What determines static friction and controls the transition to sliding?

Abstract

We studied the shear response of a confined lubricant layer on approach of the transition to sliding with a surface force apparatus modified for oscillatory shear. In a given experiment, we found that the transition to sliding occurred always around the same deformation amplitude although the shear stress needed to initiate sliding varied up to a factor of two depending on sample history. This suggests the concept of deformation-controlled switching from rest to sliding. The elastic spring-constant, in the stick state, weakened with increasing deformation amplitude. This decrease can be described by a power law when plotted versus the distance to a critical deformation amplitude. The build-up of solid-like behavior after sliding stopped was also gradual and was consistent with a logarithmic time dependence. We suggest a model relating the gradual decrease of stiffness to weakening of the boundary layer, specifically to destruction of some elastic links between molecules or between molecules and the solid surfaces. Static friction (the force that must be overcome at the onset of kinetic motion) is proportional to the number of such links formed during the time of stick.

Solid friction from stick–slip down to pinning and aging

“…In all cases, strengthening is slow, namely either linear on a logarithmic t w -scale [74] [81] or somewhat faster [79]. As illustrated on figure 27, finite latency times, ranging up to about 10 seconds, previous to aging, have been observed on hexadecane [74] and several lubricants including squalane [78].…”

“…This strengthening, which reveals structural aging, has been studied quantitatively on a variety of lubricants, including hexadecane [74], squalane [78] [79] and a star-shaped polymer melt [80]. Strengthening also manifests itself through the increase with t w of the layer elastic stiffness [81]. 24 For some fluids made of short-chain molecules, e.g.…”