Frozen Adhesive Dynamics in Soft Silicone Gels

When a soft adhesive material first touches a rough surface, it initiates a cascade of dynamic mechanical processes that ultimately can establish a strong adhesive contact between initially-mismatched interfaces. Contact initiation involves complex deformation and relaxation over both short and long time scales to bring the interfaces together, making this a difficult process to capture experimentally. In this work, we use confocal microscopy to probe how compliant silicone gels mechanically deform and retain residual stress before, during, and after adhesive contact with glass microspheres. The gels consist of crosslinked polydimethyl siloxane (PDMS) networks embedded with fluorescent tracer beads, which encode the three-dimensional motion throughout the contact cycle, essentially "freezing in" the dynamics in the bead displacements. Preliminary analysis reveals a complex combination of compression under the sphere and tension outside, with a distinct no-movement zone outside the contact line, an unexpected feature given the local stress concentration. Together, these observations connect the slow deformation during contact formation with the residual stress patterns that develop after separation, offering new insight into how soft adhesives deform and relax throughout the dynamic processes of making and breaking adhesive contact.