Tribology of the lubricant quantized-sliding state

TL;DR

This paper demonstrates the existence and robustness of a quantized sliding state in a 2D model of confined lubricants, revealing low friction and the influence of solitons and lubricant properties on this phenomenon.

Contribution

It extends the understanding of quantized sliding states from 1D models to a more realistic 2D description, highlighting the role of solitons and lubricant softness.

Findings

Quantized sliding state is robust against thermal fluctuations and disorder.

Low kinetic friction is associated with the quantized sliding state.

Transition to unpinned sliding involves lubricant melting at high speeds.

Abstract

In the framework of Langevin dynamics, we demonstrate clear evidence of the peculiar quantized sliding state, previously found in a simple 1D boundary lubricated model [Phys. Rev. Lett. 97, 056101 (2006)], for a substantially less idealized 2D description of a confined multi-layer solid lubricant under shear. This dynamical state, marked by a nontrivial ``quantized'' ratio of the averaged lubricant center-of-mass velocity to the externally imposed sliding speed, is recovered, and shown to be robust against the effects of thermal fluctuations, quenched disorder in the confining substrates, and over a wide range of loading forces. The lubricant softness, setting the width of the propagating solitonic structures, is found to play a major role in promoting in-registry commensurate regions beneficial to this quantized sliding. By evaluating the force instantaneously exerted on the top plate, we find that this quantized sliding represents a dynamical ``pinned'' state, characterized by significantly low values of the kinetic friction. While the quantized sliding occurs due to solitons being driven gently, the transition to ordinary unpinned sliding regimes can involve lubricant melting due to large shear-induced Joule heating, for example at large speed.