Effective interactions between inclusions in an active bath of chiral swimmers

We investigate the effective two- and three-body interactions mediated between non-active colloidal inclusions immersed in an active bath of chiral or non-chiral self-propelled particles (swimmers). We perform Brownian Dynamics simulations within a standard model comprising hard inclusions and swimmers in two spatial dimensions. In the absence of chirality, we corroborate previous findings by showing that strong, repulsive, two-body forces of medium range (up to surface separations of a few swimmer radii) emerge between colloidal inclusions in the active bath. We provide further insight into this behavior by demonstrating that the two-body force profiles exhibit finer details with a primary maximum and a secondary hump at larger separations between the inclusions, stemming from the formation of circular high-density layers (or 'rings') of swimmers around the inclusions at high enough self-propulsion strength. The swimmer rings around the inclusions extend to radial distances of a few swimmer radii from the inclusion surface, giving the hard-core inclusions a soft repulsive 'shoulder'. It is the overlap (multiple overlaps) between these shoulders that leads to strong effective two- (three-) body interactions over an extended range between the inclusions in an active bath. We show that chirality results in significant swimmer depletion with depletion zones of large thickness (comparable to the inclusion size, or several swimmer radii) around the inclusions and thus, eventually, in a relatively long-ranged, active, chirality-induced depletion attraction at large enough swimmer chirality strengths. We show that the effective (non-pairwise) three-body force generated between the inclusions in the active bath can be quite large and even exceed the two-body force in magnitude, displaying also distinct repulsive and attractive regimes at intermediate to large swimmer self-propulsion strengths.