Isotropic active colloids: explicit vs. implicit descriptions of propulsion mechanisms

TL;DR

This study compares explicit and implicit modeling approaches for isotropic active colloids driven by solute interactions, revealing that implicit models may fail to capture collective effects observed in explicit simulations.

Contribution

It introduces a detailed explicit model of active colloids based on solute interactions and compares it with an implicit model derived from the explicit one.

Findings

Effective diffusion decreases with density in explicit models.

Implicit models do not capture the density-dependent diffusion reduction.

Classical models may fail to describe collective dynamics in such systems.

Abstract

Modeling the couplings between active particles often neglects the possible many-body effects that control the propulsion mechanism. Accounting for such effects requires the explicit modeling of the molecular details at the origin of activity. Here, we take advantage of a recent two-dimensional model of isotropic active particles whose propulsion originates from the interactions between solute particles in the bath. The colloid catalyzes a chemical reaction in its vicinity, which results in a local phase separation of solute particles, and the density fluctuations of solute particles cause the enhanced diffusion of the colloid. In this paper, we investigate an assembly of such active particles, using (i) an explicit model, where the microscopic dynamics of the solute particles is accounted for; and (ii) an implicit model, whose parameters are inferred from the explicit model at infinite dilution. In the explicit solute model, the effective diffusion coefficient of the active colloids strongly decreases with density, an effect which is not captured by the derived implicit model. This suggests that classical models, which usually decouple pair interactions from activity, fail to describe collective dynamics in active colloidal systems driven by solute-solute interactions.