Symmetric shear bands and collective swarming of bacterial suspensions*

Active fluids are a novel class of non-equilibrium complex fluids with examples across a wide range of biological and physical systems such as flocking animals, swarming microorganisms, vibrated granular rods, and suspensions of synthetic colloidal swimmers. Different from familiar non-equilibrium systems where free energy is injected from boundaries, an active fluid is a dispersion of large numbers of self-propelled units, which convert the ambient/internal free energy and maintain non-equilibrium steady states at microscopic scales. Due to this distinct feature, active fluids exhibit fascinating and unusual flow behaviors unseen in conventional complex fluids. Here, by combining high-speed confocal microscopy, rheological measurements and biochemical engineering, we experimentally investigate the flow behaviors of E. coli suspensions, a premier example of active fluids. In particular, we show the microscopic dynamics of bacterial suspensions associated with the abnormal rheology and the emergence of collective swarming. Using theoretical tools of fluid mechanics and statistical mechanics, we develop a quantitative understanding of these interesting behaviors. Our study shows the general organizing principles of active fluids that can be exploited for designing “smart” fluids with controllable fluid properties. Our results also provide new insights into the fundamental transport processes of microbiological systems.