Characterization of bioconvection patterns formed by magnetotactic bacteria under a uniform magnetic field

Magnetospirillum gryphiswaldense is a magnetotactic bacterium that synthesizes a linear array of magnetic nanoparticles within its cytoplasm, called magnetosome. With a uniform external magnetic field, the motion of magnetotactic bacteria can be directed without applying forces thanks to the dipole moment granted by the magnetosome. We confine a dense suspension of M. gryphiswaldense between two parallel plane walls and apply a homogeneous magnetic field perpendicular to the confining walls. In this geometry, bacteria accumulate in both walls, depending on their general swimming direction (parallel or anti-parallel to magnetic field lines), and generate clusters due to hydrodynamic interactions. The flow produced by the confined swimming bacteria generate recirculating flows that result in bioconvection. Multiple of these recirculation cells alternate between the confining walls, with a characteristic separation length scale. In this work, we characterize the flow patterns formed by these swimmers, describing their formation time, characteristic wavelength, cluster size, and fluid flow produced around them.