Hidden Dynamics of Static Friction
Sam Dillavou, Harvard

Static friction, a force fundamental to nearly every mechanical system from micro-machines to basketball shoes to tectonic plates, is in fact anything but static. In contrast to the simplified version taught in high-school, static friction is actually extremely dynamic, complex, and still poorly understood. Due to small-scale roughness, even seemingly flat interfaces actually consist of sparse, scattered contact points, which comprise only a few percent or less of the interfacial area. These contact points vary in shape and size, exist under extreme pressures, and form an ensemble that evolves continuously in time. This is true for a vast range of scales and in nearly all materials, from rock to plastic to paper to metal.

We measure the evolution of this contact ensemble, and demonstrate that multi-contact interfaces (MCIs) store a memory of the pressures they experienced. Unlike simple relaxation, e.g. a spring and dashpot system, which depends only on its current state, MCIs evolve according to their entire loading history. Their evolution closely resembles the slow relaxations of a class of disordered and glassy systems that includes crumpled paper, elastic foams, polymer glasses, granular piles, and more. In fact, using a model built for these seemingly unrelated systems, we are able to reproduce the observed interfacial dynamics, implying that putting two solids in contact generates a new glassy system.