(99d) Life in Complex Fluids

Life immersed in a fluid is nothing unusual for an organism. They cope and take advantage of water or wind currents to move, feed, and reproduce. 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 motility behavior of the bacterium E. coli, the green algae C. reinhardtii, and sperm cells. 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 by viscous stresses while 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. These results demonstrate the intimate link between swimming kinematics, biology, and fluid rheology and present an exciting research direction for chemical engineers.