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3D electrowetting-on-dielectric actuation
Accepted Manuscript Title: 3D Electrowetting-on-dielectric Actuation Author: Jeong Byung Chae Seung Jun Lee Jinseung Yang Sang Kug Chung PII: DOI: Reference:
S0924-4247(15)30121-7 http://dx.doi.org/doi:10.1016/j.sna.2015.09.004 SNA 9299
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
15-10-2014 31-7-2015 3-9-2015
Please cite this article as: Jeong Byung Chae, Seung Jun Lee, Jinseung Yang, Sang Kug Chung, 3D Electrowetting-on-dielectric Actuation, Sensors and Actuators: A Physical http://dx.doi.org/10.1016/j.sna.2015.09.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
3D Electrowetting-on-dielectric Actuation Jeong Byung Chae†,1, Seung Jun Lee†,2, Jinseung Yang1 and Sang Kug Chung1,* 1
Department of Mechanical Engineering, Myongji University, Yongin 449-728, South Korea
2
SAITSamsung Advanced Institute of Technology, Yongin, Gyeonggido, South Korea
*
Corresponding author: [email protected]
†
These authors equally contributed to this work.
Highlights 3D digital microfluidics based on electrowetting-on-dielectric (EWOD) actuation is proposed and experimentally verified. The dynamic behavior of a droplet under various EWOD conditions such as different applied frequencies and switching times is studied using high-speed imaging The first complete 3D droplet manipulation through the combination of horizontal and vertical actuation between two parallel plates is experimentally achieved by sole EWOD actuation.
Abstract
This paper describes three-dimensional (3D) digital microfluidics based on an alternating current electrowetting-on-dielectric (AC-EWOD) principle. To achieve 3D manipulation of a droplet between two parallel plates, the horizontal and vertical actuation of a droplet is investigated separately. The dynamic behavior of a water droplet actuated by AC-EWOD in various conditions is studied using high-speed imaging. Optimum actuation parameters such as applied frequency, switching time, and the gap between the top and bottom plates are obtained. For horizontal actuation, a high frequency (1 kHz) is used for small-droplet deformation; on the other hand, for vertical actuation, a low frequency (44 Hz) is used for large-droplet deformation. Finally, as a proof of concept, 3D manipulation of a droplet with two parallel plates is demonstrated by the combination of horizontal and vertical AC-EWOD actuation, resulting in increased space for digital microfluidics.
Keywords: Microfluidics, Lab-on-a-chip, Alternating Current Electrowetting-on-Dielectric (AC-EWOD)
Email: [email protected]
I. INTRODUCTION
The development of reliable bio-chip and lab-on-a-chip systems is highly anticipated for biomedical and genomics applications such as cell manipulation and polymerase chain reaction (PCR) [1-4]. Bio-chip and labon-a-chip systems consist of microfluidic components such as extremely small channels, pumps, and sensors. These systems allow us to incorporate many biochemical laboratory functions on a single chip [5-8]. In the development of these systems, microfluidic technology is essential in the handling of small volumes of fluids (less than a picoliter).
As the size of a fluidic system decreases, the surface area to volume ratio is linearly increases. It makes the effects of viscosity on the fluidic system dominate. From a fluid mechanics perspective, the Reynolds number, the ratio of inertia forces to viscous forces, is commonly used to characterize flows and to verify governing forces in a fluidic system [8-10]. In a microfluidic system, the effect of the viscous forces is dominant, because of the small Reynolds number, which is inversely proportional to the size of the fluidic system. Therefore, the fluid driving techniques often used in macrofluidic systems may perform inadequately for pumping fluids in microfluidic systems, because of the high fluidic resistance generated by the viscous force. As alternatives, a variety of microfluidic technologies based on capillary forces and electrokinetic forces such as electroosmosis, electrophoresis, and dielectrophoresis have been developed [11-15]. Electrowetting-on-dielectric (EWOD) is one of these microfluidic techniques based on discrete liquids (most commonly droplets) without channel networks [16, 17]. This is fundamentally different from other existing techniques relying on complicated microchannels. EWOD has been established as one of the most efficient and feasible microfluidic technologies, because of its outstanding advantages such as fast response time, low power consumption, and robust operation [8, 18]. There have been great strides in the development of modern EWOD technology. About a hundred years ago, Gabriel Lippmann first studied the electrocapillarity, the basis of the present EWOD principle, in which the interfacial tension of mercury in contact with electrolyte solutions could be varied by applying an electric potential between the mercury and the solution [19]. He not only established a theory of the electrocapillarity, but also developed some applications such as a sensitive electrometer and a motor [16, 17]. However, electrolysis induced by the electric current flow through electrolyte solutions restricted broader applications. A century later, Berge et al. conducted a sessile droplet experiment using an electrode covered with a thin hydrophobic dielectric layer [20]. He
achieved not only a large variation in the contact angle but also a reversible droplet operation that minimized the electrolysis problem. In 1998, Washizu demonstrated the electrostatic actuation of unconfined liquid droplets by introducing arrays of patterned electrodes covered by a hydrophobic layer [21]. Early in 2000, Fair and Kim’s research groups achieved the two-dimensional (2D) manipulation of droplets confined with two parallel plates, which initiated the study of 2D digital microfluidics for biomedical applications [22-24]. As the popularity of bio-chip and lab-on-a-chip systems increases, new demands for improving chip density and analysis speed have arisen. Song et al. and Nelson et al. developed a scaling model for EWOD actuation and monolithic fabrication for manipulating picoliter droplets, respectively [25, 26]. And Welch et al. applied EWOD actuation to DNA sequencing chemistry [27]. Nevertheless, the extension of current 2D digital