ABSTRACT: A flagellated bacterium exhibits fascinating swimming behaviors both as an individual and as a member of collectively moving swarm. I discuss two recent experiments on the swimming behaviors of Escherichia coli, a prominent example of flagellated bacteria. First,  we find that bacteria in dilute colloidal suspensions display the same motile behaviors as those in polymer solutions, where a size-dependent motility enhancement up to 80% is observed accompanied by a suppression of bacterial wobbling. This striking similarity resolves the long-standing controversy over bacterial motility enhancement in complex fluids, and challenges existing theories using polymer dynamics in addressing bacterial swimming. Second, we study the collective motion of dense bacterial suspensions as an active fluid model. Using a light-powered E. coli strain, we, for the first time, map the phase diagram of bacterial flows and image the transition of bacterial suspensions towards collective motions.

BIOSKETCH: The Cheng group studies experimental soft materials physics, with a special focus on the emergent flow behaviors of soft materials and their associated mesoscopic structures and dynamics. Particularly, his research interests include the rheology of colloidal suspensions and granular flows, hydrodynamics of active fluids and dynamics of liquid-drop impact processes. Dr. Cheng has received several academic awards, including Arthur B. Metzner Early Career Award from Society of Rheology, NSF Career Award, Packard Fellowship, DARPA Young Faculty Award, 3M non-tenured faculty award and McKnight Land-Grant Professorship.