(322b) Quantifying and Understanding Velocity and Stress Correlations in Suspensions of Swimming Microorganisms

Large collections of swimming microorganisms are able to produce collective motions on a scale much larger than the scale of a single organism. In particular, the collective behavior leads to fluid structures larger than the size of an organism, enhanced transport in the fluid, and enhanced stress fluctuations which produce altered rheological properties. It is important to understand the mechanism for these collective behavior because of the possible importance in swarming, biofilm formation, and bacterial infections. A better understanding would also allow for design of artificial systems consisting of interacting swimming agents.

We have developed a new theory which shows how some of these phenomena arise from the hydrodynamic interactions between the organisms and compare the predictions with the results from experiments and computer simulations. In this way we can understand how the behavior scales with concentration, the importance of the method of swimming used, the influence of run-and-tumble like motions of the organisms, and how the interactions can lead to large-scale fluid structures. For example, in periodic geometries, the orientational correlations of the microorganisms lead to large-scale fluid structures that depend logarithmically on the simulation box size. These orientational correlations seem to dominate other proposed mechanisms such as quadrupole interactions. For organisms that do not tumble these interactions occur even for very low concentrations, consistent with microrheology experiments.