microfluidic platforms to a 3D space is seen as a necessity for further enhancement of the chip density and analysis speed [28]. To realize the 3D manipulation of biological and chemical droplets, vertical actuation is essential for transporting the droplets vertically from a bottom plate to the top and vice versa in EWOD chips. Thus far, few attempts at vertical actuation have been made. Takeda et al. demonstrated droplet jumping on a super-hydrophobic surface by using strong electric fields (~1 MVm-1) and potentials (~9 kV) [29]. Roux et al. also succeed in droplet jumping by electrostatic actuation and demonstrated 3D droplet manipulation by incorporating with EWOD actuation; however, the droplet manipulation was conducted in oil circumstances, which are not preferable in many biochemical applications [30, 31]. Later, Lee et al. demonstrated droplet jumping using a similar super-hydrophobic surface by EWOD actuation with relatively low potential (~100 V) [32]. However, to lift sessile droplets in air, the high energy barrier due to the interfacial and potential energy difference between the sessile and airborne states has to be overcome. In this study, we first present the complete 3D droplet manipulation through the combination of horizontal and vertical droplet actuation between two parallel plates, based on sole alternative current electrowetting-on-dielectric (AC-EWOD) actuation, as shown in Fig. 1. In contrast to previous works, we demonstrate vertical transportation of a droplet without jumping by electrically controlling the droplet shape and the gap between the top and bottom plates. This works in current EWOD system platforms without additional actuation methods and complex microfabrication processes for the super-hydrophobic surface. We also consider the advanced 3D EWOD platform consisting of the closed and open systems connected together
for future work. Hence, in the closed system, confined droplets could be split or created; whereas, in the open system, unconfined droplets could be horizontally and vertically transported for various fluidic operations. Note that a preliminary report on this work was presented at the International Conference on Solid-State Sensors, Actuators and Microsystems held in Barcelona, Spain [33].
II. THEORETICAL BACKGROUND
Electrowetting-on-dielectric (EWOD) is a prevalent method for manipulating the interfacial tension between two fluids using an external electric potential. When an electric potential is applied between a conductive droplet and an electrode covered by a hydrophobic dielectric layer, electrical current cannot flow, but electrical charges in the droplet accumulate around a triple contact line (TCL), since the dielectric layer behaves as a capacitor, as shown in Fig. 2(a). As a result, the interfacial tension is modified, which causes the apparent contact angle of the droplet to decrease, making the droplet spread out on the electrode. This phenomenon is called electrowetting-on-dielectric (EWOD) [4, 16]. Berge et al. first derived a Lippmann–Young equation based on the energy minimization method to explain the relationship between the modified contact angle and applied electric potential as follows [16, 17]:
0 dV 2 cos cose 2 td
(1)
where θ is the contact angle under the externally applied electric potential V, θe is the equilibrium contact angle at V = 0 V, ε 0 is the permittivity of vacuum, εd is the permittivity of the dielectric layer, γ is the interfacial tension between the droplet and the surrounding insulating fluid, and td is the thickness of the dielectric layer. Later, Jones and Kang also confirmed this equation’s validity using an electromechanical approach [34, 35]. The EWOD principle has been used in various applications such as lab-on-a-chip systems, electrical switches, liquid lenses, microprism arrays, electrowetting displays, and others [36-43].
III. FABRICATION OF TEST DEVICES AND EXPERIMENTAL SETUP
In order to test the 3D manipulation of a droplet, test devices, mainly consisting of two parallel plates (top and bottom), were microfabricated using standard lithography processes, as shown in Fig. 3. Both the top and bottom plates contained the same array of circular coplanar electrodes, as shown in Fig. 4(a). The main fabrication processes consisted of two steps: metallizing and patterning of electrodes, as in Fig. 3(a–f), and the depositing of the dielectric and hydrophobic layers, shown in Fig. 3(g, h), respectively. For the EWOD electrodes, a chrome layer of 300 Å as an adhesion layer and a gold layer of 1000 Å as a structure layer were sequentially deposited on a glass plate by sputtering and then patterned by wet etching. For the dielectric
layer, a polyimide (Durimide 7505, Fujifilm Co.) layer of 1.6 m was spin-coated over the entire surface of the plate. For the hydrophobic layer, a Teflon (AF Teflon® 1600, DupontTM Co.) layer of 1000 Å was also spincoated on top of the dielectric layer. The last step of the fabrication processes was to integrate the two plates, as shown in Fig. 3(i). After putting a water droplet injected by a syringe (600 Series MICROLITERTM Syringes model 65, Hamilton Co.) on the bottom plate, the top plate was gently pressed against the spacers on each corner of the bottom plate. A stack of 250 m thick double-sided tapes was used for the spacers. The gap between the top and bottom plates was adjusted by the number of the stacked tape layers.
For EWOD actuation, a sinusoidal wave voltage was generated by a function generator (33210A, Agilent Co.), and amplified up to 70 volts by a voltage amplifier (PZD700, Trek Co.). The amplified voltage signal was transmitted to the EWOD electrodes through photo-coupled relays (PhotoMos®, AQW614EH, Aromat Co.) controlled by a digital I/O board (DAQpad-6229 BNC, NI Co.) along with a programmed LabVIEW code. Note that the diagram of the signal flow and configuration of the photocoupled relay are described in Fig. 4. Images of the experiments were captured by a charge coupled device (CCD) camera (EO-1312C, Edmund Optics) as well as a high-speed camera (Phantom Miro eX2, Vision Research) integrated with a zoom lens (VZMTM 450i eo, Edmund Optics) and saved on a PC, as shown in Fig. 5. Note that every test was conducted within a minute to avoid droplet evaporation. For more details on the EWOD chip fabrication and actuation, refer to Chung et al. [44].
