Liquid-grain mixing suppresses droplet spreading and splashing during impact

TL;DR

This study investigates how liquid impacts on granular surfaces, revealing that liquid-grain mixing influences droplet spreading and splashing, with larger grains and higher wettability suppressing splashing through a transition from capillary to viscous regimes.

Contribution

It introduces the concept of liquid-grain mixing as a key mechanism affecting droplet impact dynamics on granular media, supported by experimental evidence.

Findings

Larger grain size and increased wettability suppress splashing.

Transition from capillary to viscous regime observed with changing parameters.

Effective viscosity explains the impact behavior.

Abstract

Would a raindrop impacting on a coarse beach behave differently from that impacting on a desert of fine sand? We study this question by a series of model experiments, where the packing density of the granular target, the wettability of individual grains, the grain size, the impacting liquid, and the impact speed are varied. We find that by increasing the grain size and/or the wettability of individual grains the maximum droplet spreading undergoes a transition from a capillary regime towards a viscous regime, and splashing is suppressed. The liquid-grain mixing is discovered to be the underlying mechanism. An effective viscosity is defined accordingly to quantitatively explain the observations.