Enhancement of bacterial rheotaxis in non-Newtonian fluids.

Many microorganisms exhibit upstream swimming, which is important to many biological processes and can cause contamination of biomedical devices and the infection of organs. This process, called rheotaxis, has been studied extensively in Newtonian fluids. However, most microorganisms thrive in non-Newtonian fluids that contain suspended polymers such as mucus and biofilms. Here, we investigate the rheotactic behavior of near walls in non-Newtonian fluids. Our experiments demonstrate that bacterial upstream swimming is enhanced by an order of magnitude in shear-thinning (ST) polymeric fluids relative to Newtonian fluids. This result is explained by direct numerical simulations, revealing a torque that promotes the alignment of bacteria against the flow. From this analysis, we develop a theoretical model that accurately describes experimental rheotactic data in both Newtonian and ST fluids.