplet charge converges to the perfect conductor theory value (Qth ) regardless of the angles and distances due to the nearly uniform electric field. As the ratio decreases, the droplet charge increases due to the highly focused electric field near the pin electrode. In particular, the increase in the droplet charge is greater for electrode systems with greater distances between the electrodes (16 mm) than that for closer distances (2 mm) in the sharper pin electrode system. www.electrophoresis-journal.com
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Figure 4. Experimental results of the dimensionless droplet charge versus the distance between the electrodes. The droplet volume is 500 nL, the electric field is 3 kV/cm, and the electrode radii are 325 m. The angle is fixed as 0° when the distance is varied and the distance is fixed as 8 mm when the angle is varied. The dashed lines indicate the linear fit (red line: slope = 0.49512/mm, blue line: slope = 0.00183/°). The number of measurements for each data point is 25. The error bars represent one standard deviation.
Furthermore, the different angles have little effect on the droplet charge in all aspect ratios and distances. This implies that if the sizes of electrodes and target droplets for actuation are chosen, the charge amount of a droplet can be optimized with adjusting the distance between electrodes. The droplet charge is also measured experimentally with various distances and angles to confirm the numerical simulations. In the middle region of the two electrodes, the droplet velocity is constant and the force balance is set using electrostatic force (FE ) and viscous drag force (FD ) according to the drag force of Hadamard–Rybczynski solution [43]: FT otal = F E + F D = QE − 4aUc = 0
(4)
where c = (3+2)/2(+1), = w /, Q is the net charge of the droplet, E is the local electric field in the middle region of the two electrodes, is the viscosity of the outer medium, w is the viscosity of water droplet, a is the droplet radius, and U is the droplet velocity at the middle of the two electrodes (consult supplementary information for details). Therefore, the droplet charge can be estimated from the velocity, which is measured using image analyses, as follows: Q=
4aUc E
(5)
Figure 4 shows the droplet charge estimated using Eq. (5) versus the distance and angle between the electrodes. As shown in Fig. 4, the droplet charge is directly proportional to the distance between the two electrodes (red symbols). As the distance increases, the droplet obtains more dimensionless charge from the charging electrode, even under the C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
same applied electric field (applied voltage/distance), and the results correspond precisely with the simulation results. The angle effect also corresponds to the simulation results as shown in Fig. 4 (blue symbols). The droplet obtained a similar charge from the charging electrode regardless of the change in the angle. However, there is slight difference between the experimental and simulation results. When the angle of the two facing electrodes is 180°, the charge of the droplet decreases slightly. The charge amount of the droplet at 180° is approximately 10% lower than the other scenarios. If the two electrodes are parallel, the shortest path between the electrodes is the side of the electrodes and the droplet is gradually attracted to the side near the top surface of the electrode. Because a uniform electric field is formed between the sides of the two parallel electrodes, the droplet experiences a relatively less focused electric field at the side of the pin electrode compared with the other angle scenarios. As a result, the droplet obtains slightly less charge from the electrode in the parallel electrode system. Despite the 180° angle scenario, the overall experimental results exhibit good agreement with the simulation results according to the distance and angle between the electrodes. From a design viewpoint, both the force exerted on the droplet and the droplet charge acquired from an electrode are important in controlling a droplet in microfluidic applications. Figure 5 shows the electric forces on the droplet with representative droplet sizes. The DEP force on the droplet due to the nonuniform electric field is also calculated without charge on the droplet at each position. Because the horizontal total driving force on the droplet consists of the DEP force and Coulomb force, the Coulomb force can be obtained using the difference between the total force and the DEP force from the numerical calculations. As a result, the derived Coulomb force includes all influences from the electric field distortion that result from the charged droplet near the electrodes. The forces in the middle region between the electrodes present little difference at both angles of 0° and 180° as shown in Fig. 5. Because the DEP force is negligible in the middle region, the total force is the same as the Coulomb force. This adequately supports that balance of Eq. (4) is valid. Near the electrodes, however, the tendency of the force differs between the 0° and 180° angles as shown in Fig. 5A. If the droplet radius is greater than the electrode radius, all forces in the 0° angle scenario are much stronger than those in the 180° scenario near the electrodes. The Coulomb force and DEP force are directly related to the electric field strength and the bigger droplet in the 0° angle scenario feels more concentrated electric field according to the electric field distribution around the droplet as shown in Fig. 5A. This effect resulting from the concentrated electric field diminishes as the droplet size decreases. Figure 5B exhibits a smaller difference in the forces near the electrodes with changes in the angle because the smaller droplet experiences relatively similar electric field gradients near the electrodes. This implies that for droplets much smaller than electrodes, a more consistent actuation velocity is expected regardless of the angle between the electrodes. www.electrophoresis-journal.com
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Figure 5. Numerical results of forces exerted on the droplet. The volume of a droplet is (A) 1000 nL and (B) 100 nL, the electric field is 3 kV/cm, the electrode radii are 325 m, and the distance between the electrodes is 3 mm. The direction of force is right when the force is positive and left when the force is negative.