IV. RESULTS AND DISCUSSION
The horizontal actuation of a droplet is first demonstrated using an array of circular coplanar electrodes, as shown in Fig. 6(c). Note that the coplanar electrode has a pair of electrodes on the same plane for EWOD actuation without a cover plate [45]. A water droplet (3 l) is initially dispensed by a syringe on an electrode near the edge of the bottom plate, as in Fig. 6(a1). When the adjacent electrode of the droplet is activated at 1 kHz and 70 V, the droplet is immediately transported to the activated electrode in Fig. 6(a2), based on the EWOD principle. By shifting and repeating this procedure on each set of electrodes, using a series of photocoupled relays controlled by a digital I/O board and a custom-programmed LabVIEW code, the droplet is successfully transported to the right end of the electrode, as shown in Fig. 6(a). The same test for a droplet hanging on the top plate can also be successfully conducted using the array of circular coplanar electrodes shown in Fig. 6(b).
To achieve the vertical transportation of a droplet between two parallel plates, the dynamic behaviors, especially the droplet morphology, of a water droplet actuated by AC-EWOD are
investigated with high-speed images. When a water droplet (3 l) sitting on the circular coplanar electrode detailed in Fig. 6(c) is actuated by AC-EWOD from 20 Hz to 50 Hz at 2 Hz intervals for three different voltages (50, 60, and 70 V), the shape of the modified droplet can be observed with a high speed camera. Note that the used frame rate of the high speed camera was 1000 frame per second (fps). The maximum height of the deformed droplet at each frequency is measured and plotted in Fig. 7. The results show that the maximum deformation of the droplet depends on the applied frequency and is proportional to the applied voltage. The maximum height of the droplet occurs at its natural frequency (44 Hz, 70 V), and at the time the height is about 2.5 mm. For the measurement reliability the tests are repeated at least 5 times and the data in the graph are the averaged values with the errors (50 m). Note that the x-axis in the graph is not drawn to scale. The instantaneous height change of the deformed droplet actuated by AC-EWOD with respect to time is also investigated to determine the optimum switching time for the vertical transportation of the droplet between two parallel plates, as shown in Fig. 8. As the droplet is actuated by AC-EWOD at 44 Hz, it repeats stretching and releasing processes while its deformation increases. In addition, the height of the droplet reaches its maximum (about 2.5 mm) at 40 ms. Based on these results, actuation conditions such as applied frequency, switching time, and the gap between the top and bottom plates are determined for the vertical transportation of a droplet. Note that the height of the droplet is periodically changed every 40 ms. The vertical actuation of a droplet is also tested with two parallel plates containing circular coplanar electrodes for EWOD actuation. When an electric potential (44 Hz and 70 V) is applied to the bottom electrode beneath the droplet (3 l), it begins to oscillate in place, harmonically with the applied frequency in Fig. 9(a2). After 40 ms, the oscillating droplet reaches maximum deformation and contacts the top plate (2.5 mm above the bottom plate, as shown in Fig. 9(a3)). At this time, the electric potential on the bottom electrode is turned off and the top electrode is activated to modify its surface property from hydrophobic to hydrophilic, because the droplet prefers to stay on a hydrophilic surface. Note that without the electrical signal switching processes, the droplet often forms a bridge between the top and bottom plates, resulting in a failed vertical transportation. Transportation of the droplet from the top plate to the bottom plate is also demonstrated, as shown in Fig. 9(b). When the same electric potential is applied to the top electrode where the droplet sits, it simultaneously oscillates and reaches maximum deformation in 40 ms. The droplet is then
successfully transported from the top plate to the bottom plate by the same electrical signal switching processes, as shown in Fig. 9(b2, b3). Note that the droplet oscillation amplitude is proportional to the droplet radius and inversely proportional to the droplet viscosity [46, 47]. As the droplet size decreases the damping effect due to viscous dissipation and friction increases. Hence, the vertical actuation may become less effective for droplets with small volumes. As a proof of concept, the sequential 3D manipulation of a droplet between two parallel plates with a 2.5 mm gap is achieved by the combination of horizontal and vertical AC-EWOD actuation through switching the applied frequency in Fig. 10, providing more space for digital microfluidics. A droplet (3 l) injected by a syringe is initially placed on an electrode on one end of the bottom plate. To transport the droplet from the bottom plate to the top plate, an electric potential (44 Hz and 70 V) is applied to the circular coplanar electrode where the droplet sits. The droplet coincidentally oscillates and is transported to the top plate in 40 ms. To transport the droplet to the electrode on the opposite end of the top plate, an electrode adjacent to the droplet is activated at 1 kHz and 70 V. The droplet is immediately transported to the activated electrode in 60 ms, shown in Fig. 10(e). By shifting and repeating this procedure to the next electrode in line, the droplet is successfully transported to the top electrode furthest from the starting, bottom electrode, as shown in Fig. 10(d– g). For vertically transporting the droplet from the top plate to the bottom plate, the same electrical signal conditions and switching processes as before are applied, resulting that the droplet is successfully transported to the top plate in Fig. 10(g–j). Finally, the droplet returns to its original position by moving horizontally to the starting electrode. Note that a high frequency (1 kHz) is applied for small-droplet deformation in the horizontal actuation, whereas a low frequency (44 Hz) is applied for large-droplet deformation in vertical actuation. Although simultaneous manipulation of multiple droplets on the top and bottom plates may be limited by the gap, this method works in current EWOD system platforms without additional complex microfabrication processes for the super-hydrophobic surface.
