  Hydrogel consists of a crosslinked polymer matrix imbibed with a solvent such
as water at volume fractions that can exceed 90\%. They are important in many
scientific and engineering applications due to their tunable physiochemical
properties, bio-compatibility, and ultra-low friction. Their multiphase
structure leads to a complex interfacial rheology, yet a detailed, microscopic
understanding of hydrogel friction is still emerging. Using a custom-built
tribometer, here we identify three distinct regimes of frictional behavior for
polyacrylic acid (PAA), polyacrylamide (PAAm), and agarose hydrogel spheres on
smooth surfaces. We find that 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 between the gel and surface that obeys
elastohydrodynamic theory. Between these regimes, the frictional force
decreases by an order of magnitude and displays slow relaxation over several
minutes. Our results can be interpreted as an interfacial shear thinning of the
polymers with an increasing relaxation time due to the confinement of
entanglements. This transition can be tuned by varying the solvent salt
concentration, solvent viscosity, and sliding geometry at the interface.
