Magnetotactic bacteria droplets : a controllable motor

We confine magnetotactic bacteria in spherical droplets suspended in oil. The application of a uniform and constant magnetic field induces a focusing of the bacteria at the North and South poles of the droplets due to both alignment with the magnetic field and confinement by the droplet curvature. The focusing of the bacteria at these particular points can eventually create a vortex when the bacteria are densely packed. The spatial characteristics of the vortex flow are extracted with PIV analysis. The dependences of the vortex flow with respect to the magnetic field strength and the droplets radii are quantitatively analyzed and correlated to the flow created outside the droplet by the collective rotation of bacteria. The outstanding applications allowed by such a system are finally discussed in terms of energy harvesting.