The analysis of the alignment effect is applied to actual digital microfluidic applications. Recently, the electrophoretic motion of a charged droplet was investigated and the electrophoresis of a charged droplet (ECD) method was developed as a novel droplet manipulation method [28]. Figure 6A shows a representative integrated ECD chip system (left) and time-lapse images of the motion of a water droplet in the system (right). To construct the ECD chip system, the electrodes were chosen very carefully because the electrode array is the key part of the system and the actuation of a droplet can be affected significantly by the electrode alignment and geometry. The target droplet size for actuation was approximately 300 nL. The electrode radius was chosen as 325 m in order to create a ratio of 0.8 between the electrode radius and droplet radius. The blue circle in Fig. 3 represents the scale of this system. As shown in Fig. 3, if the ratio is greater than 0.8, the charging efficiency of the droplet is decreased and, if the ratio is less than 0.8, the charging efficiency of the droplet increases only slightly despite the increased difficulty in assembling the system due to the smaller system scale. Therefore, if the electrode radius is 325 m, the ECD chip device is optimized practically. Moreover, the parallel alignment allows easy actuation of the 2D droplet motions as shown in Fig. 6B. For the ECD-based microfluidic devices with pin electrodes, the greatest distance that the system allows is better and the angle can be adjusted unrestrictedly. As in this
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Figure 6. (A) Integrated ECD chip system and time-lapse images of the motion of a 300 nL water droplet (side view). The time step of the droplet image is 0.1 s. (B) Two-dimensional motions of a 300 nL water droplet infused with colored ink (top view). The droplets are actuated with the applied voltage, 300 V.
practical application, the principle of the direct charging of a droplet is simple and straightforward to understand, and the analysis of the effects of the electrode alignment can provide guidelines for the design of electrodes for use in microfluidic devices. It also can be applied to improve the actuation
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efficiency of water-in-oil droplets in microchannels [1, 5, 22, 30] as well as the ECD-based microfluidic devices [28].
4 Concluding remarks From the numerical and experimental investigations, the effects of electrode alignment on the charging and actuation of a droplet have been analyzed according to the distance and angle between the two electrodes. As the distance between electrodes increases, the droplet charge acquired from the electrified electrode increases even under the same applied electric field (applied voltage/distance) and the effect of the angle between electrodes is insignificant in the charging of a droplet. This implies that the primary consideration of electrode alignment in microfluidic devices is the distance between electrodes and the insignificant effect of the angle provides a great degree of freedom in designing such devices. From an actuation viewpoint, if the radius of a droplet is greater than that of the electrode, the forces exerted on the droplet exhibit notable differences near the electrodes with different angles due to the DEP force. However, if the radius of a droplet is smaller than that of the electrode, the forces are always consistent even when the angle changes. The findings of this work show that it is simple and straightforward to design electrodes for microfluidic devices utilizing direct contact charging. It is expected that this fundamental understanding of the charging characteristics of a droplet on an electrified electrode in droplet-based microfluidic systems together with previous work [38] will offer a complete guideline for the design of electrodes in those devices. This research was supported by the grants (NRF2015R1D1A3A01019112, NRF-2014M1A8A1074941, and No. 2013R1A1A2011956) funded through the National Research Foundation of Korea (NRF). M. M. Ahn was also supported by the BK21 program of Korea. The authors have declared no financial/commercial conflict of interest.
5 References
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[7] Ahn, K., Kerbage, C., Hunt, T. P., Westervelt, R. M., Link, D. R., Weitz, D. A., Appl. Phys. Lett. 2006, 88, 024104–024103. [8] Wang, W., Yang, C., Liu, Y., Li, C. M., Lab Chip 2010, 10, 559–562. [9] Li, M., Li, S., Li, W., Wen, W., Alici, G., Electrophoresis 2013, 34, 952–960. [10] Li, Y., Dalton, C., Crabtree, H. J., Nilsson, G., Kaler, K. V. I. S., Lab Chip 2007, 7, 239–248. [11] Pethig, R., Menachery, A., Pells, S., De Sousa, P., J. Biomed. Biotechnol. 2010, 2010, 182581. ¨ ur, ¨ E., Kulah, ¨ [12] Demircan, Y., Ozg H., Electrophoresis 2013, 34, 1008–1027. [13] Cho, S. K., Moon, H., Chang-Jin, K., J. Microelectromech. Syst. 2003, 12, 70–80. [14] Wheeler, A. R., Science 2008, 322, 539–540. [15] Fair, R. B., Khlystov, A., Tailor, T. D., Ivanov, V., Evans, R. D., Griffin, P. B., Vijay, S., Pamula, V. K., Pollack, M. G., Zhou, J., IEEE Des. Test. Comput. 2007, 24, 10–24. [16] Dooraghi, A. A., Keng, P. Y., Chen, S. P., Javed, M. R., Kim, C. J., Chatziioannou, A. F., van Dam, R. M., Analyst 2013, 138, 5654–5664. [17] Hayes, R. A., Feenstra, B. J., Nature 2003, 425, 383– 385. [18] Roques-Carmes, T., Hayes, R. A., Feenstra, B. J., Schlangen, L. J. M., J. Appl. Phys. 2004, 95, 4389– 4396. [19] Hase, M., Watanabe, S. N., Yoshikawa, K., Phys. Rev. E 2006, 74, 046301. [20] Niu, X., Zhang, M., Peng, S., Wen, W., Sheng, P., Biomicrofluidics 2007, 1, 044101–044112. [21] Takinoue, M., Atsumi, Y., Yoshikawa, K., Appl. Phys. Lett. 2010, 96. [22] Ahn, B., Lee, K., Panchapakesan, R., Oh, K. W., Biomicrofluidics 2011, 5, 024113. [23] Im, D. J., Noh, J., Moon, D., Kang, I. S., Anal. Chem. 2011, 83, 5168–5174. [24] Lee, C.-P., Chang, H.-C., Wei, Z.-H., Appl. Phys. Lett. 2012, 101, 014103–014104. [25] Lee, D. W., Im, D. J., Kang, I. S., Appl. Phys. Lett. 2012, 100, 221602. [26] Hokmabad, B. V., Sadri, B., Charan, M. R., Esmaeilzadeh, E., Colloids Surf. A 2012, 401, 17–28.
