Magnetic control of magnetotactic bacteria swarms

Controllable swarms of active particles are considered prospective for object manipulation in general and as payload carriers in fluidic microenvironments in particular. However, a theoretical description of finite-sized swarms is missing, as opposed to continuous fields of active particles. Here, we thus develop a torque dipole-based hydrodynamics with spin model for the motion of active particle swarms in an external magnetic field. We apply the developed model to swarms of magnetotactic bacteria. We experimentally observe the motion of magnetotactic bacteria swarms perpendicular to the applied in-plane magnetic field. We use our model to explain that the swarms move according to the left-hand rule in combined vertical and lateral magnetic fields and that this movement originates from hydrodynamic interactions within the torque dipole ensembles. We also report the linear scaling of the swarm motion velocity with the driving magnetic field magnitude, and are able to derive the propulsion torque of the active particles from the experiments. Our results are applicable to other types of active particles and driving fields, and will pave the way for further research into harnessing active particle swarms as collective robots.