From Flocking Birds to Swarming Bacteria: A Study of the Dynamics of Active Fluids

Xiang Cheng, Assistant Professor, Chemical Engineering and Materials Science, University of Minnesota

Link to Video

https://umn.webex.com/umn/ldr.php?RCID=2b94b5fda91f2dde19db0b6d126600ec

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
behaviors unseen in conventional complex fluids. Here, by combining high-speed confocal microscopy,
holographic imaging, rheological measurements and biochemical engineering, we experimentally
investigate the dynamics of active fluids. In particular, we use E. coli suspensions as our model system
and illustrate three unique properties of active fluids, i.e., (i) abnormal rheology, (ii) enhanced diffusion
of passive tracers and (iii) emergence of collective swarming. Based on theoretical tools of fluid
mechanics and statistical mechanics, we develop a quantitative understanding of these interesting
behaviors. Our study illustrates the general organizing principles of active fluids that can be exploited for
designing “smart” fluids with controllable fluid properties. Our results also shed new light on
fundamental transport processes in microbiological systems.