[1] Link, D. R., Grasland-Mongrain, E., Duri, A., Sarrazin, F., Cheng, Z., Cristobal, G., Marquez, M., Weitz, D. A., Angew. Chem. Int. Ed. 2006, 45, 2556–2560.
[27] Liu, T. T., Seiffert, S., Thiele, J., Abate, A. R., Weitz, D. A., Richtering, W., Proc. Natl. Acad. Sci. USA 2012, 109, 384–389.
[2] Fidalgo, L. M., Whyte, G., Bratton, D., Kaminski, C. F., Abell, C., Huck, W. T. S., Angew. Chem. Int. Ed. 2008, 47, 2042–2045.
[28] Im, D. J., Yoo, B. S., Ahn, M. M., Moon, D., Kang, I. S., Anal. Chem. 2013, 85, 4038–4044.
[3] Ahn, B., Lee, K., Louge, R., Oh, K. W., Biomicrofluidics 2009, 3, 044102. [4] Priest, C., Herminghaus, S., Seemann, R., Appl. Phys. Lett. 2006, 89, 134101.
[29] Kurimura, T., Ichikawa, M., Takinoue, M., Yoshikawa, K., Phys. Rev. E 2013, 88, 042918. [30] Drews, A. M., Lee, H.-Y., Bishop, K. J. M., Lab Chip 2013, 13, 4295–4298.
[5] Zhou, H., Yao, S., Lab Chip 2013, 13, 962–969.
[31] Rajabi, N., Dolatabadi, A., Colloids Surf. A 2010, 365, 230–236.
[6] Chabert, M., Dorfman, K. D., Viovy, J.-L., Electrophoresis 2005, 26, 3706–3715.
[32] Lin, J. T. Y., Yeow, J. T. W., Biomed. Microdevices 2007, 9, 823–831.
C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.electrophoresis-journal.com
Electrophoresis 2015, 36, 2086–2093
Microfluidics and Miniaturization
2093
[33] Lienemann, J., Greiner, A., Korvink, J. G., IEEE T. Comput. Aid. D 2006, 25, 234–247.
[38] Ahn, M. M., Im, D. J., Kang, I. S., Analyst 2013, 138, 7362–7368.
[34] Kwon, J. S., Maeng, J. S., Chun, M. S., Song, S., Microfluid. Nanofluid. 2008, 5, 23–31.
[39] Im, D. J., Ahn, M. M., Yoo, B. S., Moon, D., Lee, D. W., Kang, I. S., Langmuir 2012, 28, 11656–11661.
[35] Li, M., Li, S. B., Cao, W. B., Li, W. H., Wen, W. J., Alici, G., Microfluid. Nanofluid. 2013, 14, 527–539.
[40] Hua, C. K., Lee, D. W., Kang, I. S., Colloids Surf. A 2010, 372, 86–97.
[36] Khoshmanesh, K., Nahavandi, S., Baratchi, S., Mitchell, A., Kalantar-zadeh, K., Biosens. Bioelectron. 2011, 26, 1800–1814.
[41] Khayari, A., Perez, A. T., IEEE T. Dielec. Elec. Insul. 2002, 9, 589–595.
[37] Tang, S. Y., Zhang, W., Yi, P., Baratchi, S., Kalantarzadeh, K., Khoshmanesh, K., Electrophoresis 2013, 34, 1407–1414.
C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
[42] Atkinson, W. J., Young, J. H., Brezovich, I. A., J. Phys. A 1983, 16, 2837. [43] Jung, Y. M., Oh, H. C., Kang, I. S., J. Colloid Interface Sci. 2008, 322, 617–623.
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of Chemical Engineering, Pohang University of Science and Technology, Pohang, South Korea 2 Department of Chemical Engineering, Pukyong National University, Nam-Gu. Busan, South Korea
Received March 18, 2015 Revised May 6, 2015 Accepted May 6, 2015
Electrophoresis 2015, 36, 2086–2093
Research Article
Characterization of electrode alignment for optimal droplet charging and actuation in droplet-based microfluidic system The actuation method using electric force as a driving force is utilized widely in dropletbased microfluidic systems. In this work, the effects of charging electrode alignment on direct charging of a droplet on electrified electrodes and a subsequent electrophoretic control of the droplet are investigated. The charging characteristics of a droplet according to different electrode alignments are quantitatively examined through experiments and systematic numerical simulations with varying distances and angles between the two electrodes. The droplet charge acquired from the electrified electrode is directly proportional to the distance and barely affected by the angle between the two electrodes. This implies that the primary consideration of electrode alignment in microfluidic devices is the distance between electrodes and the insignificant effect of angle provides a great degree of freedom in designing such devices. Not only the droplet charge acquired from the electrode but also the force exerted on the droplet is analyzed. Finally, the implications and design guidance for microfluidic systems are discussed with an electrophoresis of a charged droplet method-based digital microfluidic device. Keywords: Droplet actuation / Droplet charging / Electrode alignment DOI 10.1002/elps.201500141
Additional supporting information may be found in the online version of this article at the publisher’s web-site
1 Introduction
Correspondence: Professor Do Jin Im, Department of Chemical Engineering, Pukyong National University, 45 Yongso-ro, Nam-Gu. Busan 608737, South Korea E-mail: [email protected]
electric field and have been used in various chemical and biological applications [10–12]. Electrowetting on dielectric (EWOD) is another significant actuation method for the control of conducting droplets electrically [13, 14]. Because EWOD requires relatively low electric potential and has precise control of individual droplets, it is used in a wide range of applications in chemical and biological fields [15, 16] as well as in the display and liquid lens industry [17,18]. Recently, the direct charging of a droplet in a dielectric medium with an electrified bare electrode has been proposed as a novel actuation method of droplets [19–24]. Due to its essential merits, such as the intuitive and straightforward actuation and minimal contact with solid substrates, the direct charging method has been used in a growing number of studies [25–30]. In microfluidic systems using electric force as the driving force of the actuation method, the electric field distribution is a critical influence on the motion of a droplet; thus, electrodes are a significant part of the microfluidic devices because the electrified electrodes generate the electric field distribution. Therefore, the effects of the electrode geometry and alignment have been investigated in well-known droplet actuation
Abbreviations: ECD, electrophoresis of a charged droplet; EWOD, electrowetting on dielectric
Colour Online: See the article online to view Figs. 1–6 in colour.
