Universal deformation of soft substrates near a contact line and the direct measurement of solid surface stresses

TL;DR

This study reveals a universal deformation shape of soft substrates near contact lines, enabling direct measurement of solid surface stresses and showing deviations from Young's law at small scales.

Contribution

It introduces a universal shape for substrate deformation near contact lines and a new method to measure solid surface stresses directly.

Findings

Deformation shape is universal near contact line, independent of droplet size and substrate thickness.

Surface stresses can be directly measured from deformation shape.

Young's law fails for small droplets near an elastocapillary length.

Abstract

Droplets deform soft substrates near their contact lines. Using confocal microscopy, we measure the deformation of silicone gel substrates due to glycerol and fluorinated-oil droplets for a range of droplet radii and substrate thicknesses. For all droplets, the substrate deformation takes a universal shape close to the contact line that depends on liquid composition, but is independent of droplet size and substrate thickness. This shape is determined by a balance of interfacial tensions at the contact line and provides a novel method for direct determination of the surface stresses of soft substrates. Moreover, we measure the change in contact angle with droplet radius and show that Young's law fails for small droplets when their radii approach an elastocapillary length scale. For larger droplets the macroscopic contact angle is constant, consistent with Young's law.