The equilibrium contact angle of a droplet resting on a solid substrate can reveal essential properties of the solid’s surface. However, when the motion of a droplet on a surface shows significant hysteresis, it is generally accepted that the solid’s equilibrium properties cannot be determined. Here, we describe a method to measure surface tensions of soft solids with strong wetting hysteresis. With independent knowledge of the surface tension of the wetting fluid and the linear-elastic response of the solid, the solid deformations under the contact line and the contact angle of a single droplet together reveal the difference in surface tension of the solid against the liquid and vapor phases If the solid’s elastic properties are unknown, then this surface tension difference can be determined from the change in substrate deformations with contact angle. These results reveal an alternate equilibrium contact angle, equivalent to the classic form of Young-Dupré, but with surface tensions in place of surface energies. We motivate and apply this approach with experiments on gelatin, a common hydrogel.

中文翻译：

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用大接触角滞后测量软固体的表面张力

停留在固体基底上的液滴的平衡接触角可以揭示固体表面的基本特性。然而，当液滴在表面上的运动表现出明显的滞后时，通常认为无法确定固体的平衡特性。在这里，我们描述了一种测量具有强润湿滞后的软固体的表面张力的方法。通过独立了解润湿液的表面张力和固体的线弹性响应，接触线下的固体变形和单个液滴的接触角共同揭示了固体对液体和液体的表面张力的差异。气相 如果固体的弹性性质未知，那么这种表面张力差异可以从基材变形随接触角的变化来确定。这些结果揭示了替代的平衡接触角，相当于 Young-Dupré 的经典形式，但表面张力代替了表面能。我们通过对明胶（一种常见的水凝胶）的实验来激发并应用这种方法。