Hydrodynamic interactions in active suspensions*

Fluid-mediated interactions lead to new modes of locomotion in suspensions of swimming cells, from small bacteria in confined spaces to larger aquatic organisms in bulk fluids. In this talk, I will present recent work in my group where we study theoretically the impact the hydrodynamic interactions on motility. First, in collaboration with C. Fradin and K. Dalnoki-Veress at McMaster University, I will report on a new type of collective motion where a uniform distribution of magnetotactic bacteria is rendered unstable by a magnetic field due solely to hydrodynamic interactions among the cells and with boundaries. I will next explain how hydrodynamic interactions between swimmers and surfaces can be used to separate motile cells that have different types of far-field hydrodynamic signatures. I will finally consider biased swimmers (e.g. phototactic or gyrotactic) and show how this bias can lead to new instabilities in cell suspensions.