Anomalous rheology of puller-type microswimmer suspensions

We explore the mechanism underlying the anomalous rheology of puller-type microswimmer suspensions through direct hydrodynamic simulations. Puller-type swimmers generate contractile flow fields along their swimming direction, leading to hydrodynamic interactions that promote a tendency for head-to-tail alignment among the swimmers. Our simulations reveal that this alignment effect, along with the resultant orientational order of swimming motion, becomes particularly pronounced near boundary walls, where local swimmer density is amplified, predominantly controlling the overall swimming dynamics and rheological properties of the suspension. These findings contrast with our previous simulations of pusher-type swimmers, which hydrodynamically interact through extensile flow fields, whereby they exhibit weak orientational order in the bulk region, which primarily determines their steady-state properties. Furthermore, we demonstrate that the steady-state behavior near the walls is strongly influenced by the aspect ratio of the microswimmers and the degree of confinement between the walls. Our results highlight the crucial role of microswimmer characteristics, such as shape and swimming mechanisms, in determining the rheological properties of active suspensions.