Influence of non-conservative optical forces on the dynamics of optically trapped colloidal spheres: The fountain of probability

TL;DR

This paper shows that colloidal spheres in optical tweezers do not reach equilibrium but instead exhibit a steady-state vortex motion caused by non-conservative optical forces, acting as Brownian motors.

Contribution

It provides both experimental and theoretical evidence that non-conservative optical forces induce non-equilibrium steady states in trapped colloidal particles.

Findings

Colloidal spheres in optical tweezers exhibit vortex-like probability flux.

Non-conservative forces bias thermal fluctuations, preventing equilibrium.

Trapped spheres act as Brownian motors due to optical forces.

Abstract

We demonstrate both experimentally and theoretically that a colloidal sphere trapped in a static optical tweezer does not come to equilibrium, but rather reaches a steady state in which its probability flux traces out a toroidal vortex. This non-equilibrium behavior can be ascribed to a subtle bias of thermal fluctuations by non-conservative optical forces. The circulating sphere therefore acts as a Brownian motor. We briefly discuss ramifications of this effect for studies in which optical tweezers have been treated as potential energy wells.