Droplets sliding on soft solids shed elastocapillary rails

TL;DR

This study investigates how droplets slide on soft solids, revealing the formation of elastocapillary rails and different dissipation behaviors at varying speeds, advancing understanding of droplet-soft substrate interactions.

Contribution

It experimentally demonstrates the formation of elastocapillary rails during droplet sliding and analyzes the associated dissipation mechanisms at different velocities.

Findings

At low speeds, dissipation increases logarithmically with velocity.

At high speeds, contact lines become bullet-shaped and dissipation levels off.

Droplets shed elastocapillary rails that fade behind, favoring stationary ridges.

Abstract

The surface tension of partially wetting droplets deforms soft substrates. These deformations are usually localized to a narrow region near the contact line, forming a so-called `elastocapillary ridge.' When a droplet slides along a substrate, the movement of the elastocapillary ridge dissipates energy in the substrate and slows the droplet down. Previous studies have analyzed isotropically spreading droplets and found that the advancing contact line `surfs' the elastocapillary ridge, with a velocity determined by a local balance of capillary forces and bulk rheology. Here, we experimentally explore the dynamics of a droplet sliding across soft substrates. At low velocities, the contact line is nearly circular, and dissipation increases logarithmically with speed. At higher droplet velocities, the contact line adopts a bullet-like shape, and the dissipation levels off. At the same time, droplets shed a pair of `elastocapillary rails' that fade away slowly behind it. These results suggest that droplets favor sliding along a stationary ridge over surfing atop a translating one.