Non-contact Friction and Relaxational Dynamics of Surface Defects

TL;DR

This paper investigates non-contact friction in cantilever-surface interactions, proposing a defect relaxation mechanism that explains large observed damping and distance dependence, aligning well with experimental data.

Contribution

It introduces a novel mechanism based on surface defect relaxation dynamics to explain non-contact friction, surpassing electromagnetic fluctuation theories.

Findings

The proposed model accounts for the magnitude of NCF observed.

It explains the distance dependence of non-contact friction.

The model shows good agreement with experimental results.

Abstract

Motion of cantilever near sample surfaces exhibits additional friction even before two bodies come into mechanical contact. Called non-contact friction (NCF), this friction is of great practical importance to the ultrasensitive force detection measurements. Observed large NCF of a micron-scale cantilever found anomalously large damping that exceeds theoretical predictions by 8-11 orders of magnitude. This finding points to contribution beyond fluctuating electromagnetic fields within van der Waals approach. Recent experiments reported by Saitoh et al. (Phys. Rev. Lett. 105, 236103 (2010)) also found nontrivial distance dependence of NCF. Motivated by these observations, we propose a mechanism based on the coupling of cantilever to the relaxation dynamics of surface defects. We assume that the surface defects couple to the cantilever tip via spin-spin coupling and their spin relaxation dynamics gives rise to the backaction terms and modifies both the friction coefficient and the spring constant. We explain the magnitude, as well as the distance dependence of the friction due to these backaction terms. Reasonable agreement is found with the experiments.