  Suspensions of self-propelled particles, such as swimming micro-organisms,
are known to undergo complex dynamics as a result of hydrodynamic interactions.
This fluid dynamics video presents a numerical simulation of such a suspension,
based on a kinetic theory recently developed by Saintillan and Shelley (Physics
of Fluids, 20, 123403, 2008). Starting from a nearly uniform and isotropic
initial distribution, our simulations show the formation of strong density
fluctuations, which merge and break up in time in a quasiperiodic fashion.
These fluctuations are found to occur on the size of the simulation box, in
agreement with a prediction from a linear stability analysis. In addition, the
dynamics are characterized by complex chaotic flow fields, which result in
efficient fluid mixing.
