Static Friction Coefficient Depends on Geometry

Abstract Classical theories state that the friction coefficient is independent of geometry, depending only on the real contact area determined by roughness. However, experimental evidence shows significant differences between static and dynamic friction coefficients. Recent models introduce an energetic theory for friction, analogous to the Griffith theory for fracture, resulting in a constant dynamic friction coefficient at heavy loads but a larger static coefficient at low loads. We show that, for power-law punches, the ratio of static to dynamic friction coefficients at low normal loads is higher for flatter profiles and, in principle, grows unbounded at zero load. This may explain the observed variability in friction ratios. The model’s predictions align well with recent experimental results.