Article pubs.acs.org/ac
Digital Electrophoresis of Charged Droplets Do Jin Im,*,† Byeong Sun Yoo,† Myung Mo Ahn, Dustin Moon, and In Seok Kang* Department of Chemical Engineering, Pohang University of Science and Technology, San31 Hyoja-dong, Nam-Gu, Pohang, Gyeongbuk, 790-784, South Korea S Supporting Information *
ABSTRACT: A digital microfluidic system based on a direct electric charging and subsequent electrophoretic manipulation of droplets is made by simple fabrication at low cost. Digitally controlled twodimensional droplet motions are realized by digital polarity control of an array of electrodes. By independent control of droplets and colorimetric detection, the coalescence and mixing of droplets is analyzed quantitatively. The gelation of sodium alginate and the crystallization of calcium carbonate by multiple droplet translations and coalescence and the actuation of glassy carbon beads are demonstrated to show the versatile manipulation capability of the proposed technology. Finally, we discuss the implications and potentials of the present technology.
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studies on controlling an individual droplet without a microchannel and carrier fluid showed some potential for the ECD as a digital microfluidic actuation method.6,24 However, in the previous work, the system size was too large and the actuation voltage was too high (few kilovolts), which limits the applicability of the technology.25 For that reason, there is a preconception that a high actuation voltage on the order of kilovolts is required for ECD actuation,11 which is not the case for smaller systems.7 In this work, a miniaturized digital microfluidic system based on ECD is demonstrated under moderate actuation voltages (few hundred volts). Using an array of electrodes made of commercially available pin header sockets, a much smaller microfluidic system is made by a simple fabrication process at low cost. Digitally controlled one- and two-dimensional translations, coalescence, and mixing of droplets are tested and analyzed experimentally and numerically. The gelation of sodium alginate and the crystallization of calcium carbonate are demonstrated by multiple droplet translations and coalescence to show the versatile manipulation capability of the ECD technology. Finally, we discuss the implications of the current achievements.
roplet microfluidics, which handles droplets as individual microreactors, has emerged as a versatile tool for various applications in chemistry and biology.1 Droplets can be generated and manipulated in a microchannel2 or discretely controlled on an open surface using an electric field. Particularly, digital microfluidics (DMF) capable of manipulating individual droplets on an array of electrodes by applying software-controlled electric potentials has been viewed as a distinct paradigm that offers unique benefits to this field.3 Consequently, an increasing number of DMF applications in broader areas have been reported, mostly based on the electrowetting-on-dielectric (EWOD) principle.3 On the one hand, the direct charging and subsequent electrophoretic control of a droplet has been studied by a number of researchers as a novel droplet manipulation method.4−17 The electrophoresis of a charged droplet (ECD) has some advantages as a droplet actuation method: a simple and straightforward principle, minimum contact with solid surfaces, easy coalescence, and fast movement.10,13 The biocompatibility of ECD has been verified not only for mammalian cells18 but also for human cells.15 One distinguishable feature of ECD compared with conventional methods is the existence of a considerable amount of charge on a droplet. Because of the charge, the electrophoretic actuation can be stronger and more consistent, which enables the actuation of a droplet containing a solid precipitate.10 This can be helpful for the on-demand synthesis of biologically useful hydrogels (alginate and collagen)19 and for the formation of a crystalline precipitate.20 Moreover, the charge on a droplet can also be used for detecting droplet’s position.21 Most research on direct charging and manipulation of droplets has focused on the coalescence,4,22 sorting,9,23 trapping,8 and fission4 of droplets in a microchannel rather than on the individual actuation without a carrier fluid. Recent © 2013 American Chemical Society
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EXPERIMENTAL SECTION Figure 1a shows the microfluidic system for the charging and electrophoretic actuation of droplets. The array of electrodes is simply made by assembling two (5 × 4) or three (5 × 6) pin header sockets (2 × 5 pins with 2.54 mm pitch,
