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Optofluidic devices and their applications
Published in Guangya Zhou, Chengkuo Lee, Optical MEMS, Nanophotonics, and Their Applications, 2017
Figure 14.7 presents the working principle of electrowetting. A liquid droplet sits on a solid electrode plate coated with a hydrophobic and dielectric layer, forming its initial contact angle θ0 in the absence of a bias voltage application (Figure 14.7a). When an electric potential is applied between a liquid and a solid electrode (Figure 14.7b), the charge redistribution modifies the surface tension from γ0_SL to γSL at the solid-liquid interface where the like charges repulsion decreases the work by expanding the surface area. The resulting contact angle (θ) of a liquid droplet can be mathematically estimated with the applied electric potential (V) by using the popular Young–Lippmann equation [91–93] cosθ=cosθ0+12γLGε0εrtV2 where γLG is the surface tension between two immiscible fluids, ε0 is the permittivity of free space, εr is the dielectric constant, and t is the thickness of the capacitor formed between a liquid and a solid electrode. Droplet actuation in electrowetting devices is typically accomplished by electric activation on pixelated electrodes, while optoelectrowetting (OEW) utilizes optical activation either on patterned electrodes or a featureless photoconductive thin film to induce the electrowetting effect. As a result, OEW technology fully eliminates the fabrication issues of complex wiring and interconnection when a large number of electrodes or droplets need to be controlled in parallel without interference.
Low-cost digital microfluidic approach on thin and pliable polymer films
Published in Instrumentation Science & Technology, 2022
Dongping Chai, Jiaxi Jiang, Yiqiang Fan
Droplets on the digital microfluidic chips may be driven with several physical approaches, including electrowetting on the dielectric layer (EWOD),[10] dielectrophoresis (DEP),[11] optoelectrowetting forces,[12] magnetic fields,[13] thermocapillary effect,[14] and surface acoustic waves.[15] Among these methods, electrowetting is the most widely used for droplet manipulation in various applications, with advantages of rapid switching response, low power consumption, and the ability to drive droplets in micro- or even millimeter-scales.[16] Thus, the electrowetting approach was employed in this study to drive the droplets on a tilted or curved thin film.