Title: Effective Properties and Collective Dynamics in Bacterial Suspensions
Speaker: Shawn D. Ryan (Postdoctoral Research Associate, Department of Mathematical Sciences / Liquid Crystal Institute, Kent State University)
Abstract: The study of collective motion in bacterial suspensions has been of significant recent interest. A simple PDE/ODE model for the suspension is introduced to better understand the non-trivial spatial and temporal correlations emerging through the course of collective swimming. A bacterium is represented as a point force dipole subject to two types of interactions: hydrodynamic and excluded volume (collisions). The results of direct particle simulations confirm striking experimental observations: correlations are independent of the concentration and swimming speed past the critical concentration threshold for collective motion. Instead, correlation properties are solely determined by the particle size and shape. Analysis of the model shows that the alignment of asymmetrical particles and the presence of self-propulsion also gives rise to a drastic reduction in the effective viscosity of the suspension. An explicit asymptotic formula for the effective viscosity in terms of known physical parameters is derived using a kinetic approach. This work exemplifies the delicate balance between hydrodynamic interactions and collisions governing collective motion in bacterial suspensions and provides important insights into its mesoscopic nature.