Wetting and phase separation in soft adhesion

TL;DR

This paper investigates how surface stress influences wetting and phase separation in soft adhesion, revealing that soft gels form complex contact zones and that adhesion theories must consider gel compressibility and surface stress effects.

Contribution

It introduces a new understanding of wetting and phase separation in soft gels, emphasizing the roles of surface stress and gel compressibility in adhesion.

Findings

Contact angle depends on surface functionalization, not sphere size.

Phase separation occurs near the contact line, forming a four-phase zone.

The volume of the phase-separated region depends on indentation volume.

Abstract

In the classic theory of solid adhesion, surface energy drives deformation to increase contact area while bulk elasticity opposes it. Recently, solid surface stress has been shown also to play an important role in opposing deformation of soft materials. This suggests that the contact line in soft adhesion should mimic that of a liquid droplet, with a contact angle determined by surface tensions. Consistent with this hypothesis, we observe a contact angle of a soft silicone substrate on rigid silica spheres that depends on the surface functionalization but not the sphere size. However, to satisfy this wetting condition without a divergent elastic stress, the gel separates from its solvent near the contact line. This creates a four-phase contact zone with two additional contact lines hidden below the surface of the substrate. While the geometries of these contact lines are independent of the size of the sphere, the volume of the phase-separated region is not, but rather depends on the indentation volume. These results indicate that theories of adhesion of soft gels need to account for both the compressibility of the gel network and a non-zero surface stress between the gel and its solvent.