Magnetically controllable bacterial turbulence*

Self-sustained turbulent flow, the so-called active turbulence, is a hallmark of dense active matter systems where individual units are capable of self-propelling. Although controlling active turbulence has long been a challenge due to its importance in fundamental physical understanding and in diverse applications, the influence of external fields on such chaotic self-organization remains largely unexplored. Here, we show that the active turbulence of swimming bacteria can be driven into a nematic-aligned state over long distances by doping magnetic nanoparticles and applying a uniform magnetic field. This stems from the magnetic torque exerted on the rod-shaped non-magnetic void bacteria create in the magnetic fluid. Intriguingly, the nematic phase is accompanied by flows perpendicular to the magnetic field due to active stress by dipole pushers, which induce orientation undulation with a characteristic wavelength independent of the magnetic field strength. This indicates that the intrinsic nature of active turbulence is elicited by magnetically tuning alignments of bacteria. We further corroborate the experimental result with a hydrodynamic model for self-propelled particles and provide a versatile strategy to spatiotemporally control the collective states in active systems.