Electric force is widely used as a driving force in the actuation method for droplet-based microfluidic systems. Besides its intrinsic advantages such as fast response times and precise individual control of droplets, the actuation method using electric force is easily integrated with various microfluidic systems [1–6]. As a result of these advantages, many microfluidic devices, especially chip-based microfluidic platforms, have adopted electric force as an actuation method. DEP, which is the motion of uncharged polarizable particles in a nonuniform electric field, is a representative actuation method in microfluidic systems using electric force [7–9]. Due to the interaction of the electric field with the induced dipole moment, the dielectrophoretic forces can actuate droplets along the gradient of the
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methods, such as EWOD and DEP [7, 31–37]. Furthermore, the droplet motions have been examined experimentally and numerically depending on the electrode geometry. Droplets have exhibited different behaviors with different electrode geometries even in the same microfluidic system, and these results could be used to optimize the devices for better droplet actuation [33, 34]. In our previous work, the effects of the electrode geometry on direct charging of a droplet were investigated [38]. The experimental and numerical studies were performed through varying the cross-sectional area and length of charging electrodes. From those results, it was found that for a given droplet size, a long and sharp electrode is favorable for the charging and actuation of a droplet; that is, a pin electrode is more efficient for droplet charging and actuation compared with a planar one. Although these findings can provide important information for the design of electrodes for microfluidic devices, the electrode alignment including the distance and angle between electrodes should be also considered to offer a complete design guideline. However, there has not yet been a systematic study on the effect of electrode alignment on the direct charging and subsequent electrophoretic control of a droplet. In this study, the effects of charging electrode alignment on charging characteristics of a droplet are quantitatively examined through experiments and numerical simulations. Two key factors of charging electrode alignment are examined: the distance and the angle between electrodes. Although there are numerous alignment setups for microfluidic devices and it is difficult to analyze all individual electrode alignments, most alignments can be covered through varying the distance and angle between electrodes. The droplet charges are estimated from the experiments with varied distances and angles of charging electrodes and these are compared with the charges from numerical simulations. A droplet-based microfluidic application is also demonstrated and analyzed using the findings of the electrode alignment effects. Finally, the implications for basic understanding of the droplet contact charging phenomena on the electrode according to the geometry and alignment of electrodes, and for the improvement of efficiency of microfluidic devices, are discussed.
2 Materials and methods A schematic view of the experimental setup is shown in Fig. 1A. A transparent acrylic cube (internal dimensions: 5 × 5 × 5 cm3 ) was used as a test cell; it was placed between a light source and a microscope and filled with silicone oil (ShinEtsu KF-96 1000 cSt) as the dielectric suspending medium. In order to avoid thermal side effects, a cooled light-emitting diode (LED) was used as a light source. As mentioned in introduction, a pin electrode is advantageous in charging and actuation of a droplet; thus, pin electrodes were used in this study [38]. Two copper pin electrodes (radius: 0.325 mm; length: 20 mm) were fixed on the stages to change the distance and angles between electrodes; they were connected C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 1. (A) Schematic view of the experimental setup. Timelapse images of the bouncing motion of a 500 nL water droplet under 3 kV/cm when the angle between the electrodes is 0°. (B) Numerical results for the electric potential distribution around a droplet and the electrodes. The potential difference between the equi-potential lines is 100 V. (C) Definitions of the distance and the angle between two electrodes. The distance (d) is the shortest length between the centers of the facing surfaces of the two electrodes and the angle () is the sum of the individual angle of the two electrodes (i.e. = 1 + 2 ). (D) Representative angles of two electrodes: 0° and 180° setups.
