Raindrop impact on sand: a dynamic explanation of crater morphologies

TL;DR

This study investigates how raindrops impact sandy surfaces, revealing the roles of substrate deformation, droplet spreading, and liquid penetration in shaping crater morphology through high-speed measurements and a new model.

Contribution

It introduces a comprehensive model explaining crater morphology transitions by analyzing substrate deformation, droplet spreading, and liquid penetration during impact.

Findings

Critical packing fraction for substrate deformation is approximately 0.585.

Maximum droplet spreading scales with a corrected Weber number.

A new model explains the transition in crater morphology.

Abstract

As a droplet impacts on a granular substrate, both the intruder and the target deform, during which the liquid may penetrate into the substrate. {These three aspects together distinguish} it from other impact phenomena in the literature. We perform high-speed, double-laser profilometry measurements and disentangle the dynamics into three aspects: the deformation of the substrate during the impact, the maximum spreading diameter of the droplet, and the penetration of the liquid into the substrate. By systematically varying the impact speed and the packing fraction of the substrate, (i) the substrate deformation indicates a critical packing fraction $\phi^*\approx 0.585$; (ii) the maximum droplet spreading diameter is found to scale with a Weber number corrected by the substrate deformation; and (iii) a model about the liquid penetration is established and is used to explain the observed crater morphology transition.