Droplets sit and slide anisotropically on soft, stretched substrates

TL;DR

This study demonstrates that uniform, stretched soft substrates induce anisotropic droplet behavior, affecting both static shape and sliding dynamics, due to substrate deformation near the contact line, without chemical patterning.

Contribution

It reveals that uniform soft substrates under stretch can produce anisotropic wetting and droplet behavior, a phenomenon previously achieved mainly through patterning or gradients.

Findings

Droplets slide 67% faster along the stretch direction.

Static droplets are elongated parallel to the stretch.

Substrate deformation near the contact line causes anisotropic effects.

Abstract

Anisotropically wetting substrates enable useful control of droplet behavior across a range of applications. Usually, these involve chemically or physically patterning the substrate surface, or applying gradients in properties like temperature or electrical field. Here, we show that a flat, stretched, uniform soft substrate also exhibits asymmetric wetting, both in terms of how droplets slide and in their static shape. Droplet dynamics are strongly affected by stretch: glycerol droplets on silicone substrates with a 23\% stretch slide 67\% faster in the direction parallel to the applied stretch than in the perpendicular direction. Contrary to classical wetting theory, static droplets in equilibrium appear elongated, oriented parallel to the stretch direction. Both effects arise from droplet-induced deformations of the substrate near the contact line.