Magnetic control of orientational order and intrinsic hydrodynamic instability in bacterial turbulence

TL;DR

This study demonstrates how a uniform magnetic field can control bacterial turbulence and induce nematic order in Bacillus subtilis, revealing an intrinsic length scale of instability unaffected by magnetic strength.

Contribution

It introduces a novel method of controlling active bacterial turbulence using magnetic torques, linking external magnetic fields to collective bacterial behavior.

Findings

Magnetic fields induce nematic alignment in bacterial turbulence.

The length scale of undulation remains constant regardless of magnetic field strength.

Hydrodynamic model predicts an intrinsic instability length scale independent of magnetic influence.

Abstract

Highly concentrated active agents tend to exhibit turbulent flows, reminiscent of classical hydrodynamic turbulence, which has attracted considerable attention lately. Controlling the so-called active turbulence has long been a challenge, and the influence of external fields on such chaotic self-organization remains largely unexplored. Here we report on active turbulence of Bacillus subtilis bacteria controlled by a uniform magnetic field via a magnetizable medium based on magnetic nanoparticles. The rod-shaped bacteria act as non-magnetic voids in the otherwise magnetic medium, allowing magnetic torques to be generated on their bodies. This leads to an externally controllable nematic alignment constraint that further controls bacterial turbulence into a nematic state. The nematic orientational ordering in the direction parallel to the magnetic field is accompanied by transverse flows owing to active stress by dipole pushers, which induce undulation of the nematic state. Remarkably, the typical length of the undulation is almost independent of the magnetic field strength. Our theoretical model based on the hydrodynamic equations for suspensions of self-propelled particles predicts the intrinsic length scale of hydrodynamic instability independent of the magnetic field. Our findings suggest that magnetic torques are a powerful approach for controlling both individual agents and their collective states in active systems.