(Cancelled) Life in Complex Fluids

While much of our understanding of microbial swimming is derived from Newtonian fluid mechanics, many microorganisms including bacteria, algae, and sperm cells move in fluids or liquids that contain (bio)-polymers and/or solids. Examples include human cervical mucus, intestinal fluid, wet soil, and tissues. These so-called complex fluids often exhibit non-Newtonian rheological behavior due to the non-trivial interaction between the fluid microstructure and the applied stresses. In this talk, I will show how the presence of polymers in the fluid medium can strongly affect the motility behavior of microorganisms. In particular, I will focus on the effects of fluid elasticity (and viscosity) on the swimming behavior of the bacterium E. coli (“puller”) and the green algae C. reinhardtii (“pusher”). Results show that elastic and viscous stresses can significantly affect motility kinematics (speed, beating frequency and amplitude) and energetics of such microorganisms in unexpected ways. For example, the run-and-tumble mechanism characteristic of E. coli is suppressed, and its speed is enhanced by fluid elasticity (and possibly shear-thinning). On the other hand, 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” axoneme structures in complex fluids. Finally, I will discuss how even minute amounts of polymer can affect the collective motion of swimming E. coli. These results demonstrate the intimate link between swimming kinematics, biology, and fluid rheology and present an exciting research direction for physicists and engineers alike.