Enhanced upstream swimming of bacteria in complex fluids: part I, shear-thinning.

Many swimming microorganisms exhibit positive rheotaxis, i.e. the ability to swim upstream, which allows them to explore environments and enhance contamination. Our work investigates how this ability is affected by complex fluid rheology.

In this talk, we present experimental evidence that shear-thinning (ST) viscosity can significantly enhance rheotaxis of the bacteria E. coli swimming near surfaces relative to Newtonian fluids. A simple model that accounts for the modified interaction of E. coli with surface due to ST effects captures the main experimental observations, namely a transition to rheotaxis occurring at lower flow rates and an increased alignment of upstream swimmers. Importantly, this analysis singles out traits that could predict the spreading ability of a population or of individual swimming E. coli. Our results for shear-thinning fluids provide an essential step towards understanding the spreading of swimming bacteria in more complex biological fluids.