Non-equilibrium Properties of an Active Nanoparticle in a Harmonic Potential

TL;DR

This paper experimentally demonstrates that active nanoparticles in a harmonic potential exhibit non-equilibrium behaviors beyond effective temperature increases, including orbital rotations, due to self-propulsion mechanisms related to particle shape and critical solute conditions.

Contribution

The study provides the first experimental observation of non-equilibrium orbital rotations of active nanoparticles in a harmonic potential, explained through a theoretical Fokker-Planck model.

Findings

Transition from Boltzmann to non-equilibrium distribution with increased laser power

Observation of fast orbital rotations around the beam axis

Self-propulsion driven by particle shape and critical solute properties

Abstract

Active particles break out of thermodynamic equilibrium thanks to their directed motion, which leads to complex and interesting behaviors in the presence of confining potentials. When dealing with active nanoparticles, however, the overwhelming presence of rotational diffusion hinders directed motion, leading to an increase of their effective temperature, but otherwise masking the effects of self-propulsion. Here, we demonstrate an experimental system where an active nanoparticle immersed in a critical solution and held in an optical harmonic potential features far-from-equilibrium behavior beyond an increase of its effective temperature. When increasing the laser power, we observe a cross-over from a Boltzmann distribution to a non-equilibrium state, where the particle performs fast orbital rotations about the beam axis. These findings are rationalized by solving the Fokker-Planck equation for the particle's position and orientation in terms of a moment expansion. The proposed self-propulsion mechanism results from the particle's non-sphericity and the lower critical point of the solute.