Unifying Frictional Transients Reveals the Origin of Static Friction

Frictional motion is harder to initiate than to sustain, as evident when pushing a heavy object. This disparity between static and kinetic friction drives instabilities and stick-slip dynamics in systems ranging from nanodevices and MEMS to squealing brakes, glaciers and tectonic faults, yet its origin and the transition mechanism remain poorly understood. Empirical rate-and-state friction laws predict that during the static-to-kinetic transition, friction increases for nanometer-per-second slip rates, but decreases for micrometers-per-second rates and above. These transients are believed to be associated with contact strengthening (aging) at static interfaces, although their physical basis is unclear and the crossover between regimes has never been observed directly. Here we show, through nanometer-resolution sliding experiments on macroscopic rough surfaces, that these transients are segments of a single, universal non-monotonic response whose peak defines static friction. We show that this behavior arises from mechanical reorganization of interlocking surface asperities under shear, fundamentally distinct from contact aging, which is governed by thermal molecular processes. We derive, from first principles and without invoking any empirical postulates, a differential equation that quantitatively captures the friction peak. These results unify frictional transients across scales and speeds, and establish a physics-based framework for understanding frictional instabilities and failure processes in engineering and geosciences.