Drop impact on viscous liquid films

TL;DR

This study explores how viscous liquid films can repel or allow bouncing of impacting drops, revealing the roles of film thickness, viscosity, and impact dynamics through experiments, simulations, and modeling.

Contribution

It introduces a minimal model and experimental analysis of drop impact on viscous films, elucidating the transition between bouncing and non-bouncing regimes.

Findings

Bouncing is unaffected by viscous films for very viscous or thin films.

Drop viscosity above a critical value suppresses bouncing.

Thicker or less viscous films influence rebound dynamics and repellency limits.

Abstract

When a liquid drop falls on a solid substrate, the air layer in between them delays the occurrence of liquid--solid contact. For impacts on smooth substrates, the air film can even prevent wetting, allowing the drop to bounce off with dynamics identical to that observed for impacts on superamphiphobic materials. In this article, we investigate similar bouncing phenomena, occurring on viscous liquid films, that mimic atomically smooth substrates, with the goal to probe their effective repellency. We elucidate the mechanisms associated to the bouncing to non-bouncing (floating) transition using experiments, simulations, and a minimal model that predicts the main characteristics of drop impact, the contact time, and the coefficient of restitution. In the case of highly viscous or very thin films, the impact dynamics is not affected by the presence of the viscous film. Within this substrate--independent limit, bouncing is suppressed once the drop viscosity exceeds a critical value as on superamphiphobic substrates. For thicker or less viscous films, both the drop and film properties influence the rebound dynamics and conspire to inhibit bouncing above a critical film thickness. This substrate--dependent regime also admits a limit, for low viscosity drops, in which the film properties alone determine the limits of repellency.