Memory in Solid-Solid Interfaces*

The interface between two solid, static bodies - your chair and the floor, your cup and the table, two books in a stack - is in fact continuously evolving. Due to small-scale roughness, ostensibly flat surfaces typically have a real area of contact several orders of magnitude smaller than apparent area. As a result, these contact points experience enormous pressures, and slowly deform in time.

By measuring the evolution of the real area of contact, we demonstrate that these 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.

Understanding of MCIs is not only useful for knowing how much coffee has been in your cup; the real area of contact also determines the friction between two bodies. As a result, the coefficient of static friction also evolves according to this interfacial memory. These effects suggests that MCIs and frictional systems belong in a universality class of glassy systems characterized by disorder and slow relaxation.