Collective Hydrodynamics of Swimming Microorganisms: Living Fluids

Experimental observations indicate that, at sufficiently high cell densities, swimming bacteria exhibit coordinated motions on length scales (10 to 100 μm) that are large compared with the size of an individual cell but too small to yield significant gravitational or inertial effects. We discuss simulations of hydrodynamically interacting self-propelled particles as well as stability analyses and numerical solutions of averaged equations of motion for low Reynolds number swimmers. It has been found that spontaneous motions can arise in such systems from the coupling between the stresses the bacteria induce in the fluid as they swim and the rotation of the bacteria due to the resulting fluid velocity disturbances.