Geometric characterization of optimal electrode designs for improved droplet charging and actuation.

Droplet charging characteristics depending on the geometry of charging electrodes have been investigated experimentally and numerically. In the experiments, two contrasting electrode systems are examined: pin-pin versus planar-planar types. To confirm the different charging behaviours on each electrode, an asymmetric system of a pin-planar type has also been examined. From the experimental and numerical results, it has been found that the droplet charge can be significantly increased (more than four times) with pin type electrodes compared with planar ones due to the increase in surface charge density by the intensification of the electric field around the charging electrode. Moreover, as the system scale becomes smaller, the superior charging effect becomes greater. Through comprehensive numerical studies on the effects of the cross-sectional area and length of a charging electrode, we have found the optimal geometric design of an electrode for droplet charging and actuation. The implications for basic understanding of the charging phenomenon and electrode design of microfluidic systems are discussed.