Measuring surface tensions of soft solids with huge contact-angle hysteresis

TL;DR

This paper introduces a novel method to measure the surface tensions of soft solids exhibiting large contact-angle hysteresis by analyzing substrate deformations and contact angles, enabling determination of solid surface tensions despite hysteresis.

Contribution

The paper presents a new technique to determine solid surface tensions in the presence of strong hysteresis using substrate deformation and contact angle measurements.

Findings

Method accurately measures solid surface tension differences.

Applicable to soft solids like hydrogels with hysteresis.

Reveals equilibrium contact angle analogous to Young-Dupré with surface tensions.

Abstract

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-Dupr\'{e}, but with surface tensions in place of surface energies. We motivate and apply this approach with experiments on gelatin, a common hydrogel.