Interdisciplinary Applied Mathematics

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8.6.2 Optoelectrowetting

Implementation of electrowetting for multichannel liquid networks requires a large number of electrodes, often leading to a wiring bottleneck in twodimensional arrays. An alternative but conceptually similar approach is optoelectrowetting (OEW), which was first studied in (Ichimura et al., 2000; Chiou et    al.,    2003).    The    basic mechanism is    shown    in    the    sketch    of Figure

8.12, where a photoconductive material is placed under the dielectric layer of a standard electrowetting setup. The contact angle can still be computed by the Young-Lippmann equation, i.e.,

eV 2

cos в = cos во H—,


where d is the    thickness    of the    dielectric    layer.    In the    dark    state,    i.e.,    no

light source, the frequency of the AC curent is controlled so that the photoconductor dominates, and thus through a voltage divider the voltage drop will occur across the photoconducting layer. When a light source is present, the conductivity of photoconductor increases by orders of magnitude, and consequently, the voltage drop is mainly in the dielectric layer. The material used in the experiments of (Chiou et al., 2003) was amorphous silicon because of its low dark conductivity and visible light response. Its conductivity increased by almost two orders of magnitude with light intensity of 65 mW/cm2.

The liquid droplet in the demonstration experiments of (Chiou et al., 2003) was deionized water, which was placed between a top hydrophobic surface (indium tin oxide coated with Teflon), while the bottom surface of amorphous silicon was also coated with Teflon. An electrode grid was placed under the photoconducting layer. By applying light on one end of the microdroplet, the contact angle decreases and motion is induced. Velocities up

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