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.
Highlights
The simplest and most widely used technique to characterize a solid surface is the measurement of the contact angle of sessile droplets
This is typically achieved by relating the equilibrium contact angle θeq to the surface-energy difference across a contact line via the law of Young-Dupre: γlv cos θeq 1⁄4 γsv − γsl: ð1Þ
What can we extract from the shapes of the ridges? In previous work on hysteresis-free silicone gels, the angles between the three interfaces where they meet at the contact line were found to be fixed [9]
Summary
The simplest and most widely used technique to characterize a solid surface is the measurement of the contact angle of sessile droplets. We introduce a method to measure surface tension differences across contact lines of hysteretic soft materials, based on the measurement of microscopic deformations of the gel upon wetting. This is achieved by measuring θ0, the angle at which the surface tension forces acting at the contact line are in horizontal force balance. The measured value of the surface-tension difference across the contact line is very similar to the liquid-vapor surface tension of the gelatin solution just before gelation This suggests that the surface tension of the interface between a gel and a droplet of its solvent can be negligible
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