Transition from static to kinetic friction in a model lubricated system

Abstract

Molecularly thin confined fluids were deformed in shear faster than structural relaxations in response to shear could be accomplished, such that with increasing deformation the systems passed from the rest state to sliding. The response of these systems—two atomically smooth mica sheets separated by a fluid comprised of globularly shaped molecules [octamethylcyclotetrasiloxane]—was studied as a function of film thickness of the fluid (from 80 to 10 Å, i.e, from ∼8 to ∼1 molecular dimensions), as a function of normal pressure, and as a function of deformation rate, using a modified surface forces apparatus. Whereas the linear response was always liquid-like provided that the deformation rate was sufficiently slow, a “stick-slip” transition from the rest state to sliding was observed when the deformation rate was large, provided that the oscillatory frequency sufficiently exceeded the inverse intrinsic relaxation time of the confined fluid. This transition was monotonic and reversible without hysteresis for relatively thick films but for thinner films was discontinuous with hysteresis. For films thicker than 3 molecular layers (ML), two length scales in deformation were observed; the films showed nonlinear force-deformation response beginning at a deformation amplitude of 3 Å but in general showed stick slip only when the deformation was larger than this. The critical deformation at the point of stick slip decreased from 9 to 3 Å with increasing normal pressure, indicating diminished plasticity of the confined structures with increasing normal pressure. The critical film thickness of 3 ML correlates with the possibility of one rather than more slip planes. The thinnest films under the highest compressive pressures showed moderate increase of the viscous shear force with increasing effective sliding velocity, but in general the viscous force reached a plateau in which force showed almost no dependence on sliding rate. In interpreting the results in the context of friction, static friction was identified with the elastic stress at rupture and kinetic friction was identified with the limiting maximum observed level of viscous force. After normalizing friction and normal forces by the contact area, the static friction coefficient was found to be 0.44 and the kinetic friction coefficient to be 0.14. In other words, as the normal pressure increased, the elastic force needed to rupture the system increased more rapidly than the limiting shear stress. The magnitude of the limiting shear stress increased exponentially with decreasing film thickness with a decay length of 1 molecular dimension. This decay length correlates well with the known exponential decay of oscillations in the static force–distance profile. The critical shear amplitude of 3 Å, relative to the molecular dimension of ≈9 Å, is reminiscent of early estimates by Frenkel of the point of instability when planes of atoms slide over one another.