  Self-mated hydrogel contacts show extremely small friction coefficients at
low loads but a distinct velocity dependence. Here we combine mesoscopic
simulations and experiments to test the polymer-relaxation hypothesis for this
velocity dependence, where a velocity-dependent regime emerges when the
perturbation of interfacial polymer chains occurs faster than their relaxation
at high velocity. Our simulations reproduce the experimental findings, with
speed-independent friction at low velocity, followed by a friction coefficient
that rises with velocity to some power of order unity. We show that the
velocity-dependent regime is characterized by reorientation and stretching of
polymer chains in the direction of shear, leading to an entropic stress that
can be quantitatively related to the shear response. The detailed exponent of
the power law in the velocity dependent regime depends on how chains interact:
We observe a power close to $1/2$ for chains that can stretch, while pure
reorientation leads to a power of unity. Our simulations quantitatively match
experiments and show that the velocity dependence of hydrogel friction at low
loads can be firmly traced back to the morphology of near-surface chains.
