Abstract

The way microorganisms swim in concentrated polymer solutions has important biomedical implications, e.g. that is how pathogens invade the mucosal lining of mammal guts. Based on early measurements, it is widely believed that the swimming speed of many flagellated bacteria initially increases when high-molecular weight linear polymers are added to their aqueous medium, before eventually slowing down at high polymer concentrations. Pores in the polymer solution were suggested as the explanation. Quantifying this picture led to a theory that predicted such non-monotonic speed-concentration curves. Using new, high-throughput methods for characterising motility, we have measured the swimming speed and the angular frequency of cell-body rotation of motile Escherichia coli as a function of polymer concentration in polyvinylpyrrolidone (PVP) and Ficoll solutions of different molecular weights. We find that non-monotonic speed-concentration curves are the result of low-molecular weight impurities. Removing such impurities by dialysis allows us to observe two distinct behaviors in Newtonian and Non-Newtonian solutions. Results at low molecular weight can be explained by Newtonian hydrodynamic theory, which brings about striking data collapse at different molecular weights. There is clear evidence for non-Newtonian effects in the highest molecular weight PVP solution suggesting that flagella can be seen as `nano-rheometers’ for probing the non-Newtonian behavior of polymer solutions on a molecular scale.