Noncontact friction: Role of phonon damping and its nonuniversality

TL;DR

This paper investigates noncontact friction mechanisms, specifically phonon radiation and damping, revealing their different scaling behaviors and nonuniversality, which impacts interpretation of experimental data and system size scaling.

Contribution

The study extends analytical models of noncontact friction, confirms results with simulations, and highlights the nonadditive nature of phonon damping versus the additive phonon radiation.

Findings

Phonon radiation dominates at large distances.

Phonon damping dominates at small distances.

Dissipation can decrease with increasing surface area.

Abstract

While obtaining theoretical predictions for dissipation during sliding motion is a difficult task, one regime that allows for analytical results is the so-called noncontact regime, where a probe is weakly interacting with the surface over which it moves. Studying this regime for a model crystal, we extend previously obtained analytical results and confirm them quantitatively via particle based computer simulations. Accessing the subtle regime of weak coupling in simulations is possible via use of Green-Kubo relations. The analysis allows to extract and compare the two paradigmatic mechanisms that have been found to lead to dissipation: phonon radiation, prevailing even in a purely elastic solid, and phonon damping, e.g., caused by viscous motion of crystal atoms. While phonon radiation is dominant at large probe-surface distances, phonon damping dominates at small distances. Phonon radiation is furthermore a pairwise additive phenomenon so that the dissipation due to interaction with different parts (areas) of the surface adds up. This additive scaling results from a general one-to-one mapping between the mean probe-surface force and the friction due to phonon radiation, irrespective of the nature of the underlying pair interaction. In contrast, phonon damping is strongly nonadditive, and no such general relation exists. We show that for certain cases, the dissipation can even {\it decrease} with increasing surface area the probe interacts with. The above properties, which are rooted in the spatial correlations of surface fluctuations, are expected to have important consequences when interpreting experimental measurements, as well as scaling with system size.