Similarity of internal and external friction: Soft matter frictional instabilities obey mean field dissipation through slip avalanches

Resistance to slip across hydrogel surfaces is typically minimal, and sliding is smooth. However, recent surface friction experiments under high applied pressures caused stick-slip behavior between a glass probe and polyacrylamide hydrogel. In this paper we analyze the surface-based interface and its behavior similar to the more internal slip avalanches that occur in plastic deformations of metallic glasses and crystals using statistical descriptions. We find that the stick-slip surface friction satisfies universal power laws and scaling functions predicted by a simple mean field theory (MFT). We provide the rescaled average avalanche slip rate profiles, or average avalanche ''shapes,'' defined as the average slip rate versus time, averaged over all avalanches with similar durations or sizes. We show that the avalanche shapes obtained in this friction experiment are consistent with MFT. This suggests the similar character of force buildup and release events in internal slip and surface friction for this unique data set, and the suitability of surface microfriction experiments to explore this space.