Contact and slip mechanics between crosslinked hydrogel surfaces using in situ microscopy

The contact mechanics of soft-soft interfaces provide a backdrop for the slip mechanics due to the conformal contact at the interface, in which any surface asperities are fully compressed, and the apparent area of contact is the real area of contact. However, assumptions regarding the nature of the soft material as a thermal, semi-dilute mesh network, poroelastic solid, or other, will control the contact mechanics, especially over time. In this work we show detailed measurements of contact areas with polyacrylamide hydrogels during microindentation and slip which are revealed through particle inclusion and/or exclusion microscopy. The particles are green fluorescent polystyrene spheres of 0.5 or 1 µm in diameter. We identify time-dependent contact mechanics in migrating, stationary, and self-mated “Gemini” contact. Our data suggest that for long times, Gemini contact approaches a constant-pressure contact model, which depends upon the equilibrium osmotic pressure of the sample. Finally, we present for the first time asymmetric contact areas as visualized by in situ particle exclusion which manifests as a flow field around the probe. The results of this work begin to connect hydrogel material properties with surface mechanics.