Role of long-range van der Waals interaction in the coefficient of static friction

TL;DR

This paper derives an analytical model showing how long-range van der Waals interactions influence the static friction coefficient between layered materials, emphasizing geometric factors over dielectric properties.

Contribution

It introduces a perturbative analytical expression for static friction based on surface corrugations and van der Waals forces, applicable to 2D materials like graphene.

Findings

The static friction coefficient depends on geometry, not dielectric properties.

Predicted $$ values align with experimental data for graphene and hBN.

Model can be extended to include other interlayer forces.

Abstract

To investigate the role of long-range van der Waals interactions in static friction, we derive an analytic expression for the coefficient of static friction $\mu_s$ between two thin layers of polarizable materials under zero load. For simplicity, we model the surface roughness with sinusoidal corrugations and calculate the interaction energy perturbatively up to the second order in corrugation amplitude. The ratio of corresponding maximum lateral Casimir force to normal Casimir force is defined as the coefficient of static friction, which is found to be independent of the dielectric properties of the materials. It depends on the geometric properties, like interlayer separation, corrugation amplitude, and wavelength of the corrugation. As a proof of concept, our predicted values of $\mu_s$ for the 2D van der Waals materials graphene and hexagonal boron nitride are in reasonable agreement with previously reported values in the literature. This simplistic model could be generalized by incorporating other forces, such as the frequency-dependent contributions of van der Waals interactions and electrostatic interactions.