V. CONCLUSION
This paper presents a technique in three-dimensional (3D) digital microfluidics using alternating current electrowetting-on-dielectric (AC-EWOD) actuation. The horizontal manipulation of a water droplet is demonstrated on a microfluidic chip consisting of an array of circular coplanar EWOD electrodes. To extend this work in 3D space, the dynamic behavior of a droplet actuated by AC-EWOD was investigated using high-speed imaging. The conditions for vertical actuation such as the applied frequency (44 Hz), switching time (40 ms), and the gap (2.5 mm) between the top and bottom plates are obtained. The vertical manipulation of a droplet is also achieved based on actuation conditions. Finally, the 3D manipulation of a droplet between two parallel plates (top and bottom) is experimentally demonstrated by the combination of horizontal and vertical ACEWOD actuation. This method works in current EWOD system platforms without additional microfabrication processes for the super-hydrophobic surface.
ACKNOWLEDGEMENT This research was partially supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0025039).
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Biographies of authors
Jeong Byung Chae received the Bachelor’s degree of mechanical engineering from Myongji University in 2012. He currently is a graduate student in Myongji University and his research interests lie on the development of microfluidic applications based on an electrowetting-on-dielectric principle.
Seung Jun Lee is a research staff member at the Samsung Advanced Institute of Technology (SAIT) in Korea. He received the B.S. degree in the department of mechanical engineering from Pohang University of Science and Technology (POSTECH), Pohang, Korea, in 2007 and the Ph.D. degree from the same department in 2013. He had worked as postdoctoral associate in the department of Printed Electronics from Korea Institute of Machinery & Materials (KIMM). Then, he joined a research staff member at SAIT in 2013. His research interests are in microfluidics and MEMS, including the design and fabrication of micro systems and the electrowetting-driven droplet manipulation.
Jinseung Yang is a professor of the department of mechanical engineering at the Myongji University in Korea. He received his Ph.D. degree in 1991 from the Georgia Institute of Technology, Atlanta, U.S.A. He received his B.S. and M.S. degrees in 1980 and 1983, respectively, from the Seoul National University. He had worked at Hyundai Motor from 1991 to 1994. Upon joining the faculty at Myongji University in 1996, he has directed the Myongji University Tribology Laboratory. His research is in tribology and MEMS.
Sang Kug Chung (M’09) is an associate professor of the department of mechanical engineering at the Myongji University in Korea. He received the Ph.D. degree in Mechanical Engineering and Materials Science from the University of Pittsburgh in 2009 along with the Graduate Research Excellence Award. He received the M.S. degree from Pohang University of Science and Technology (POSTECH) and B.S. from Myongji University. He had worked for the development of the world first Liquid Lens at Samsung Electro-Mechanics from 2003 to 2009. Upon joining the faculty at Myongji University in 2009, he has directed the Microsystems Laboratory. And he has also served as a principal investigator in the Advanced Microfluids Engineering Research Laboratory (AMERL) since 2013. His research is in microfluidics and MEMS, including design and fabrication of micro/nano actuators and systems.
FIGURES CAPTION
Figure 1 Experimental schematic for the 3D manipulation of a droplet between parallel plates consisting of patterned arrays of EWOD electrodes: (a) simulated 3D rendering; (b) 2D droplet motion images.
Figure 1 Experimental schematic for the 3D manipulation of a droplet between parallel plates consisting of patterned arrays of EWOD electrodes: (a) simulated 3D rendering; (b) 2D droplet motion images.
Figure 2 Schematic diagram of Electrowetting-on-dielectric(EWOD): (a) 1 kHz; (b) 44 Hz.
Figure 3 Schematic image of microfabrication. Figure 4 Schematic image of chip integration and experimental setup: (a) Chip integration and experimental setup, (b) Configuration of the photo-coupled relay, (c) The relay working condition.
Figure 4 Schematic image of chip integration and experimental setup: (a) Chip integration and experimental setup, (b) Configuration of the photo-coupled relay, (c) The relay working condition.
Figure 5 Experimental setup.
Figure 6 Sequential snapshots of the horizontal transportation of a droplet by AC-EWOD actuation: (a) EWOD actuation on the bottom plate; (b) EWOD actuation on the top plate; (c) Top view of the array of circular coplanar electrodes. Note that red dots and arrows in images indicate the droplet initial position and its moving distance, respectively.
Figure 7 Measurement of the height of a droplet actuated by AC-EWOD in different frequencies and voltages.
Figure 8 Sequential high speed images of the deformed shape of a droplet actuated by AC-EWOD at 70 V and 44 Hz with respect to time. Note that it takes 40 ms to reach the maximum droplet deformation.
Figure 9 Sequential snapshots of vertical actuation of a droplet in parallel plates using AC-EWOD actuation.
Figure 10 3D manipulation of a droplet in parallel plates using AC-EWOD actuation: (a–d, g–j) Vertical transportation of the droplet actuated by AC-EWOD at 44 Hz; (d–g, j–a) Horizontal transportation of the droplet actuated by AC-EWOD at 1 kHz. Note that it takes 40 ms for the vertical transportation and 60 ms for the horizontal transportation in each step.
