Rotation reversal of chiral bacterial vortices

It is well established that many flagellated bacteria, such as Escherichia coli, swim in clockwise circles above rigid surfaces. However, in a cylindrical microwell with asymmetric top-bottom boundary conditions, such that bacteria segregate into two populations of differing sizes at opposing flat boundaries, the smaller bacterial vortex has been observed to rotate in the opposite direction to that expected in the absence of the other population [K. Beppu, Z. Izri, T. Sato, Y. Yamanishi, Y. Sumino and Y. T. Maeda, Proc. Natl. Acad. Sci. U. S. A., 2021, 118, e2107461118]. Motivated by these observations, we employ flow singularities to investigate the motion of a population of chiral swimmers near one flat boundary of a cylindrical geometry, subject to the flows generated by a bacterial vortex at the opposing surface. We show numerically that, purely due to hydrodynamic interactions, the rotational direction of the bacterial population reverses in the presence of a sufficiently large vortex on the opposite boundary. Our numerical results are fully explained by an analytical theory in the continuum limit, which captures the essential hydrodynamic interactions governing the observed reversal.