Viscous droplet impingement on soft substrates

TL;DR

This study investigates how viscous droplets behave upon impacting soft substrates, revealing complex transitions between bouncing, wetting, and hovering, influenced by substrate elasticity, impact velocity, and liquid viscosity.

Contribution

It provides the first systematic experimental analysis of impact behaviors on soft substrates, highlighting the role of Weber and Ohnesorge numbers in predicting wetting transitions.

Findings

Weber number at wetting transition decreases with Ohnesorge number for rigid substrates.

For softer substrates, Weber number at wetting transition first increases then decreases.

Maximum Weber number correlates with liquid repellency across various surface types.

Abstract

Viscous droplets impinging on soft substrates may exhibit several distinct behaviours including repeated bouncing, wetting, and hovering, i.e., spreading and retracting after impact without bouncing back or wetting. We experimentally study the conditions enabling these characteristic behaviours by systematically varying the substrate elasticity, impact velocity and the liquid viscosity. For each substrate elasticity, the transition to wetting is determined as the dependence of the Weber number We, which measures the droplet's kinetic energy against its surface energy, on the Ohnesorge number Oh, which compares viscosity to inertia and capillarity. We find that while We at the wetting transition monotonically decreases with Oh for relatively rigid substrates, it exhibits a counter-intuitive behaviour in which it first increases then gradually decreases for softer substrates. We experimentally determine the dependence of the maximum Weber number allowing non-wetting impacts on the substrate elasticity and show that it provides an excellent quantitative measure of liquid repellency for a wide range of surfaces, from liquid to soft surfaces and non-deformable surfaces.