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

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.

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References in corpus (5)

• Self-motile colloidal particles: from directed propulsion to random walk
• Optical alignment and spinning of laser-trapped microscopic particles
• Phototaxis of synthetic microswimmers in optical landscapes
• Generalized energy equipartition in harmonic oscillators driven by active baths
• Direct measurement of thermophoretic forces

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• Emergent short-range repulsion for attractively coupled active particles
• Curvature dependent dynamics of a bacterium confined in a giant unilamellar vesicle
• Light-Activated Self-thermophoretic Janus Nanopropellers
• From Attraction to Repulsion: Emergent Interactions in Harmonically Coupled Active Binary System