Emergent collective dynamics in suspensions of swimming bacteria

Self-organization is one of the most important problems in biology and a central issue in contemporary non-equilibrium physics. The fundamental question here is how local interactions between individual components lead to the observed collective behavior.;Suspensions of swimming bacteria Bacillus subtilis were used as test systems to examine dynamic properties of emergent collective states in controlled, state-of-the-art physical experiments.;We discovered that at concentrations near the maximum allowed by steric repulsion, swimming bacteria form a dynamical state exhibiting extended spatiotemporal coherence. The concentration dependence of correlations in the collective state is probed here with a novel technique that herds bacteria into condensed populations of adjustable concentration. For the particular thin-film geometry employed, the correlation lengths vary smoothly and monotonically during the transition from individual to collective behavior. We investigated a specific model incorporating hydrodynamic interactions in thin-film geometries and showed by numerical studies that the model displays large scale persistently recurring vortices, consistent with experimental observation.;High-resolution optical coherence tomography was used to study the onset of large-scale convective motions in free-standing thin films of adjustable thickness containing the bacterial suspension. Clear evidence was found that beyond a threshold film thickness there exists a transition from quasi-two-dimensional collective swimming to three-dimensional turbulent behavior. The latter state is characterized by enhanced diffusivities of oxygen and bacteria. These results emphasize the impact of self-organized large-scale bacterial locomotion on the onset of three-dimensional dynamics.;Furthermore, measurements of the shear viscosity in suspensions of swimming bacteria in free standing liquid films have revealed that the viscosity can decrease by up to a factor of seven compared to the viscosity of the same liquid without bacteria or with non-motile bacteria. The reduction in viscosity was observed in two complimentary experiments: studying the decay of a large vortex and measuring the viscous torque on a rotating magnetic particle. The measured viscosity depends on the concentration and swimming speed of the bacteria.