Pore-size dependence and glassy behavior of hydrogel friction on smooth surfaces

Hydrogels are important in many scientific and engineering applications due to their tunable physiochemical properties and bio-compatibility. Bulk hydrogels consist of a crosslinked polymer matrix imbibed with water or a similar solvent at volume fractions that can exceed 90%, leading to a rich spectrum of interfacial rheological behaviors. Using a custom-built, continuous pin-on-disc tribometer, here we identify three distinct regimes of frictional behavior for both polyacrylamide (PAAm) and agarose hydrogel spheres on smooth surfaces. At low velocities, friction is controlled by hydrodynamic flow through the porous hydrogel network, and is inversely proportional to the characteristic pore size. At high velocities, a mesoscopic, lubricating liquid film forms obeying elastohydrodynamic theory. In between these regimes, the frictional force sharply decreases with velocity and simultaneously displays time-dependent behavior characteristic of glassy, slow relaxation over several minutes. This relaxation strongly depends on the fluid shear rate, and the transition can be tuned by varying the solvent salt concentration, solvent viscosity, and sliding geometry at the interface.