Rheotaxis in viscoleastic fluids: Experiments with swimming E. coli

The positive rheotaxis of microorganisms in Newtonian fluids encompasses the spontaneous orientation of individual swimmers against a unidirectional flow. The mechanism is governed mainly by the positioning of the swimmer at an angle in the high shear flow region close to solid boundaries. While much is known about rheotaxis in simple Newtonian liquids, our understanding of such process in complex fluids is still in its infancy. In this talk, we present experiments on positive rheotaxis of the swimming bacterium E. coli in viscoelastic fluids. We find that the flux upstream is increased by an order of magnitude in viscoelastic shear-thinning fluids as opposed to strictly Newtonian viscous fluids. The analysis of E.Coli orientation distribution allows us to quantify the relative contribution of the elastic and viscous stresses on the observed phenomena.