Memory effects in friction: the role of sliding heterogeneities

We report on memory effects involved in the unsteady-state frictional response of a contact interface between a silicone rubber and a spherical glass probe when it is perturbed by changes in the orientation of the driving motion or by velocity steps. From measurements of the displacement fields at the interface, we show that observed memory effects can be accounted for by the non-uniform distribution of the sliding velocity within the contact interface. As a consequence of these memory effects, the friction force may no longer be aligned with respect to the sliding trajectory. In addition, stick-slip motions with a purely geometrical origin are also evidenced. These observations are adequately accounted for by a friction model that takes into account heterogeneous displacements within the contact area. When a velocity dependence of the frictional stress is incorporated in this model, unsteady-state regimes induced by velocity steps are also adequately described. The good agreement between the model and experiments outlines the role of space heterogeneities in memory effects involved in soft matter friction.