Poroelastic sliding of thin grafted hydrogel layers

We investigate the sliding friction of thin (µm), water swollen, hydrogel layers grafted onto glass substrate with an emphasis on the contribution to friction of stress-induced poroelastic drainage of the hydrogel network. Friction experiments using poly(dimethylacrylamide) hydrogels films are carried out with the contact immersed in water. In addition to friction force measurements, an optical set-up allows imaging the contact. The velocity-dependence of friction force F_t and contact shape is found to be controlled by a Péclet number Pe corresponding to the ratio of advective (sliding) to diffusive components (fluid drainage). When Pe<1, the equilibrium circular contact achieved under 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 velocity is increased. These findings are discussed in the light of a poroelastic model based on a thin film approximation. This model indicates that changes in contact geometry are due to the development of a pore pressure imbalance when Pe>1. An order of magnitude estimate of F_t and its dependence on normal load and velocity is provided under the assumption that most of frictional energy is dissipated by poroelastic flow.