Formation of Active Two-Dimensional Fluids by Multicellular Magnetotactic Bacteria

We characterize a new phase of active matter composed of Multicellular Magnetotactic Bacteria (MMB). After colliding with a surface in a weak magnetic field, MMB escape into the bulk fluid at random angles; MMB are exponentially distributed about the surface. At high magnetic fields, MMB self organize as an active two-dimensional fluid on the surface. Unlike previously reported microbial active fluids, contact with the surface enhances rotational diffusion by a factor of eight and velocity fluctuations are Laplace distributed. The active fluid undergoes a percolation transition at a critical magnetic field. Supercritical active fluids exhibit dynamic voids, which limit the relaxation of density fluctuations.