Elastocapillary Adhesion of Soft Gel Microspheres*

While elasticity has long been understood to be important in determining the relative “stickiness” between non-conformal surfaces, in recent years capillary forces have emerged as playing key roles in adhesion with highly compliant materials. For example, recent studies have demonstrated that solid surface tension can compete with or dominate over bulk elasticity in governing contact mechanics on small length scales, and mounting evidence suggests that the internal free fluid phase of compliant polymer gels also contributes significantly to mechanical response via both poroelasticity and classic capillary wetting. In this work, we experimentally investigate the adhesion between polydimethylsiloxane gel microspheres and rigid glass substrates. By varying the asperity size, adhesion energy, and gel material properties, we observe a continuous transition from classic elastic to capillary-like adhesion, with contact line deformation mediated by a fluid contact zone that phase separates from the gel. Interestingly, the intermediate-compliance gels demonstrate a broad range of equilibrium contact deformations, reflecting a shallow energetic minimum that likely contributes to the robustness of everyday adhesives. We develop a new model incorporating elastocapillary and poroelastic mechanics that captures the complete range of adhesive behavior.