The Geometric State of a Solid-Solid Interface

The evolution of a static frictional interface is surprisingly elaborate. It is well known that the frictional strength of a solid-solid interface changes with normal load, but it also evolves in another way: slowly over time, in a process known as ‘aging.’ The effects of aging and a change in normal load on an interface are traditionally seen as unimportant, if they are assumed to exist at all. In a glass-silicone rubber interface, we hold the real area of contact constant over time and demonstrate the existence of differences between these two types of evolution as well as their dependence on contact geometry. This suggests that a frictional interface cannot be fully described by the real area of contact, or any such instantaneous variable, as different configurations of the same interface could all have the same total real area of contact. As the real area of contact is often used as a proxy for frictional strength, these results have important implications for contemporary models of friction, such as the widely used Rate and State Laws.