Controlling stability and transport of magnetic microswimmers by an external field

TL;DR

This paper studies how external magnetic fields influence the stability and transport of magnetic microswimmers, revealing conditions for stability, instability, and reentrant behavior through combined theoretical and simulation approaches.

Contribution

It introduces a kinetic theory framework to analyze the hydrodynamic stability and transport of magnetic microswimmers under external fields, highlighting novel stability regimes.

Findings

Homogeneous polar state becomes unstable at high activity and magnetic field strengths.

External fields can cause partial depolarization and reduce transport speed.

Reentrant stability occurs at higher field strengths, restoring transport efficiency.

Abstract

We investigate the hydrodynamic stability and transport of magnetic microswimmers in an external field using a kinetic theory framework. Combining linear stability analysis and nonlinear 3D continuum simulations, we show that for sufficiently large activity and magnetic field strengths, a homogeneous polar steady state is unstable for both puller and pusher swimmers. This instability is caused by the amplification of anisotropic hydrodynamic interactions due to the external alignment and leads to a partial depolarization and a reduction of the average transport speed of the swimmers in the field direction. Notably, at higher field strengths a reentrant hydrodynamic stability emerges where the homogeneous polar state becomes stable and a transport efficiency identical to that of active particles without hydrodynamic interactions is restored.