Role of offset during drop-on-drop impact dynamics on a superhydrophobic substrate

Abstract

Droplet impacting on another deposited droplet (drop-on-drop) is present in various applications. However, the actual impact configuration deviates from completely aligned case. In this work, the drop-on-drop impact with an offset between sessile and impacting droplet on a superhydrophobic substrate is investigated. First droplet is designated as Droplet #1, and oncoming droplet as Droplet #2. Two boundary conditions of Droplet #1 deposition are highlighted: gentle deposition and impact deposition. There is pining of merged droplet during receding phase with impact deposition, while the combined droplet bounces off entirely with gentle deposition. All further investigation of offset and We number are performed with gentle deposition of Droplet #1. Two regimes namely: merging and non-merging, at lower We number of Droplet #2 are observed for all offset values. There is single merging outcome at higher We numbers. The droplets merges, spreads, recedes, and bounces-off. The offset present between two droplets lead to asymmetrical interfacial shape evolution. The variation of pressure and velocity is also presented for understanding the mechanism of interfacial evolution. There is additional rotational behaviour after bounce-off of the merged droplet. The time lag in spreading and receding phases towards right and left sides (of orthogonal to the droplet base profile) is attributed to the presence of rotational behaviour. The time of bounce-off is also higher with offset, as compared to the completely aligned case. This study delineates major differences in drop-on-drop impact dynamics with presence of offset between Droplet #1 and Droplet #2.