The Role of Extracted Free Chains in Adhesion on Soft Gel Surfaces

Adhesion on soft polymeric materials is crucial for many engineering applications. However, the substantial surface deformations of compliant substrates present challenges for classical contact theory in describing the adhesion behavior. Furthermore, surface contact can drive the migration of free solvent within polymer networks, further complicating the governing physics of soft adhesion. In this work, we used dual-wavelength confocal microscopy to characterize how contact-induced extraction of free chains affects the adhesion profiles of microspheres on soft silicone gels with varying cross-linking densities. Locally, a phase separation region emerged near the contact points while the balance of surface stresses was preserved. Globally, the indentation depth and contact radius of the spheres were described by incorporating the competition between adhesion energy and surface stress into classical contact models. Over extended measurement periods, we imaged in situ a slow increase in the indentation depth of attached spheres in all experiments. This finding indicates a gradual rise in adhesion energy, occurring on a time scale consistent with poroelastic diffusion in soft silicone gels.