S0924-4247(15)30121-7 http://dx.doi.org/doi:10.1016/j.sna.2015.09.004 SNA 9299
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
15-10-2014 31-7-2015 3-9-2015
Please cite this article as: Jeong Byung Chae, Seung Jun Lee, Jinseung Yang, Sang Kug Chung, 3D Electrowetting-on-dielectric Actuation, Sensors and Actuators: A Physical http://dx.doi.org/10.1016/j.sna.2015.09.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
3D Electrowetting-on-dielectric Actuation Jeong Byung Chae†,1, Seung Jun Lee†,2, Jinseung Yang1 and Sang Kug Chung1,* 1
Department of Mechanical Engineering, Myongji University, Yongin 449-728, South Korea
2
SAITSamsung Advanced Institute of Technology, Yongin, Gyeonggido, South Korea
*
Corresponding author: [email protected]
†
These authors equally contributed to this work.
Highlights 3D digital microfluidics based on electrowetting-on-dielectric (EWOD) actuation is proposed and experimentally verified. The dynamic behavior of a droplet under various EWOD conditions such as different applied frequencies and switching times is studied using high-speed imaging The first complete 3D droplet manipulation through the combination of horizontal and vertical actuation between two parallel plates is experimentally achieved by sole EWOD actuation.
Abstract
This paper describes three-dimensional (3D) digital microfluidics based on an alternating current electrowetting-on-dielectric (AC-EWOD) principle. To achieve 3D manipulation of a droplet between two parallel plates, the horizontal and vertical actuation of a droplet is investigated separately. The dynamic behavior of a water droplet actuated by AC-EWOD in various conditions is studied using high-speed imaging. Optimum actuation parameters such as applied frequency, switching time, and the gap between the top and bottom plates are obtained. For horizontal actuation, a high frequency (1 kHz) is used for small-droplet deformation; on the other hand, for vertical actuation, a low frequency (44 Hz) is used for large-droplet deformation. Finally, as a proof of concept, 3D manipulation of a droplet with two parallel plates is demonstrated by the combination of horizontal and vertical AC-EWOD actuation, resulting in increased space for digital microfluidics.
Keywords: Microfluidics, Lab-on-a-chip, Alternating Current Electrowetting-on-Dielectric (AC-EWOD)
Email: [email protected]
I. INTRODUCTION
The development of reliable bio-chip and lab-on-a-chip systems is highly anticipated for biomedical and genomics applications such as cell manipulation and polymerase chain reaction (PCR) [1-4]. Bio-chip and labon-a-chip systems consist of microfluidic components such as extremely small channels, pumps, and sensors. These systems allow us to incorporate many biochemical laboratory functions on a single chip [5-8]. In the development of these systems, microfluidic technology is essential in the handling of small volumes of fluids (less than a picoliter).
As the size of a fluidic system decreases, the surface area to volume ratio is linearly increases. It makes the effects of viscosity on the fluidic system dominate. From a fluid mechanics perspective, the Reynolds number, the ratio of inertia forces to viscous forces, is commonly used to characterize flows and to verify governing forces in a fluidic system [8-10]. In a microfluidic system, the effect of the viscous forces is dominant, because of the small Reynolds number, which is inversely proportional to the size of the fluidic system. Therefore, the fluid driving techniques often used in macrofluidic systems may perform inadequately for pumping fluids in microfluidic systems, because of the high fluidic resistance generated by the viscous force. As alternatives, a variety of microfluidic technologies based on capillary forces and electrokinetic forces such as electroosmosis, electrophoresis, and dielectrophoresis have been developed [11-15]. Electrowetting-on-dielectric (EWOD) is one of these microfluidic techniques based on discrete liquids (most commonly droplets) without channel networks [16, 17]. This is fundamentally different from other existing techniques relying on complicated microchannels. EWOD has been established as one of the most efficient and feasible microfluidic technologies, because of its outstanding advantages such as fast response time, low power consumption, and robust operation [8, 18]. There have been great strides in the development of modern EWOD technology. About a hundred years ago, Gabriel Lippmann first studied the electrocapillarity, the basis of the present EWOD principle, in which the interfacial tension of mercury in contact with electrolyte solutions could be varied by applying an electric potential between the mercury and the solution [19]. He not only established a theory of the electrocapillarity, but also developed some applications such as a sensitive electrometer and a motor [16, 17]. However, electrolysis induced by the electric current flow through electrolyte solutions restricted broader applications. A century later, Berge et al. conducted a sessile droplet experiment using an electrode covered with a thin hydrophobic dielectric layer [20]. He
achieved not only a large variation in the contact angle but also a reversible droplet operation that minimized the electrolysis problem. In 1998, Washizu demonstrated the electrostatic actuation of unconfined liquid droplets by introducing arrays of patterned electrodes covered by a hydrophobic layer [21]. Early in 2000, Fair and Kim’s research groups achieved the two-dimensional (2D) manipulation of droplets confined with two parallel plates, which initiated the study of 2D digital microfluidics for biomedical applications [22-24]. As the popularity of bio-chip and lab-on-a-chip systems increases, new demands for improving chip density and analysis speed have arisen. Song et al. and Nelson et al. developed a scaling model for EWOD actuation and monolithic fabrication for manipulating picoliter droplets, respectively [25, 26]. And Welch et al. applied EWOD actuation to DNA sequencing chemistry [27]. Nevertheless, the extension of current 2D digital microfluidic platforms to a 3D space is seen as a necessity for further enhancement of the chip density and analysis speed [28]. To