Curvature effect of electrowetting-induced droplet detachment

Abstract

Harnessing detachment of an aqueous droplet via electrowetting on a flat surface has been of considerable interest for potential practical applications, ranging from self-cleaning to novel optical and digital microfluidic devices, due to the wettability of the droplet on a solid substrate enhanced by applying an electric voltage between the droplet and the insulated substrate. However, a quantitative understanding of the detachment process and an accurate prediction on the thresholds of applied voltage for droplet detachment on curved surfaces are still lacking. In this paper, based on energy conservation, we derive a critical condition theoretically for electrowetting-induced droplet detachment from a hydrophobic curved surface. Furthermore, phase diagrams are constructed in terms of droplet volume, viscosity, the Ohnesorge number, friction coefficient at contact line, surface curvature, surface wettability, and electrowetting number. The deduced critical condition offers a general and quantitative prediction on when the detachment occurs, a criterion enabling us to gain more insights into how to accurately manipulate the electrowetting-induced detachment of an aqueous droplet on a curved surface. The results obtained in this paper also imply that the detachable regimes of the phase diagrams can be enlarged through increasing droplet volume and surface curvature and reducing liquid viscosity, friction coefficient, the Ohnesorge number, and wettability of substrates.