Actuated rheology of magnetic micro-swimmers suspensions : emergence of motor and brake states

TL;DR

This paper investigates how magnetic fields influence the rheological behavior of micro-swimmer suspensions, revealing controllable motor and brake states driven by magnetic actuation and swimmer activity.

Contribution

It introduces a model for magnetic micro-swimmers under shear, demonstrating how magnetic fields induce a shear stress that can act as a motor or brake depending on conditions.

Findings

Magnetic field induces a shear stress dependent on swimmer activity.

Low magnetic fields produce a linear shear stress response.

The system allows control of fluid rheology via magnetic actuation.

Abstract

We study the effect of magnetic field on the rheology of magnetic micro-swimmers suspensions. We use a model of a dilute suspension under simple shear and subjected to a constant magnetic field. Particle shear stress is obtained for both pusher and puller types of micro-swimmers. In the limit of low shear rate, the rheology exhibits a constant shear stress, called actuated stress, which only depends on the swimming activity of the particles. This stress is induced by the magnetic field and can be positive (brake state) or negative (motor state). In the limit of low magnetic fields, a scaling relation of the motor-brake effect is derived as a function of the dimensionless parameters of the model. In this case, the shear stress is an affine function of the shear rate. The possibilities offered by such an active system to control the rheological response of a uid are finally discussed.