Anomalous rheology of puller-type microswimmer suspensions

TL;DR

This study uses hydrodynamic simulations to uncover how puller-type microswimmers' contractile flow fields lead to unique alignment and rheological behaviors, especially near boundaries, contrasting with pusher-type swimmers.

Contribution

It reveals the impact of swimmer type, shape, and confinement on suspension rheology, highlighting the importance of microswimmer characteristics in active fluid dynamics.

Findings

Puller swimmers align vertically due to hydrodynamic interactions.

Boundary effects amplify local swimmer density and influence rheology.

Swimmer aspect ratio and confinement significantly affect steady-state behavior.

Abstract

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 cause the swimmers to align vertically. 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.