Depletion effect in an active chiral system†

Abstract

We investigate the dynamics of ring-shaped chiral particles immersed in a monolayer of hard spherical beads, confined between two vertically vibrating plates. At high bead concentrations, we observe depletion-like attraction between the rings in both co-rotating and counter-rotating configurations, leading to the formation of a dimer. Notably, the range of this interaction extends over several bead diameters, in contrast to passive systems where interactions are typically limited to the particle size. More intriguingly, at low bead concentrations, the rings exhibit mutual repulsion in both cases, which we attribute to a localized high bead concentration between them—suggesting a mechanism opposite to conventional depletion interactions. Additionally, we examine the diffusive properties of dimerized rings and compare them with those of an isolated ring. While the angular diffusion of the dimer is significantly suppressed compared to that of a single ring, its translational diffusion remains nearly unchanged.