Elastocapillary Adhesion of Compliant Gel Microspheres*

Capillary forces can play an important role in adhesion between rigid surfaces and highly compliant polymer gels in multiple ways. Recent studies have demonstrated that solid surface tension can compete with or even dominate over bulk elasticity on small length scales, leading to phenomena unanticipated by classic solid mechanics. Furthermore, mounting evidence suggests that the internal free fluid phase of gels can also contribute significantly to the mechanical response of highly compliant materials, from phase separation in static equilibria to the most extreme dynamics. In this work, we investigate the adhesion between polydimethylsiloxane (PDMS) gel microspheres and rigid glass substrates. We vary the stiffness and size of the microspheres as well as the surface energy of the substrate and directly image the adhered microspheres using brightfield microscopy. Our measurements show a range of adhesive contact geometries, from classic elastic to quantitatively wetting-like behavior. We understand our data with a model that incorporates adhesion, elasticity, surface tension, and phase separation.