Measuring Surface Tensions of Soft Solids with Huge Contact-Angle Hysteresis

Abstract

The equilibrium contact angle of a droplet resting on a solid substrate can\nreveal essential properties of the solid's surface. However, when the motion of\na droplet on a surface shows significant hysteresis, it is generally accepted\nthat the solid's equilibrium properties cannot be determined. Here, we describe\na method to measure surface tensions of soft solids with strong wetting\nhysteresis. With independent knowledge of the surface tension of the wetting\nfluid and the linear-elastic response of the solid, the solid deformations\nunder the contact line and the contact angle of a single droplet together\nreveal the difference in surface tension of the solid against the liquid and\nvapor phases. If the solid's elastic properties are unknown, then this surface\ntension difference can be determined from the change in substrate deformations\nwith contact angle. These results reveal an alternate equilibrium contact\nangle, equivalent to the classic form of Young-Dupr\\'{e}, but with surface\ntensions in place of surface energies. We motivate and apply this approach with\nexperiments on gelatin, a common hydrogel.\n