Bouncing-to-wetting transition for impact of water droplets on soft solids

TL;DR

This study investigates how the interaction between trapped air films and cavities influences whether water droplets bounce or wet soft hydrophobic surfaces, revealing a phase diagram and detailed dynamics of rupture and jet formation.

Contribution

It introduces a systematic phase diagram linking air film rupture mechanisms to bouncing and wetting behaviors on soft surfaces, highlighting the role of dimple inversion.

Findings

Air film rupture by dimple inversion separates bouncing and wetting regimes.

Dimple inversion influences air cavity collapse and jet formation.

A detailed characterization of cavity collapse and jetting dynamics is provided.

Abstract

Soft surfaces impacted by liquid droplets trap more air underneath than their rigid counterpart. The extended lifetime of the air film not only facilitates bouncing behaviours of the impacting droplets but also increases the possibility of an interaction between the air film itself and the air cavity formed inside the droplets by capillary waves. Such interaction may cause rupture of the trapped air film by a so-called dimple inversion phenomenon and suppress bouncing. In this work, we systematically investigate the relation between air cavity collapse and air film rupture for water droplets impacting on soft, hydrophobic surfaces. By constructing a bouncing-to-wetting phase diagram based on the rupturing dynamics of the trapped air film, we observe that the regime in which air film rupture is induced by dimple inversion consistently separates the bouncing regime and the one in which wetting is caused by random rupture. We also find that air film rupture by dimple inversion, in-turn, affects both the collapsing dynamics of the air cavity and the resulting high-speed jet. We then provide a detailed characterisation of the collapsing dynamics of the air cavity and subsequent jetting.