Light-switchable propulsion of active particles with reversible interactions

TL;DR

This study introduces a light-controlled active particle system with reversible propulsion and interactions, enabling dynamic assembly, disassembly, and control over collective behaviors in active matter.

Contribution

We developed a photocatalytic active particle system with reversible propulsion and switchable interactions, allowing real-time control over collective phenomena.

Findings

Reversal of propulsion direction changes hydrodynamic interactions from attractive to repulsive.

Particles can induce fusion and fission in assemblies.

Active colloids can act as nucleation sites for passive particles.

Abstract

Active systems such as microorganisms and self-propelled particles show a plethora of collective phenomena, including swarming, clustering, and phase separation. Control over the propulsion direction and switchability of the interactions between the individual self-propelled units may open new avenues in designing of materials from within. Here, we present a self-propelled particle system, consisting of half-gold coated titania ($\mathrm{TiO}_2$) particles, in which we can fast and on-demand reverse the propulsion direction, by exploiting the different photocatalytic activities on both sides. We demonstrate that the reversal in propulsion direction changes the nature of the hydrodynamic interaction from attractive to repulsive and can drive the particle assemblies to undergo both fusion and fission transitions. Moreover, we show these active colloids can act as nucleation sites, and switch rapidly the interactions between active and passive particles, leading to reconfigurable assembly and disassembly. Our experiments are qualitatively described by a minimal hydrodynamic model.