Enhanced microcooling by electrically induced droplet oscillation

Abstract

The rapid oscillation of a micro-litre sized droplet on application of a periodic DC electric field in an EWOD (electrowetting on dielectric) configuration, consisting of an ITO (Indium Tin Oxide – In₂O₃/SnO₂) coated glass substrate coated with polydimethylsiloxane (PDMS) and Teflon, has been studied. The physics of the wetting and dewetting phenomena during the oscillation process is analyzed extensively as a function of the time dependent applied voltage. DC pulse functions with voltages ranging from 150 V to 300 V and delay time from 25 ms to 200 ms are imposed on the droplet and the subsequent amplitude, frequency of oscillation, contact angle change and hysteresis are measured. It has been demonstrated that the oscillation and the ensuing mixing inside the droplet result in the augmentation of the transport process. The enhancement of the cooling rate of a hot spot on a substrate due to droplet oscillation and its variation with droplet volume and delay time are evaluated experimentally. The increase is a result of oscillation induced internal circulation inside the evaporating droplet and can be used as a tool for specific micro-cooling applications; e.g., cooling of a hot-spot.