Spontaneous propulsion of an isotropic colloid in a phase-separating environment

TL;DR

This paper demonstrates how isotropic colloids can spontaneously propel in a phase-separating environment due to interactions with solute particles, leading to directed motion and enhanced diffusion, challenging the need for anisotropy in active colloids.

Contribution

It introduces a minimal model showing spontaneous propulsion of isotropic colloids driven by environmental interactions and phase separation effects.

Findings

Spontaneous phase transition near the colloid induces directed motion.

Interactions cause significant enhancement in colloid diffusion.

Effective Langevin equation captures self-propulsion and diffusion contributions.

Abstract

The motion of active colloids is generally achieved through their anisotropy, as exemplified by Janus colloids. Recently, there was a growing interest in the propulsion of isotropic colloids, which requires some local symmetry breaking. Although several mechanisms for such propulsion were proposed, little is known about the role played by the interactions within the environment of the colloid, which can have a dramatic effect on its propulsion. Here, we propose a minimal model of an isotropic colloid in a bath of solute particles that interact with each other. These interactions lead to a spontaneous phase transition close to the colloid, to directed motion of the colloid over very long timescales and to significantly enhanced diffusion, in spite of the crowding induced by solute particles. We determine the range of parameters where this effect is observable in the model, and we propose an effective Langevin equation that accounts for it and allows one to determine the different contributions at stake in self-propulsion and enhanced diffusion.