Elastocapillary adhesion of soft gel microspheres

Softer means stickier for solid adhesives, because material compliance facilitates close contact between non-conformal surfaces. Recent discoveries have revealed that soft materials can exhibit a rich array of new physics arising from competing effects of continuum elasticity, fluid-like surface mechanics, and internal poroelastic flows, all of which can directly impact interfacial interactions. In this work, we investigate this complex interplay across several orders of magnitude of elastic stiffness by measuring the complete adhesive contact geometry between compliant silicone gel microspheres and flat, rigid substrates. We observe a continuous elastocapillary transition in adhesion mechanics, with novel features revealed by both the breadth of data and the detailed contact geometries. Importantly, soft gel spheres exhibit a remarkably broad range of near-equilibrium contact morphologies and their contact line deformation is always mediated by a fluid contact zone that phase separates from the gel. To explain this, we develop a new model incorporating elastocapillary and poroelastic mechanics that predicts the complete range of adhesive behavior and elucidates energetic tradeoffs. The data and model together reveal a shallow energy landscape that may contribute to the robustness of everyday adhesives.