Which Interactions Dominate in Active Colloids?

TL;DR

This paper introduces a simplified model showing that phoretic interactions, driven by gradients used for swimming, are crucial and often dominate hydrodynamic interactions in active colloids, leading to dynamic clustering.

Contribution

The authors derive a pair-interaction model for phoretic interactions controlled mainly by swimming speed, highlighting their importance over hydrodynamics in active colloids.

Findings

Phoretic interactions are generally significant unless strongly screened.

The model predicts dynamic clustering at low densities.

Clustering results from screened phoretic attractions and active diffusion.

Abstract

Despite a mounting evidence that the same gradients which active colloids use for swimming, induce important cross-interactions (phoretic interaction), they are still ignored in most many-body descriptions, perhaps to avoid complexity and a zoo of unknown parameters. Here we derive a simple model, which reduces phoretic far-field interactions to a pair-interaction whose strength is mainly controlled by one genuine parameter (swimming speed). The model suggests that phoretic interactions are generically important for autophoretic colloids (unless effective screening of the phoretic fields is strong) and should dominate over hydrodynamic interactions for the typical case of half-coating and moderately nonuniform surface mobilities. Unlike standard minimal models, but in accordance with canonical experiments, our model generically predicts dynamic clustering in active colloids at low density. This suggests that dynamic clustering can emerge from the interplay of screened phoretic attractions and active diffusion.