to a direct current power supply (Trek Models 610E) and the opposite sides of the electrodes were soaked in the middle of the test cell. After a single deionized water droplet was dispensed from a micro-pipette between the two pin electrodes in the test cell, a horizontal bouncing motion of the droplet was recorded using a high speed camera (Photron Fastcam 1024 PCI model 100K) mounted on the microscope as shown in the bottom of Fig. 1A. The recorded images were analyzed using a LabVIEW Vision Assistant Toolkit. In order to exclude deformation and wall effects, the radius and horizontal velocity of the droplet were only measured in the middle region between the two electrodes. From the measured data, the droplet charge was estimated using the force balance between the electrical and hydrodynamic drag forces (consult the reference [39] and supplementary www.electrophoresis-journal.com
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information for details). Because the electric field between two pin electrodes is not uniform unlike a uniform electric field between two planar electrodes, numerical simulations were performed to obtain the electric field distribution between the two pin electrodes and to find a theoretical charge amount. Three-dimensional numerical simulations were performed to obtain the electric field distribution between the two pin electrodes and to calculate the droplet charge amount as shown in Fig. 1B. The water droplet in the present experimental conditions can be assumed to be a perfect conductor regardless of the electrical double layer inside the droplet as a result of the previous research [40]. Silicone oil can be assumed to be a perfect dielectric due to the relatively long charge relaxation time (100 s) compared with the bouncing time (1 s) of the droplet. Therefore, the electric field distribution of the system can be calculated using the following Laplace equation: ∇2 = 0
(1)
where is the electric potential. The equation was solved using the electrostatic module of a commercial numerical analysis program (COMSOL Multiphysics, v4.2) using appropriate boundary conditions (i.e. positive voltage and negative voltage at the electrodes, ∂/∂n = 0 at the test cell walls). The droplet charge was obtained from the integration of the surface charge density over the surface of the droplet in contact with the electrified electrode. The force exerted on the droplet was calculated using the following equation: (2) F = n · TdS S
where n is the outward normal vector, S the droplet surface, and T the Maxwell stress tensor. These calculations were conducted with varying distances and angles between the two electrodes. Consult supplementary information for numerical details. For effective comparison, the charges of the droplets in the simulations and experiments were normalized with the theoretical value Qth for a perfectly conductive sphere in contact with an electrified infinite planar electrode given as follows [41]: Qth =
2 4a 2 ε E 0 6
(3)
where a is the sphere radius, ε the permittivity of the surrounding medium, and E0 applied electric field strength (applied voltage/distance). The normalized charge (dimensionless charge, Q/Qth ) explains how it differs from the above theory. The distance and angle were defined as shown in Fig. 1C. The distance was defined as the shortest length between the centers of the facing surfaces of the two electrodes, which was varied from 2 to 60 mm. The angle between the two electrodes was defined as the sum of the individual angle of the two electrodes; the individual angle of one electrode was defined as the acute angle between the electrode C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
and the horizontal line, which is the same straight line used to measure the distance. The angle was varied from 0°, which means that the two pin electrodes are placed in a line facing each other (left side of Fig. 1D), to 180°, which means that two pin electrodes are parallel to each other (right side of Fig. 1D).
3 Results and discussion The distance between the electrodes is a significant consideration for the alignment of electrodes in microfluidic applications because the distance is directly relevant to the scale of a microfluidic system. Figure 2A shows the dimensionless charges of a droplet on the electrode as a function of the distance between the two pin electrodes from numerical simulations. As the distance increases, the droplet charge increases even under the same applied electric field (applied voltage/distance). The droplet charge obtained from the electrode is dependent on the electric field strength around the charging electrode. If the electrodes are planar, the electric field is uniform and consistent even though the applied voltage is increased to maintain the electric field strength according to the increasing distance between electrodes. Therefore, under the same applied electric field, the droplet always obtains the same amount of charges (Qth ) from the electrode with different distances in the planar electrodes system. However, the electric field distribution is not uniform in the pin electrodes system. Hence, the electric field distribution must be analyzed first, to understand the droplet charging characteristics on the pin electrode. According to the analytic solution for the electric field distribution between two parallel disk electrodes (radii: 325 m) of infinitesimal thickness [42], the local electric field strength near the electrode is greater for longer distances as shown in Fig. 2B. The difference of electric field distribution around electrode between the pin and planar electrodes system is illustrated in Fig. 2C. As the distance increases, the electric field around the pin electrodes is more concentrated even under the same applied electric field (applied voltage/distance) whereas the electric field around the planar electrodes remains unchanged. Therefore, the droplet on the pin electrode experiences a stronger electric field with longer distances; as a result, the droplet acquires more charges from the pin electrode than from the planar electrode even under the same applied electric field (applied voltage/distance). The effect of the distance between two electrodes with constant applied voltage is also investigated for comparison as shown in Supporting Information Fig. 4. The other significant consideration for the electrode alignment in microfluidic devices is the angle between the electrodes, because it is important to the arrangement and structure of the components of microfluidic devices. As shown in the inset graph of Fig. 2A, the droplet charge is barely affected by the angle between the two electrodes. Under the same distance between electrodes and the same applied voltage, the amount of droplet charge on the electrode www.electrophoresis-journal.com
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Table 1. Numerical results of the average dimensionless charges with various angles between two electrodes from 0° to 180° (The gap is 20°)
Distance between electrodes (mm)
Dimensionless droplet charge (Q/Qth )
2 4 6 8 10
1.557 ± 0.025 2.633 ± 0.008 3.663 ± 0.022 4.668 ± 0.040 5.647 ± 0.049
The droplet volume was 100 nL, the electric field is 3 kV/cm, and the electrode radii were 325 m.
Figure 3. Numerical results for the effects of the angles and distances between electrodes. The dimensionless charges of a droplet on the electrode are plotted as a function of the ratio between the electrode radius and the droplet radius. The droplet volume is 100 nL, the electric field is 3 kV/cm, and the electrode radii are 325 m.
