Les bactéries magnétotactiques en tant que fluide actif dirigé : du comportement individuel vers des effets collectifs

.We report the work we lead on magneto tactic bacteria, from the point of view of active matter. The ability of this bacterium to swim at 100µm/s directed by the magnetic field makes it a good candidate to study driven active matter. Indeed, in this configuration, the self-propelled system is not dragged by an external force, and its directed motion comes from its biased orientation. We choose the strain MC-1 for our study, for the robustness of its individual behavior and its swimming speed. We studied the individual behavior, confirming previous results where the bacteria passively aligns on the magnetic field being disoriented solely by the magnetic field, but also succeeded in triggering activity in their reorientation, suspending it in different chemical environments, or directing them against a solid interface, where this bacteria could tumble. This tumbling behavior, very common amongst non-magnetic bacteria, was not reported for Mangetotactic bacteria. These new results leaded us to develop a model of Run and Tumble under a magnetic field. We studied their behavior when densely concentrated in a micro-channel, in jammed configuration, using standard microfluidics tools. We observed their motion in hour glass shaped micro-channels, without any flow, and characterized the chronology of the jamming process. We investigated the interaction of their swim with a shear, in a counter flow experiments, where MC-1 would be directed against a Poiseuille flow. Due to equilibrium between the magnetic torque and the hydrodynamic shear, bacteria would focus instantly in the middle of the channel. We studied this phenomenon theoretically, and checked our model with the experiments. We discovered a instability of a new kind in the same configuration, for high magnetic fields. Indeed, beyond a threshold the focused suspension would become wavy to end up in segregated droplets of bacteria. We characterized experimentally this phenomenon which reminds us of Rayleigh-Plateau and Kelvin-Helmholtz instabilities, varying the flow rate, the Magnetic field and the density of the suspension. Recirculation in the droplets is observed and explained. We interpret these convection droplets as the source of the instability of the focused suspensio