Non-monotonic motion of sliding droplets on strained soft solids

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.