Figure 2. (A) The dependency of the dimensionless droplet charges on the distance between electrodes. The droplet volume is 100 nL; the electric field is 3 kV/cm; and the electrode radii are 325 m. The red line indicates a linear fit line (slope: 0.4663; intercept: 1.017). (B) The electric field strength distribution according to the distance between two electrodes from the analytic solution. Eavg denotes the average electric field (3 kV/cm) that is the same as applied electric field. In each distance case, the electric potential is set differently in order to maintain the average electric field, e.g. 3 kV/cm = 1200 V/4 mm. (C) Electric potential distribution around the electrodes. Black lines are equi-potential lines with potential difference of 100 V.
is almost identical even though the angle is varied from 0° to 180° as shown in Table 1 and Supporting Information Fig. 5. Therefore, the effect of the angle between the electrodes is insignificant for the droplet charging on the electrode. In other words, this result provides a great degree of C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
freedom in the design of electrodes for microfluidic devices utilizing droplet contact charging. For a more comprehensive understanding of these results, a more systematic analysis was conducted according to the system scale. In order to demonstrate the effects of the distance and angle between electrodes simultaneously, the dimensionless charge of a droplet is plotted with representative angles and distances with variations in the aspect ratio between the electrode radius and droplet radius in Fig. 3. In Figure 3, a greater aspect ratio indicates that the electrode is more planar and a lesser aspect ratio indicates that the electrode becomes a sharper pin. If the radius of the electrode is approximately 10 times greater than the droplet, the droplet charge converges to the perfect conductor theory value (Qth ) regardless of the angles and distances due to the nearly uniform electric field. As the ratio decreases, the droplet charge increases due to the highly focused electric field near the pin electrode. In particular, the increase in the droplet charge is greater for electrode systems with greater distances between the electrodes (16 mm) than that for closer distances (2 mm) in the sharper pin electrode system. www.electrophoresis-journal.com
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Figure 4. Experimental results of the dimensionless droplet charge versus the distance between the electrodes. The droplet volume is 500 nL, the electric field is 3 kV/cm, and the electrode radii are 325 m. The angle is fixed as 0° when the distance is varied and the distance is fixed as 8 mm when the angle is varied. The dashed lines indicate the linear fit (red line: slope = 0.49512/mm, blue line: slope = 0.00183/°). The number of measurements for each data point is 25. The error bars represent one standard deviation.
Furthermore, the different angles have little effect on the droplet charge in all aspect ratios and distances. This implies that if the sizes of electrodes and target droplets for actuation are chosen, the charge amount of a droplet can be optimized with adjusting the distance between electrodes. The droplet charge is also measured experimentally with various distances and angles to confirm the numerical simulations. In the middle region of the two electrodes, the droplet velocity is constant and the force balance is set using electrostatic force (FE ) and viscous drag force (FD ) according to the drag force of Hadamard–Rybczynski solution [43]: FT otal = F E + F D = QE − 4aUc = 0
(4)
where c = (3+2)/2(+1), = w /, Q is the net charge of the droplet, E is the local electric field in the middle region of the two electrodes, is the viscosity of the outer medium, w is the viscosity of water droplet, a is the droplet radius, and U is the droplet velocity at the middle of the two electrodes (consult supplementary information for details). Therefore, the droplet charge can be estimated from the velocity, which is measured using image analyses, as follows: Q=
4aUc E
(5)
Figure 4 shows the droplet charge estimated using Eq. (5) versus the distance and angle between the electrodes. As shown in Fig. 4, the droplet charge is directly proportional to the distance between the two electrodes (red symbols). As the distance increases, the droplet obtains more dimensionless charge from the charging electrode, even under the C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
same applied electric field (applied voltage/distance), and the results correspond precisely with the simulation results. The angle effect also corresponds to the simulation results as shown in Fig. 4 (blue symbols). The droplet obtained a similar charge from the charging electrode regardless of the change in the angle. However, there is slight difference between the experimental and simulation results. When the angle of the two facing electrodes is 180°, the charge of the droplet decreases slightly. The charge amount of the droplet at 180° is approximately 10% lower than the other scenarios. If the two electrodes are parallel, the shortest path between the electrodes is the side of the electrodes and the droplet is gradually attracted to the side near the top surface of the electrode. Because a uniform electric field is formed between the sides of the two parallel electrodes, the droplet experiences a relatively less focused electric field at the side of the pin electrode compared with the other angle scenarios. As a result, the droplet obtains slightly less charge from the electrode in the parallel electrode system. Despite the 180° angle scenario, the overall experimental results exhibit good agreement with the simulation results according to the distance and angle between the electrodes. From a design viewpoint, both the force exerted on the droplet and the droplet charge acquired from an electrode are important in controlling a droplet in microfluidic applications. Figure 5 shows the electric forces on the droplet with representative droplet sizes. The DEP force on the droplet due to the nonuniform electric field is also calculated without charge on the droplet at each position. Because the horizontal total driving force on the droplet consists of the DEP force and Coulomb force, the Coulomb force can be obtained using the difference between the total force and the DEP force from the numerical calculations. As a result, the derived Coulomb force includes all influences from the electric field distortion that result from the charged droplet near the electrodes. The forces in the middle region between the electrodes present little difference at both angles of 0° and 180° as shown in Fig. 5. Because the DEP force is negligible in the middle region, the total force is the same as the Coulomb force. This adequately supports that balance of Eq. (4) is valid. Near the electrodes, however, the tendency of the force differs between the 0° and 180° angles as shown in Fig. 5A. If the droplet radius is greater than the electrode radius, all forces in the 0° angle scenario are much stronger than those in the 180° scenario near the electrodes. The Coulomb force and DEP force are directly related to the electric field strength and the bigger droplet in the 0° angle scenario feels more concentrated electric field according to the electric field distribution around the droplet as shown in Fig. 5A. This effect resulting from the concentrated electric field diminishes as the droplet size decreases. Figure 5B exhibits a smaller difference in the forces near the electrodes with changes in the angle because the smaller droplet experiences relatively similar electric field gradients near the electrodes. This implies that for droplets much smaller than electrodes, a more consistent actuation velocity is expected regardless of the angle between the electrodes. www.electrophoresis-journal.com
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Figure 5. Numerical results of forces exerted on the droplet. The volume of a droplet is (A) 1000 nL and (B) 100 nL, the electric field is 3 kV/cm, the electrode radii are 325 m, and the distance between the electrodes is 3 mm. The direction of force is right when the force is positive and left when the force is negative.
