Kinetics and Order-Disorder Transitions in Hydrodynamically Self-Assembled Magnetotactic Bacteria

Magnetotactic bacteria are inherently magnetic motile micro-organisms. Hence, their swimming orientations are readily controlled by external magnetic fields making them a valuable model active matter system. When oriented perpendicular to a surface, they experience attractive hydrodynamic interactions that result in spontaneous self-organization (clustering). By tuning the cell density and magnetic field strength, this ordering may be systematically removed and restored, permitting construction of a phase boundary defining the onset of ordering and experimental exploration of cluster kinetics. The self-organization process is quantified using an order-parameter independent approach based on lossless compression algorithms (Lempel-Ziv), as well as a more conventional method employing the radial distribution function. These analyses reveal that the clusters scale logarithmically in time, representing a non-equilibrium analog of the “self-focusing” regime of charged colloids, and implying that the interplay between hydrodynamic attraction and magnetic repulsion control the kinetics. Furthermore connections between experimentally determined pair-wise interactions and the many-body dynamics, as well as the role viscous dissipation plays in stabilizing the structures will be discussed.