Conditions for the propulsion of a colloid surrounded by a mesoscale phase separation

TL;DR

This paper investigates how a chemically active colloid can propel itself through local phase separation of solute particles, with conditions mapped out via simulations, and extends findings to three-dimensional systems.

Contribution

It introduces a model linking colloid propulsion to local phase separation driven by chemical reactions and provides a comprehensive state diagram of activity conditions.

Findings

Propulsion depends on the filling fraction of the reaction area.

Activity occurs under specific parameter regimes identified in the state diagram.

Propulsion is also observed in three-dimensional geometries.

Abstract

We study a two-dimensional model of an active isotropic colloid whose propulsion is linked to the interactions between solute particles of the bath. The colloid catalyzes a chemical reaction in its vicinity, that yields a local phase separation of solute particles. The density fluctuations of solute particles result in the enhanced diffusion of the colloid. Using numerical simulations, we thoroughly investigate the conditions under which activity occurs, and we establish a state diagram for the activity of the colloid as a function of the parameters of the model. We use the generated data to unravel a key observable that controls the existence and the intensity of activity: the filling fraction of the reaction area. Remarkably, we finally show that propulsion also occurs in three-dimensional geometries, which confirms the interest of this mechanism for experimental applications.