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Hydrogel consists of a crosslinked polymer matrix imbibed with a solvent suchas water at volume fractions that can exceed 90\%. They are important in manyscientific and engineering applications due to their tunable physiochemicalproperties, bio-compatibility, and ultra-low friction. Their multiphasestructure leads to a complex interfacial rheology, yet a detailed, microscopicunderstanding of hydrogel friction is still emerging. Using a custom-builttribometer, here we identify three distinct regimes of frictional behavior forpolyacrylic acid (PAA), polyacrylamide (PAAm), and agarose hydrogel spheres onsmooth surfaces. We find that at low velocities, friction is controlled byhydrodynamic flow through the porous hydrogel network, and is inverselyproportional to the characteristic pore size. At high velocities, a mesoscopic,lubricating liquid film forms between the gel and surface that obeyselastohydrodynamic theory. Between these regimes, the frictional forcedecreases by an order of magnitude and displays slow relaxation over severalminutes. Our results can be interpreted as an interfacial shear thinning of thepolymers with an increasing relaxation time due to the confinement ofentanglements. This transition can be tuned by varying the solvent saltconcentration, solvent viscosity, and sliding geometry at the interface.

Pore-size dependence and slow relaxation of hydrogel friction on smooth surfaces