Universal deformation of soft substrates near contact line reveals solid surface stresses

We study how sessile droplets behave on soft substrates. Using confocal microscopy, we investigate how droplet surface tension (and Laplace pressure) deforms the substrate. We show that the near-tip shape of the wetting ridge is entirely determined by the surface tensions of the three contacting phases. In particular we can use this observation to (i) directly measure solid-vapour and solid-liquid surface tensions, (ii) resolve how out-of-plane force balance is ensured at the contact line.