Emergent pattern formation of active magnetic suspensions in an external field

TL;DR

This paper investigates how active magnetic suspensions self-organize under external magnetic fields, revealing various instability patterns and dynamic states through a combined analytical and simulation approach.

Contribution

It introduces a mesoscopic continuum model for active magnetic suspensions and analyzes pattern formation and stability under different activity and magnetic field conditions.

Findings

High activity and moderate magnetic fields lead to instability and pattern formation.

Different patterns emerge for pushers and pullers, including traveling bands and pillar-like regions.

Strong magnetic fields can stabilize the homogeneous polar state, causing reentrant stability.

Abstract

We study collective self-organization of weakly magnetic active suspensions in a uniform external field by analyzing a mesoscopic continuum model that we have recently developed. Our model is based on a Smoluchowski equation for a particle probability density function in an alignment field coupled to a mean-field description of the flow arising from the activity and the alignment torque. Performing linear stability analysis of the Smoluchowski equation and the resulting orientational moment equations combined with non-linear 3D simulations, we provide a comprehensive picture of instability patterns as a function of strengths of activity and magnetic field. For sufficiently high activity and moderate magnetic field strengths, the competition between the activity-induced flow and external magnetic torque renders a homogeneous polar steady state unstable. As a result, four distinct dynamical patterns of collective motion emerge. The instability patterns for pushers include traveling bands governed by bend-twist instabilities and dynamical aggregates. For pullers, finite-sized and system spanning pillar-like concentrated regions predominated by splay deformations emerge which migrate in the field direction. Notably, at very strong magnetic fields, we observe a reentrant hydrodynamic stability of the polar steady state.