Anisotropic particles near surfaces: Self-propulsion and friction

TL;DR

This paper develops a theoretical framework to analyze self-propulsion forces on anisotropic particles near surfaces due to Casimir effects in thermal non-equilibrium, revealing significant near-field forces that could be experimentally observed.

Contribution

It derives a general formula for Casimir-induced self-propulsion forces on small objects near surfaces and explores their dependence on distance and shape, linking to fluctuation-dissipation relations.

Findings

Self-propulsion force increases as the particle approaches the surface.

Lateral forces on hot spheroids can match gravitational forces.

Identifies a correction term in friction related to linear response theory.

Abstract

We theoretically study the phenomenon of self-propulsion through Casimir forces in thermal non-equilibrium. Using fluctuational electrodynamics, we derive a formula for the self-propulsion force for an arbitrary small object in two scenarios, i) for the object being isolated, and ii) for the object being close to a planar surface. In the latter case, the self-propulsion force (i.e., the force parallel to the surface) increases with decreasing distance, i.e., it couples to the near-field. We numerically calculate the lateral force acting on a hot spheroid near a surface and show that it can be as large as the gravitational force, thus being potentially measurable in fly-by experiments. We close by linking our results to well-known relations of linear response theory in fluctuational electrodynamics: Looking at the friction of the anisotropic object for constant velocity, we identify a correction term that is additional to the typically used approach.