Theory of De-Pinning of Monolayer Films Adsorbed on a Quartz Crystal Microbalance

TL;DR

This paper proposes a theoretical explanation for the unexpectedly low friction forces observed in quartz crystal microbalance experiments, suggesting that defect potential ranges greater than atomic spacing lead to negligible net forces on monolayer films.

Contribution

It introduces a new theoretical model accounting for defect potential ranges exceeding atomic spacing to explain low friction in monolayer films on substrates.

Findings

Net force on stiff films is extremely small if defect potentials are long-ranged.

Both line defects and localized defects contribute minimally to friction.

The model aligns with experimental observations of low sliding forces.

Abstract

In quartz crystal microbalance (QCM) studies of the friction between an adsorbed monolayer film and a metallic substrate, the films are observed to slide relative to the substrate under inertial forces of order $10^{-14}dyn$ per film atom, a force much smaller than all theoretical estimates of the force that surface defects are capable of exerting. In this letter we propose, in order to resolve this issue, that if the defect potentials have a range of greater than an atomic spacing, the net force on a relatively stiff film due to the defects is likely to be extremely small. Line defects (e.g., step and facet edges and grain boundaries) as well as more localized defects (e.g., vacancies) are considered.