Electrowetting-induced droplet detachment from hydrophobic surfaces.

Detachment of droplets from solid surfaces is a basic and crucial process in practical applications such as heat transfer and digital microfluidics. In this study, electrowetting actuations with square pulse signals are employed to detach droplets from a hydrophobic surface. The threshold voltage for droplet detachment is obtained both experimentally and theoretically to find that it is almost constant for various droplet volumes ranging from 0.4 to 10 μL. It is also found that droplets can be detached more easily when the width of applied pulse is well-matched to the spreading time (i.e., the time to reach the maximum spread diameter). When the droplet is actuated by a double square pulse, the threshold voltage is reduced by ∼20% from that for a single square pulse actuation. Finally, by introducing an interdigitated electrode system, it is demonstrated that droplets can be detached from the solid bottom surface without using a top needle electrode.