School of Physics Colloquium

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. 


About Paulo Arratia

Paulo E. Arratia is a Professor of Mechanical Engineering & Applied Mechanics (MEAM) and Chemical & Biological Engineering (CBE) at the University of Pennsylvania. He received a PhD from Rutgers University in 2003 and was a postdoc in the Department of Physics at Haverford College (2003-2005) and then at the University of Pennsylvania (2005-2007). Paulo is the recipient of the National Science Foundation CAREER Award, the American Chemical Society New Investigation Award, and a Fellow of the American Physical Society (APS). He is also the recipient of the Milton Van Dike Award (APS-DFD), Gallery of Fluid Motion Award (APS -DFD/GNSP), the Rutgers University Early Career Distinction Award, the Christian R. and Mary F. Lindback Award for Distinguished Teaching, the Ford Motor Company Award for Faculty Advising, and the Penn Health Pioneer Award. Paulo’s research focuses on the dynamics of complex fluids, dead or alive, from viscoelastic flows, rheology of dense colloidal suspensions & granular materials to swimming of microorganisms and active matter.

Event Details


  • Date: 
    Monday, March 25, 2024 - 3:30pm to 4:30pm

Marcus Nano 1117 - 1118