The analysis of the alignment effect is applied to actual digital microfluidic applications. Recently, the electrophoretic motion of a charged droplet was investigated and the electrophoresis of a charged droplet (ECD) method was developed as a novel droplet manipulation method [28]. Figure 6A shows a representative integrated ECD chip system (left) and time-lapse images of the motion of a water droplet in the system (right). To construct the ECD chip system, the electrodes were chosen very carefully because the electrode array is the key part of the system and the actuation of a droplet can be affected significantly by the electrode alignment and geometry. The target droplet size for actuation was approximately 300 nL. The electrode radius was chosen as 325 m in order to create a ratio of 0.8 between the electrode radius and droplet radius. The blue circle in Fig. 3 represents the scale of this system. As shown in Fig. 3, if the ratio is greater than 0.8, the charging efficiency of the droplet is decreased and, if the ratio is less than 0.8, the charging efficiency of the droplet increases only slightly despite the increased difficulty in assembling the system due to the smaller system scale. Therefore, if the electrode radius is 325 m, the ECD chip device is optimized practically. Moreover, the parallel alignment allows easy actuation of the 2D droplet motions as shown in Fig. 6B. For the ECD-based microfluidic devices with pin electrodes, the greatest distance that the system allows is better and the angle can be adjusted unrestrictedly. As in this
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Figure 6. (A) Integrated ECD chip system and time-lapse images of the motion of a 300 nL water droplet (side view). The time step of the droplet image is 0.1 s. (B) Two-dimensional motions of a 300 nL water droplet infused with colored ink (top view). The droplets are actuated with the applied voltage, 300 V.
practical application, the principle of the direct charging of a droplet is simple and straightforward to understand, and the analysis of the effects of the electrode alignment can provide guidelines for the design of electrodes for use in microfluidic devices. It also can be applied to improve the actuation
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efficiency of water-in-oil droplets in microchannels [1, 5, 22, 30] as well as the ECD-based microfluidic devices [28].
4 Concluding remarks From the numerical and experimental investigations, the effects of electrode alignment on the charging and actuation of a droplet have been analyzed according to the distance and angle between the two electrodes. As the distance between electrodes increases, the droplet charge acquired from the electrified electrode increases even under the same applied electric field (applied voltage/distance) and the effect of the angle between electrodes is insignificant in the charging of a droplet. This implies that the primary consideration of electrode alignment in microfluidic devices is the distance between electrodes and the insignificant effect of the angle provides a great degree of freedom in designing such devices. From an actuation viewpoint, if the radius of a droplet is greater than that of the electrode, the forces exerted on the droplet exhibit notable differences near the electrodes with different angles due to the DEP force. However, if the radius of a droplet is smaller than that of the electrode, the forces are always consistent even when the angle changes. The findings of this work show that it is simple and straightforward to design electrodes for microfluidic devices utilizing direct contact charging. It is expected that this fundamental understanding of the charging characteristics of a droplet on an electrified electrode in droplet-based microfluidic systems together with previous work [38] will offer a complete guideline for the design of electrodes in those devices. This research was supported by the grants (NRF2015R1D1A3A01019112, NRF-2014M1A8A1074941, and No. 2013R1A1A2011956) funded through the National Research Foundation of Korea (NRF). M. M. Ahn was also supported by the BK21 program of Korea. The authors have declared no financial/commercial conflict of interest.
