Control of cohesive states in colloidal chiral fluids through chemical interactions

Ensembles of spinning particles suspended within a fluid medium represent a distinctive class of active matter systems known as chiral fluids. Recently, a densely packed chiral fluid has been realized, featuring spinning colloidal magnets driven by an externally applied rotating magnetic field. These colloidal particles engage in complex interactions, combining magnetic dipolar and hydrodynamic forces, to collectively assemble into distinct circulating clusters characterized by unidirectional edge flows. Here we present a novel approach to externally govern the collective behavior of spinning colloids by introducing additional diffusiophoretic interactions amongst geometrically anisotropic colloidal components. Specifically, ellipsoidal hematite micro-particles are demonstrated to catalyze the decomposition of H₂O₂ when exposed to UV light, instigating nearby chemical gradients. This, in turn, gives rise to an axi-asymmetric interaction around individual particles, wherein attractive dipolar interactions dominate along the direction of the magnetic moment, while repulsive diffusiophoretic interactions prevail laterally. At the collective level, this supplementary interaction induces a loss in the structural cohesiveness of the circulating clusters, thereby promoting their expansion while retaining their interconnected network architecture. Crucially, this modulation process is fully reversible, offering precise external control over the emergent dynamics in dense chiral fluids.