realize the 3D manipulation of biological and chemical droplets, vertical actuation is essential for transporting the droplets vertically from a bottom plate to the top and vice versa in EWOD chips. Thus far, few attempts at vertical actuation have been made. Takeda et al. demonstrated droplet jumping on a super-hydrophobic surface by using strong electric fields (~1 MVm-1) and potentials (~9 kV) [29]. Roux et al. also succeed in droplet jumping by electrostatic actuation and demonstrated 3D droplet manipulation by incorporating with EWOD actuation; however, the droplet manipulation was conducted in oil circumstances, which are not preferable in many biochemical applications [30, 31]. Later, Lee et al. demonstrated droplet jumping using a similar super-hydrophobic surface by EWOD actuation with relatively low potential (~100 V) [32]. However, to lift sessile droplets in air, the high energy barrier due to the interfacial and potential energy difference between the sessile and airborne states has to be overcome. In this study, we first present the complete 3D droplet manipulation through the combination of horizontal and vertical droplet actuation between two parallel plates, based on sole alternative current electrowetting-on-dielectric (AC-EWOD) actuation, as shown in Fig. 1. In contrast to previous works, we demonstrate vertical transportation of a droplet without jumping by electrically controlling the droplet shape and the gap between the top and bottom plates. This works in current EWOD system platforms without additional actuation methods and complex microfabrication processes for the super-hydrophobic surface. We also consider the advanced 3D EWOD platform consisting of the closed and open systems connected together
for future work. Hence, in the closed system, confined droplets could be split or created; whereas, in the open system, unconfined droplets could be horizontally and vertically transported for various fluidic operations. Note that a preliminary report on this work was presented at the International Conference on Solid-State Sensors, Actuators and Microsystems held in Barcelona, Spain [33].
II. THEORETICAL BACKGROUND
Electrowetting-on-dielectric (EWOD) is a prevalent method for manipulating the interfacial tension between two fluids using an external electric potential. When an electric potential is applied between a conductive droplet and an electrode covered by a hydrophobic dielectric layer, electrical current cannot flow, but electrical charges in the droplet accumulate around a triple contact line (TCL), since the dielectric layer behaves as a capacitor, as shown in Fig. 2(a). As a result, the interfacial tension is modified, which causes the apparent contact angle of the droplet to decrease, making the droplet spread out on the electrode. This phenomenon is called electrowetting-on-dielectric (EWOD) [4, 16]. Berge et al. first derived a Lippmann–Young equation based on the energy minimization method to explain the relationship between the modified contact angle and applied electric potential as follows [16, 17]:
0 dV 2 cos cose 2 td
(1)
where θ is the contact angle under the externally applied electric potential V, θe is the equilibrium contact angle at V = 0 V, ε 0 is the permittivity of vacuum, εd is the permittivity of the dielectric layer, γ is the interfacial tension between the droplet and the surrounding insulating fluid, and td is the thickness of the dielectric layer. Later, Jones and Kang also confirmed this equation’s validity using an electromechanical approach [34, 35]. The EWOD principle has been used in various applications such as lab-on-a-chip systems, electrical switches, liquid lenses, microprism arrays, electrowetting displays, and others [36-43].
III. FABRICATION OF TEST DEVICES AND EXPERIMENTAL SETUP
In order to test the 3D manipulation of a droplet, test devices, mainly consisting of two parallel plates (top and bottom), were microfabricated using standard lithography processes, as shown in Fig. 3. Both the top and bottom plates contained the same array of circular coplanar electrodes, as shown in Fig. 4(a). The main fabrication processes consisted of two steps: metallizing and patterning of electrodes, as in Fig. 3(a–f), and the depositing of the dielectric and hydrophobic layers, shown in Fig. 3(g, h), respectively. For the EWOD electrodes, a chrome layer of 300 Å as an adhesion layer and a gold layer of 1000 Å as a structure layer were sequentially deposited on a glass plate by sputtering and then patterned by wet etching. For the dielectric
layer, a polyimide (Durimide 7505, Fujifilm Co.) layer of 1.6 m was spin-coated over the entire surface of the plate. For the hydrophobic layer, a Teflon (AF Teflon® 1600, DupontTM Co.) layer of 1000 Å was also spincoated on top of the dielectric layer. The last step of the fabrication processes was to integrate the two plates, as shown in Fig. 3(i). After putting a water droplet injected by a syringe (600 Series MICROLITERTM Syringes model 65, Hamilton Co.) on the bottom plate, the top plate was gently pressed against the spacers on each corner of the bottom plate. A stack of 250 m thick double-sided tapes was used for the spacers. The gap between the top and bottom plates was adjusted by the number of the stacked tape layers.
For EWOD actuation, a sinusoidal wave voltage was generated by a function generator (33210A, Agilent Co.), and amplified up to 70 volts by a voltage amplifier (PZD700, Trek Co.). The amplified voltage signal was transmitted to the EWOD electrodes through photo-coupled relays (PhotoMos®, AQW614EH, Aromat Co.) controlled by a digital I/O board (DAQpad-6229 BNC, NI Co.) along with a programmed LabVIEW code. Note that the diagram of the signal flow and configuration of the photocoupled relay are described in Fig. 4. Images of the experiments were captured by a charge coupled device (CCD) camera (EO-1312C, Edmund Optics) as well as a high-speed camera (Phantom Miro eX2, Vision Research) integrated with a zoom lens (VZMTM 450i eo, Edmund Optics) and saved on a PC, as shown in Fig. 5. Note that every test was conducted within a minute to avoid droplet evaporation. For more details on the EWOD chip fabrication and actuation, refer to Chung et al. [44].
