Dielectrowetting for Digital Microfluidics: Principle and Application. A Critical Review
This review describes a relatively new wettability control mechanism, dielectrowetting, which originates from liquid dielectrophoresis (L-DEP). An L-DEP force generated by a non-homogeneous fringe electric field can be highly localized at the three-phase contact line and thus can enhance the wetting of dielectric fluids, or even cause superspreading (the contact angle can be lowered down to nearly zero degrees). This phenomenon, referred to as "dielectrowetting," is apparently similar to electrowetting or electrowetting on dielectric (EWOD) but its physical mechanism is different. In this review, to enhance the understanding of similarity, difference, and relation of dielectrowetting in comparison to EWOD and L-DEP, EWOD and L-DEP are first discussed briefly. The discussion includes the evolution from L-DEP and EWOD to dielectrowetting and applications of EWOD and L-DEP with parallel-plate configurations. L-DEP is based on the forces exerted on charge dipoles (induced or permanently built in dielectric liquid) under a non-homogeneous fringe electric field, while EWOD relies on the electrostatic forces acting on free charges (ions) in conductive liquids. Both L-DEP and EWOD have been widely used as an efficient tool to control dielectric or conductive microfluids. The conventional configuration for droplet manipulation is with two parallel plates, between which microfluid droplets are sandwiched and an electric potential is applied. This structure has been quite successful to create, move, divide, and merge microliter scale droplets by EWOD as well as by L-DEP for digital microfluidics. In EWOD, the change in the contact angle of the droplet is the important phenomenon to understand the driving mechanism, while in L-DEP the contact angle change of dielectric fluid is not the main interest since the dielectric fluid seems to be driven in bulk by the dielectrophoretic body force. Dielectrowetting uses a co-planar electrode design with interdigitated multiple fingers, which localizes and focuses the dielectrophoretic force at the three-phase contact line and thus changes the contact angle of fluids. As a result, dielectrowetting has both common features with and distinctions from EWOD and L-DEP. This principle induces many interesting phenomena, including nearly-complete wetting (superspreading) of dielectric droplets and large contact angle change even in conductive fluids. Dielectrowetting not only avoids using a parallel-plate channel but also significantly enhances the wetting in an open environment. In this review, the theory and principle of dielectrowetting is detailed followed by the recent progress and application of dielectrowetting in digital microfluidics. Finally, this review concludes with a summary and outlook. The unique mechanism and capability of dielectrowetting are expected to promote more applications beyond microfluidics.
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