5 References
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[7] Ahn, K., Kerbage, C., Hunt, T. P., Westervelt, R. M., Link, D. R., Weitz, D. A., Appl. Phys. Lett. 2006, 88, 024104–024103. [8] Wang, W., Yang, C., Liu, Y., Li, C. M., Lab Chip 2010, 10, 559–562. [9] Li, M., Li, S., Li, W., Wen, W., Alici, G., Electrophoresis 2013, 34, 952–960. [10] Li, Y., Dalton, C., Crabtree, H. J., Nilsson, G., Kaler, K. V. I. S., Lab Chip 2007, 7, 239–248. [11] Pethig, R., Menachery, A., Pells, S., De Sousa, P., J. Biomed. Biotechnol. 2010, 2010, 182581. ¨ ur, ¨ E., Kulah, ¨ [12] Demircan, Y., Ozg H., Electrophoresis 2013, 34, 1008–1027. [13] Cho, S. K., Moon, H., Chang-Jin, K., J. Microelectromech. Syst. 2003, 12, 70–80. [14] Wheeler, A. R., Science 2008, 322, 539–540. [15] Fair, R. B., Khlystov, A., Tailor, T. D., Ivanov, V., Evans, R. D., Griffin, P. B., Vijay, S., Pamula, V. K., Pollack, M. G., Zhou, J., IEEE Des. Test. Comput. 2007, 24, 10–24. [16] Dooraghi, A. A., Keng, P. Y., Chen, S. P., Javed, M. R., Kim, C. J., Chatziioannou, A. F., van Dam, R. M., Analyst 2013, 138, 5654–5664. [17] Hayes, R. A., Feenstra, B. J., Nature 2003, 425, 383– 385. [18] Roques-Carmes, T., Hayes, R. A., Feenstra, B. J., Schlangen, L. J. M., J. Appl. Phys. 2004, 95, 4389– 4396. [19] Hase, M., Watanabe, S. N., Yoshikawa, K., Phys. Rev. E 2006, 74, 046301. [20] Niu, X., Zhang, M., Peng, S., Wen, W., Sheng, P., Biomicrofluidics 2007, 1, 044101–044112. [21] Takinoue, M., Atsumi, Y., Yoshikawa, K., Appl. Phys. Lett. 2010, 96. [22] Ahn, B., Lee, K., Panchapakesan, R., Oh, K. W., Biomicrofluidics 2011, 5, 024113. [23] Im, D. J., Noh, J., Moon, D., Kang, I. S., Anal. Chem. 2011, 83, 5168–5174. [24] Lee, C.-P., Chang, H.-C., Wei, Z.-H., Appl. Phys. Lett. 2012, 101, 014103–014104. [25] Lee, D. W., Im, D. J., Kang, I. S., Appl. Phys. Lett. 2012, 100, 221602. [26] Hokmabad, B. V., Sadri, B., Charan, M. R., Esmaeilzadeh, E., Colloids Surf. A 2012, 401, 17–28.
[1] Link, D. R., Grasland-Mongrain, E., Duri, A., Sarrazin, F., Cheng, Z., Cristobal, G., Marquez, M., Weitz, D. A., Angew. Chem. Int. Ed. 2006, 45, 2556–2560.
[27] Liu, T. T., Seiffert, S., Thiele, J., Abate, A. R., Weitz, D. A., Richtering, W., Proc. Natl. Acad. Sci. USA 2012, 109, 384–389.
[2] Fidalgo, L. M., Whyte, G., Bratton, D., Kaminski, C. F., Abell, C., Huck, W. T. S., Angew. Chem. Int. Ed. 2008, 47, 2042–2045.
[28] Im, D. J., Yoo, B. S., Ahn, M. M., Moon, D., Kang, I. S., Anal. Chem. 2013, 85, 4038–4044.
[3] Ahn, B., Lee, K., Louge, R., Oh, K. W., Biomicrofluidics 2009, 3, 044102. [4] Priest, C., Herminghaus, S., Seemann, R., Appl. Phys. Lett. 2006, 89, 134101.
[29] Kurimura, T., Ichikawa, M., Takinoue, M., Yoshikawa, K., Phys. Rev. E 2013, 88, 042918. [30] Drews, A. M., Lee, H.-Y., Bishop, K. J. M., Lab Chip 2013, 13, 4295–4298.
[5] Zhou, H., Yao, S., Lab Chip 2013, 13, 962–969.
[31] Rajabi, N., Dolatabadi, A., Colloids Surf. A 2010, 365, 230–236.
[6] Chabert, M., Dorfman, K. D., Viovy, J.-L., Electrophoresis 2005, 26, 3706–3715.
[32] Lin, J. T. Y., Yeow, J. T. W., Biomed. Microdevices 2007, 9, 823–831.
C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.electrophoresis-journal.com
Electrophoresis 2015, 36, 2086–2093
Microfluidics and Miniaturization
2093
[33] Lienemann, J., Greiner, A., Korvink, J. G., IEEE T. Comput. Aid. D 2006, 25, 234–247.
[38] Ahn, M. M., Im, D. J., Kang, I. S., Analyst 2013, 138, 7362–7368.
[34] Kwon, J. S., Maeng, J. S., Chun, M. S., Song, S., Microfluid. Nanofluid. 2008, 5, 23–31.
[39] Im, D. J., Ahn, M. M., Yoo, B. S., Moon, D., Lee, D. W., Kang, I. S., Langmuir 2012, 28, 11656–11661.
[35] Li, M., Li, S. B., Cao, W. B., Li, W. H., Wen, W. J., Alici, G., Microfluid. Nanofluid. 2013, 14, 527–539.
[40] Hua, C. K., Lee, D. W., Kang, I. S., Colloids Surf. A 2010, 372, 86–97.
[36] Khoshmanesh, K., Nahavandi, S., Baratchi, S., Mitchell, A., Kalantar-zadeh, K., Biosens. Bioelectron. 2011, 26, 1800–1814.
[41] Khayari, A., Perez, A. T., IEEE T. Dielec. Elec. Insul. 2002, 9, 589–595.
[37] Tang, S. Y., Zhang, W., Yi, P., Baratchi, S., Kalantarzadeh, K., Khoshmanesh, K., Electrophoresis 2013, 34, 1407–1414.
C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
[42] Atkinson, W. J., Young, J. H., Brezovich, I. A., J. Phys. A 1983, 16, 2837. [43] Jung, Y. M., Oh, H. C., Kang, I. S., J. Colloid Interface Sci. 2008, 322, 617–623.
www.electrophoresis-journal.com
