Synopsis: Wetting Physics on a Squishy Surface

A solid’s surface tension is usually difficult to separate from its bulk elasticity, but a new technique determines this property by measuring the contact angle of small droplets on a soft surface.
Synopsis figure
Courtesy E. R. Dufresne/Yale University

It is well known that the surface tension of a liquid determines how it wets an underlying solid, but the wetting characteristics also depend on surface stresses in the solid itself. Surface tension in a solid is, however, notoriously difficult to measure because a solid’s bulk elasticity dominates its response to external forces. In a paper in Physical Review Letters, Robert Style and colleagues at Yale University, Connecticut, demonstrate a new technique to measure surface tension in soft surfaces that, unlike existing methods, does not require knowledge of the bulk properties of the underlying solid.

Style et al. study how a soft surface bends when a liquid droplet rests on it. Typically, the angle at which the surfaces of a droplet and a solid meet—the contact angle—is determined by Young’s Law, but the authors show that on soft surfaces the measured contact angle deviates from the predicted one by as much as 30 degrees. They argue that this failure of Young’s Law on a soft surface occurs because the liquid’s surface tension is able to significantly deform a soft solid.

Using confocal microscopy, the authors study glycerol and oil droplets of various sizes resting on silicone gel of different thicknesses. They show that there is a very small region around the contact line where the deformation is universal in the sense that it depends only on the surface tension in the underlying solid. Style et al.’s approach is the first that allows surface stresses to be measured in any soft solid, regardless of its bulk rheological properties. – Sami Mitra


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