Rubber friction: from steady sliding to stick slip and squeaking*

Even though friction is responsible for keeping your car on the road, your coffee mug in your hand and on larger scales governs earthquakes, many aspects of it are still poorly understood. In particular, for soft materials moving on stiff ones there are still a lot of open questions. Here, we study both experimentally and numerically the frictional motion at the interface between a soft and a rigid body for driving velocities spanning four orders of magnitude. We find that, while the applied far field velocity is uniform, steady sliding at the frictional interface is not always dynamically stable. For driving velocities lower than a critical velocity we observe homogeneous steady sliding of the entire frictional interface. For larger velocities, however, slip is spatially localized along fronts. These fronts nucleate at the trailing edge of the soft sample, then propagate dynamically at speeds approaching the shear wave speed of the soft material. Finally, for driving velocities of the order of 1 m/s, sliding is associated with a distinct squeaking sound which frequency is correlated to the slip fronts.