Life in Complex Fluids: Swimming with polymers*

Much of our understanding of microbial transport is derived from Newtonian fluid mechanics, but many microorganisms swim in complex non-Newtonian fluids. Examples include sperm cells in cervical mucus, beneficial bacteria through intestinal fluid, and ground worms within aqueous soils. Intriguingly, a non-trivial relationship exists: the presence of organisms changes the bulk properties of the fluids, just as the properties of a fluid affect the way organisms swim. In this talk, I will show how elasticity, introduced by the presence of polymers, in the fluid medium strongly affect the motility of E. coli (“puller”) and green algae C. reinhardtii (“pusher”). Changes in swimming energetics and kinematics, such as speed, beating frequency, and amplitude will be explored. For example, the run-and-tumble mechanism characteristic of E. coli is suppressed, and its speed is enhanced by fluid elasticity. In contrast, elastic stresses hinder the swimming speed of both sperm cells and C. reinharditti and lead to significant hypertension in their flagellum, indicating a common trait among the “9+2” axonome structure in complex fluids. Finally, I will discuss how even minute amounts of polymer affect the collective motion of swimming E. coli. These results highlight the intimate link between swimming kinematics and fluid rheology and present an exciting research direction.