Light-switchable propulsion of active particles with reversible interactions

Active or self-propelled colloidal-particle systems are currently a subject of great interest in soft condensed matter science, owing to their ability to mimic the collective behavior of complex living systems, but also to serve as model systems to study intrinsically out-of-equilibrium systems. Self-propelled particles can exhibit rich collective behavior, such as clustering, segregation, and anomalous density fluctuations, by consuming internal energy or extracting energy from their local environment in order to generate their own motion.  Control over the propulsion direction and switchability of the interactions between the individual self-propelled units may open new avenues in the designing materials from within. In this talk, we present a unique self-propelled particle system, consisting of half-gold coated Titania 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. These results would open new possibilities to drive the arrested systems (e.g. gels, and glasses) by “stirring with light”.