Magnetically controlled bacterial turbulence

Concentrated active agents can exhibit turbulent-like flows reminiscent of hydrodynamic turbulence. Despite its importance, the influence of external fields on active turbulence remains largely unexplored. Here we demonstrate the ability to control the swimming direction and active turbulence of Bacillus subtilis bacteria using external magnetic fields. The control mechanism leverages the magnetic torque experienced by the non-magnetic, rod-shaped bacteria in a magnetizable medium containing superparamagnetic nanoparticles. This allows aligning individual bacteria with the magnetic field, leading to a nematically aligned state over millimetric scales with minute transverse undulations and flows. Turning off the field releases the alignment constraint, leading to directly observable hydrodynamic instability of the dipole pushers. Our theoretical model predicts the intrinsic length scale of this instability, independent of the magnetic field, and provides a quantitative control strategy. Our findings suggest that magnetic fields and torques can be excellent tools for controlling non-equilibrium phase transitions in active systems.