Non-monotonic motion of sliding droplets on strained soft solids

TL;DR

This study investigates how pre-strain in soft silicone gels affects droplet sliding, revealing a non-monotonic relationship where moderate compression slows droplets, but larger compression accelerates them due to viscoelastic and pressure effects.

Contribution

It uncovers the complex non-monotonic influence of pre-strain on droplet motion and links it to viscoelasticity and surface instabilities in soft solids.

Findings

Mild compression decelerates droplets

Stronger compression accelerates droplets

Creasing instability causes 'run-and-stop' motion

Abstract

Soft materials are ubiquitous in technological applications that require deformability, for instance, in flexible, water-repellent coatings. However, the wetting properties of pre-strained soft materials are only beginning to be explored. Here we study the sliding dynamics of droplets on pre-strained soft silicone gels, both in tension and in compression. Intriguingly, in compression we find a non-monotonic strain dependence of the sliding speed: mild compressions decelerate the droplets, but stronger compressions lead again to faster droplet motion. Upon further compression, creases nucleate under the droplets until finally, the entire surface undergoes the creasing instability, causing a ``run-and-stop" motion. We quantitatively elucidate the speed modification for moderate pre-strains by incremental viscoelasticity, while the acceleration for larger pre-strains turns out to be linked to the solid pressure, presumably through a lubrication effect of expelled oligomers.