Theory of noncontact friction for atom-surface interactions

TL;DR

This paper analyzes noncontact (van der Waals) friction between atoms and surfaces, highlighting the dominance of backaction effects over direct electromagnetic fluctuation contributions across various materials and atom types.

Contribution

It provides a detailed theoretical investigation of noncontact friction, emphasizing the significance of backaction effects and comparing different atom-surface material interactions.

Findings

Backaction friction dominates over direct electromagnetic fluctuation friction.

Friction coefficients vary significantly among materials, with gold showing much smaller values.

Theoretical predictions cover hydrogen, helium, and metastable helium atoms with multiple surfaces.

Abstract

The noncontact (van der Waals) friction is an interesting physical effect which has been the subject of controversial scientific discussion. The "direct" friction term due to the thermal fluctuations of the electromagnetic field leads to a friction force proportional to 1/Z^5 where Z is the atom-wall distance). The "backaction" friction term takes into account the feedback of thermal fluctuations of the atomic dipole moment onto the motion of the atom and scales as 1/Z^8. We investigate noncontact friction effects for the interactions of hydrogen, ground-state helium and metastable helium atoms with alpha-quartz (SiO_2), gold (Au) and calcium difluorite (CaF_2). We find that the backaction term dominates over the direct term induced by the thermal electromagnetic fluctuations inside the material, over wide distance ranges. The friction coefficients obtained for gold are smaller than those for SiO_2 and CaF_2 by several orders of magnitude.