  We report on memory effects involved in the transient 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 which takes into account
heterogeneous displacements within the contact area. When a velocity dependence
of the frictional stress is incorporated in this the model, transient 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.
