  We report on the frictional behaviour of thin poly(dimethylacrylamide) (PDMA)
hydrogels films grafted on glass substrates in sliding contact with a glass
spherical probe. Friction experiments are carried out at various velocities and
applied normal loads with the contact fully immersed in water. In addition to
friction force measurements, a novel optical set-up is designed to image the
shape of the contact under steady-state sliding. The velocity-dependence of
both friction force $F_t$ and contact shape is found to be controlled by a
P\'eclet number Pe defined as the ratio of the time $\tau$ needed to drain the
water out of the contact region to a contact time $a/v$, where $v$ is the
sliding velocity and $a$ is the contact radius. When Pe<1, the equilibrium
circular contact achieved under static normal indentation remains unchanged
during sliding. Conversely, for Pe>1, a decrease in the contact area is
observed together with the development of a contact asymmetry when the sliding
velocity is increased. A maximum in $F_t$ is also observed at Pe~$\approx$~1.
These experimental observations are discussed in the light of a poroelastic
contact model based on a thin film approximation. This model indicates that the
observed changes in contact geometry are due to the development of a pore
pressure imbalance when Pe>1. An order of magnitude estimate of the friction
force and its dependence on normal load and velocity is also provided under the
assumption that most of the frictional energy is dissipated by poroelastic flow
at the leading and trailing edges of the sliding contact.
