Electrowetting dynamics facilitated by pulsing

Electrovariable optical components such as variable-focus lenses, microfluidic systems, and electronic displays exploit the electrowetting effect, where the shape of a liquid/fluid interface changes in response to an applied electric field. Low-voltage electrowetting devices cannot make use of an insulating polymeric film coating and are susceptible to contact angle hysteresis. When bare solid electrodes are used, pinning forces originate from surface roughness and heterogeneity, making hysteresis in these devices practically unavoidable. Recent experimental studies of an electrowetting system based on an interface between two immiscible electrolytic solutions (ITIES) have demonstrated that hysteresis can be eliminated by applying short voltage pulses in addition to the steady-state bias. The applied microsecond pulses activate the droplet, facilitating the depinning of the contact line and its motion. Here, a theoretical model describing the effect of electric pulses on droplet dynamics is presented. Key factors in determining the electrowetting response under pulsing are the driving force due to the applied voltage, the static friction force resulting from energetic and morphological nonuniformities of the surface, the viscous-like energy dissipation, and the inertia of the moving liquid. The significance of the present description of hysteresis-free, pulse-assisted electrowetting is not confined to the development of opto-fluidic devices using this technique. In a wider context, it provides a new framework for the theoretical analysis of wetting dynamics, pinning, and friction in systems with liquid droplets on nonideally smooth, solid substrates.