Collective Asperity Dynamics and the Origin of Static Friction

Solid interfaces resist sliding up to a threshold shear force, called static friction, beyond which they start moving and their resistance drops to the kinetic friction. Static friction at rough interfaces has long been described empirically using system-specific coefficients tabulated in engineering handbooks. Here, through nanometer-resolution sliding experiments, we show that it is set by a friction overshoot during the onset of sliding. We demonstrate that this overshoot originates from the collective configurational evolution of surface asperities under shear, and derive a minimal differential equation governing this evolution. Our theory predicts that such overshoots generically emerge when an athermal frictional system evolves smoothly toward a unique steady-state kinetic friction. These results show that static friction is not an intrinsic material property, but an emergent consequence of collective asperity dynamics.