Frictional Transition from Superlubric Islands to Pinned Monolayers

TL;DR

This study combines experiments and simulations to investigate the transition from superlubricity to pinning in adsorbed monolayers, revealing an abrupt, adhesion-driven change in sliding behavior as coverage increases.

Contribution

It provides a detailed joint experimental and theoretical analysis of the frictional transition from superlubric islands to pinned monolayers, highlighting the role of coverage and density compression.

Findings

Superlubricity occurs in large incommensurate Xe islands.

An abrupt transition to a pinned monolayer is observed near full coverage.

Hysteresis and density compression drive the transition.

Abstract

The inertial sliding of physisorbed submonolayer islands on crystal surfaces contains unexpected information on the exceptionally incommensurate smooth sliding state associated with superlubricity and on the mechanisms of its disappearance. Here we show in a joint quartz crystal microbalance (QCM) and molecular dynamics (MD) simulation case study of Xe on Cu(111) how superlubricity emerges in the large size limit of naturally incommensurate Xe islands. Theory also predicts, as coverage approaches a full monolayer, an abrupt adhesion-driven 2D density compression of order several %, implying here a hysteretic jump from superlubric free islands to a pressurized commensurate immobile monolayer. This scenario is fully supported by QCM data showing remarkably large slip times at increasing submonolayer coverages, signaling superlubricity, followed by a dramatic drop to zero for the dense commensurate monolayer. Careful analysis of this variety of island sliding phenomena will be mandatory in future applications of friction of crystal-adsorbate interfaces.