Friction on layered media: How deep do phonons reach?

TL;DR

This paper theoretically investigates how phonons contribute to friction on coated surfaces, revealing that different phonon modes—traveling and evanescent—affect energy dissipation differently depending on coating thickness and material properties.

Contribution

It introduces a viscoelastic theory to analyze phonon modes involved in friction on layered media, distinguishing the roles of various excitations and their dependence on coating parameters.

Findings

Traveling (3D) spherical waves carry energy away from the probe.

Evanescent waves convert energy into heat in the near-field.

Friction behavior varies with coating thickness and material properties.

Abstract

We theoretically study the frictional damping of a small probe object on a coated planar surface, analyzing the resulting phonon modes via a theory of viscoelasticity. Three different types of excitations are found to contribute to friction in distinct ways: traveling (3D) spherical waves, traveling (2D) surface waves, and evanescent waves. While traveling waves transport energy away from the probe, determined by long range elastic properties (wavelength), evanescent waves transform energy into heat in a near-field range, characterized by the size of the probe. Thus, fundamentally different behaviors are predicted, depending on coating thickness and material properties.