IV. RESULTS AND DISCUSSION
The horizontal actuation of a droplet is first demonstrated using an array of circular coplanar electrodes, as shown in Fig. 6(c). Note that the coplanar electrode has a pair of electrodes on the same plane for EWOD actuation without a cover plate [45]. A water droplet (3 l) is initially dispensed by a syringe on an electrode near the edge of the bottom plate, as in Fig. 6(a1). When the adjacent electrode of the droplet is activated at 1 kHz and 70 V, the droplet is immediately transported to the activated electrode in Fig. 6(a2), based on the EWOD principle. By shifting and repeating this procedure on each set of electrodes, using a series of photocoupled relays controlled by a digital I/O board and a custom-programmed LabVIEW code, the droplet is successfully transported to the right end of the electrode, as shown in Fig. 6(a). The same test for a droplet hanging on the top plate can also be successfully conducted using the array of circular coplanar electrodes shown in Fig. 6(b).
To achieve the vertical transportation of a droplet between two parallel plates, the dynamic behaviors, especially the droplet morphology, of a water droplet actuated by AC-EWOD are
investigated with high-speed images. When a water droplet (3 l) sitting on the circular coplanar electrode detailed in Fig. 6(c) is actuated by AC-EWOD from 20 Hz to 50 Hz at 2 Hz intervals for three different voltages (50, 60, and 70 V), the shape of the modified droplet can be observed with a high speed camera. Note that the used frame rate of the high speed camera was 1000 frame per second (fps). The maximum height of the deformed droplet at each frequency is measured and plotted in Fig. 7. The results show that the maximum deformation of the droplet depends on the applied frequency and is proportional to the applied voltage. The maximum height of the droplet occurs at its natural frequency (44 Hz, 70 V), and at the time the height is about 2.5 mm. For the measurement reliability the tests are repeated at least 5 times and the data in the graph are the averaged values with the errors (50 m). Note that the x-axis in the graph is not drawn to scale. The instantaneous height change of the deformed droplet actuated by AC-EWOD with respect to time is also investigated to determine the optimum switching time for the vertical transportation of the droplet between two parallel plates, as shown in Fig. 8. As the droplet is actuated by AC-EWOD at 44 Hz, it repeats stretching and releasing processes while its deformation increases. In addition, the height of the droplet reaches its maximum (about 2.5 mm) at 40 ms. Based on these results, actuation conditions such as applied frequency, switching time, and the gap between the top and bottom plates are determined for the vertical transportation of a droplet. Note that the height of the droplet is periodically changed every 40 ms. The vertical actuation of a droplet is also tested with two parallel plates containing circular coplanar electrodes for EWOD actuation. When an electric potential (44 Hz and 70 V) is applied to the bottom electrode beneath the droplet (3 l), it begins to oscillate in place, harmonically with the applied frequency in Fig. 9(a2). After 40 ms, the oscillating droplet reaches maximum deformation and contacts the top plate (2.5 mm above the bottom plate, as shown in Fig. 9(a3)). At this time, the electric potential on the bottom electrode is turned off and the top electrode is activated to modify its surface property from hydrophobic to hydrophilic, because the droplet prefers to stay on a hydrophilic surface. Note that without the electrical signal switching processes, the droplet often forms a bridge between the top and bottom plates, resulting in a failed vertical transportation. Transportation of the droplet from the top plate to the bottom plate is also demonstrated, as shown in Fig. 9(b). When the same electric potential is applied to the top electrode where the droplet sits, it simultaneously oscillates and reaches maximum deformation in 40 ms. The droplet is then
successfully transported from the top plate to the bottom plate by the same electrical signal switching processes, as shown in Fig. 9(b2, b3). Note that the droplet oscillation amplitude is proportional to the droplet radius and inversely proportional to the droplet viscosity [46, 47]. As the droplet size decreases the damping effect due to viscous dissipation and friction increases. Hence, the vertical actuation may become less effective for droplets with small volumes. As a proof of concept, the sequential 3D manipulation of a droplet between two parallel plates with a 2.5 mm gap is achieved by the combination of horizontal and vertical AC-EWOD actuation through switching the applied frequency in Fig. 10, providing more space for digital microfluidics. A droplet (3 l) injected by a syringe is initially placed on an electrode on one end of the bottom plate. To transport the droplet from the bottom plate to the top plate, an electric potential (44 Hz and 70 V) is applied to the circular coplanar electrode where the droplet sits. The droplet coincidentally oscillates and is transported to the top plate in 40 ms. To transport the droplet to the electrode on the opposite end of the top plate, an electrode adjacent to the droplet is activated at 1 kHz and 70 V. The droplet is immediately transported to the activated electrode in 60 ms, shown in Fig. 10(e). By shifting and repeating this procedure to the next electrode in line, the droplet is successfully transported to the top electrode furthest from the starting, bottom electrode, as shown in Fig. 10(d– g). For vertically transporting the droplet from the top plate to the bottom plate, the same electrical signal conditions and switching processes as before are applied, resulting that the droplet is successfully transported to the top plate in Fig. 10(g–j). Finally, the droplet returns to its original position by moving horizontally to the starting electrode. Note that a high frequency (1 kHz) is applied for small-droplet deformation in the horizontal actuation, whereas a low frequency (44 Hz) is applied for large-droplet deformation in vertical actuation. Although simultaneous manipulation of multiple droplets on the top and bottom plates may be limited by the gap, this method works in current EWOD system platforms without additional complex microfabrication processes for the super-hydrophobic surface.
