Regulating droplet rebound by square-pulse electrowetting techniques

Abstract

Electrowetting presents a powerful technique for manipulating droplets, but its potential to enhance post-impact droplet rebound remains insufficiently understood and underutilized. In this study, we realize the regulation of rebound enhancement and suppression in impacting Galinstan and water droplets using square pulse electrowetting techniques. We numerically investigate the effects of pulse width, surface wettability, and liquid properties on rebound characteristics and demonstrate a phase diagram of rebound modes. Our findings reveal that a moderate pulse width facilitates rebound enhancement, whereas excessively small or large pulse widths lead to rebound suppression. Notably, a fascinating bubble entrapment phenomenon is identified under moderate pulse width, resulting in a distinctive tooth-like rebound shape and secondary liquid–solid contact. Contrary to conventional beliefs, we discover that the optimal rebound velocity occurs at approximately 1.5 times the spreading time, rather than solely at one spreading time. Through unraveling the energy conversion mechanism, we attribute this deviation to the trade-off between additional surface energy and total energy loss. Furthermore, this study highlights that compared to water droplets, the ultra-high surface tension of Galinstan increases additional surface energy while diminishing the viscous effect, leading to heightened rebound velocity, reduced contact time, and an expanded range of pulse widths for rebound enhancement.