Control of cohesive states in colloidal chiral fluids*

Active matter systems are composed of autonomous interacting units that continuously dissipate energy, exerting mechanical forces and stresses. Several non-equilibrium phenomena emerge in these systems, governed by the interplay between self-propulsion, thermal fluctuations, and pairwise interactions. In contrast to self-propelling particles, spinning particles in fluids constitute a new class of active matter systems which exhibit coherent dynamical structures through hydrodynamic interactions. A recent experimental realization is a dense chiral fluid composed of spinning colloidal magnets driven by a uniform external rotating magnetic field. These particles couple both via dipolar and hydrodynamic interactions and organize into circulating clusters with unidirectional edge flows. Here we report a mechanism to externally control the collective states of spinning magnetic particles by introducing additional diffusio-osmotic interactions. At a collective scale, we show that this additional interaction leads to a loss in cohesivity in circulating clusters and promotes reversible expansion of the rotating cluster vortex. We identify an activity induced colloidal chain-branching mechanism that mediates the transition between a circulating cluster and its expanded state, which is responsible for the loss in cohesivity. Introduction of chemical activity-based interactions in chiral fluids paves the way for a new paradigm of self-organization routes in chiral active matter.