V. CONCLUSION
This paper presents a technique in three-dimensional (3D) digital microfluidics using alternating current electrowetting-on-dielectric (AC-EWOD) actuation. The horizontal manipulation of a water droplet is demonstrated on a microfluidic chip consisting of an array of circular coplanar EWOD electrodes. To extend this work in 3D space, the dynamic behavior of a droplet actuated by AC-EWOD was investigated using high-speed imaging. The conditions for vertical actuation such as the applied frequency (44 Hz), switching time (40 ms), and the gap (2.5 mm) between the top and bottom plates are obtained. The vertical manipulation of a droplet is also achieved based on actuation conditions. Finally, the 3D manipulation of a droplet between two parallel plates (top and bottom) is experimentally demonstrated by the combination of horizontal and vertical ACEWOD actuation. This method works in current EWOD system platforms without additional microfabrication processes for the super-hydrophobic surface.
ACKNOWLEDGEMENT This research was partially supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0025039).
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Biographies of authors
Jeong Byung Chae received the Bachelor’s degree of mechanical engineering from Myongji University in 2012. He currently is a graduate student in Myongji University and his research interests lie on the development of microfluidic applications based on an electrowetting-on-dielectric principle.
Seung Jun Lee is a research staff member at the Samsung Advanced Institute of Technology (SAIT) in Korea. He received the B.S. degree in the department of mechanical engineering from Pohang University of Science and Technology (POSTECH), Pohang, Korea, in 2007 and the Ph.D. degree from the same department in 2013. He had worked as postdoctoral associate in the department of Printed Electronics from Korea Institute of Machinery & Materials (KIMM). Then, he joined a research staff member at SAIT in 2013. His research interests are in microfluidics and MEMS, including the design and fabrication of micro systems and the electrowetting-driven droplet manipulation.
Jinseung Yang is a professor of the department of mechanical engineering at the Myongji University in Korea. He received his Ph.D. degree in 1991 from the Georgia Institute of Technology, Atlanta, U.S.A. He received his B.S. and M.S. degrees in 1980 and 1983, respectively, from the Seoul National University. He had worked at Hyundai Motor from 1991 to 1994. Upon joining the faculty at Myongji University in 1996, he has directed the Myongji University Tribology Laboratory. His research is in tribology and MEMS.
Sang Kug Chung (M’09) is an associate professor of the department of mechanical engineering at the Myongji University in Korea. He received the Ph.D. degree in Mechanical Engineering and Materials Science from the University of Pittsburgh in 2009 along with the Graduate Research Excellence Award. He received the M.S. degree from Pohang University of Science and Technology (POSTECH) and B.S. from Myongji University. He had worked for the development of the world first Liquid Lens at Samsung Electro-Mechanics from 2003 to 2009. Upon joining the faculty at Myongji University in 2009, he has directed the Microsystems Laboratory. And he has also served as a principal investigator in the Advanced Microfluids Engineering Research Laboratory (AMERL) since 2013. His research is in microfluidics and MEMS, including design and fabrication of micro/nano actuators and systems.
FIGURES CAPTION
Figure 1 Experimental schematic for the 3D manipulation of a droplet between parallel plates consisting of patterned arrays of EWOD electrodes: (a) simulated 3D rendering; (b) 2D droplet motion images.
Figure 1 Experimental schematic for the 3D manipulation of a droplet between parallel plates consisting of patterned arrays of EWOD electrodes: (a) simulated 3D rendering; (b) 2D droplet motion images.
Figure 2 Schematic diagram of Electrowetting-on-dielectric(EWOD): (a) 1 kHz; (b) 44 Hz.
Figure 3 Schematic image of microfabrication. Figure 4 Schematic image of chip integration and experimental setup: (a) Chip integration and experimental setup, (b) Configuration of the photo-coupled relay, (c) The relay working condition.
Figure 4 Schematic image of chip integration and experimental setup: (a) Chip integration and experimental setup, (b) Configuration of the photo-coupled relay, (c) The relay working condition.
Figure 5 Experimental setup.
Figure 6 Sequential snapshots of the horizontal transportation of a droplet by AC-EWOD actuation: (a) EWOD actuation on the bottom plate; (b) EWOD actuation on the top plate; (c) Top view of the array of circular coplanar electrodes. Note that red dots and arrows in images indicate the droplet initial position and its moving distance, respectively.
Figure 7 Measurement of the height of a droplet actuated by AC-EWOD in different frequencies and voltages.
Figure 8 Sequential high speed images of the deformed shape of a droplet actuated by AC-EWOD at 70 V and 44 Hz with respect to time. Note that it takes 40 ms to reach the maximum droplet deformation.
Figure 9 Sequential snapshots of vertical actuation of a droplet in parallel plates using AC-EWOD actuation.
Figure 10 3D manipulation of a droplet in parallel plates using AC-EWOD actuation: (a–d, g–j) Vertical transportation of the droplet actuated by AC-EWOD at 44 Hz; (d–g, j–a) Horizontal transportation of the droplet actuated by AC-EWOD at 1 kHz. Note that it takes 40 ms for the vertical transportation and 60 ms for the horizontal transportation in each step.