Bacterial Swimming in Newtonian and non-Newtonian Fluids

Kenny Breuer,
Brown University

There has long been controversy and conflicting data regarding the speed at which flagellated bacteria swim in fluids of different Newtonian and non-Newtonian properties. Some data indicates faster swimming in viscoelastic media, while others suggest slower swimming.  There is even controversy regarding the swimming speed in purely Newtonian media of different viscosities.  We will present data and analysis from recent experiments in which we  follow smooth-swimming and wild-type E coli using a tracking microscope as they move through different Newtonian and non-Newtonian fluids.  We find that increasing the bulk viscosity affects swimming speed in two ways - it reduces the effectiveness of flagellar propulsion, but also increases the flagellar bundling time of wild type E coli.  Cells are found to swim faster in non-Newtonian fluids, even though the bulk viscosity rises.  The dominate contribution is found to be related to the strong shear thinning property of the fluid, although the normal stresses associated with viscoelasticity can also contribute through reduced cell precession, slightly improved propulsive efficacy and faster flagellar bundling times.