Viscoplastic interactions tune collective dynamics of microswimmers

Viscoplastic fluids flow only when stress exceeds the yield stress, confining swimmer-induced flows within the yield surface and limiting far-field hydrodynamic interactions. We investigate how yield stress tunes collective dynamics of microswimmers in monolayer suspensions. Higher yield stress shifts the phase transition in puller suspensions to higher densities and weakens flocking of spherical neutral swimmers, governed by the ratio of yield stress to swimmer density. In ellipsoidal swimmers, collisions dominate, making yield stress less influential. These systems constitute a distinct category in which hydrodynamic interactions are cut off, providing new insight into the long-standing debate about the impact of far- and near-field hydrodynamics on collective dynamics of microswimmers.