Active bacterial suspensions : from microhydrodynamics to transport properties in microfluidic channels

Overview & Synopsis

We present a study of the swimming dynamics of Escherichia coli bacteria in different physical conditions. Their 3D motion is assessed by means of a device for automated 3D Lagrangian tracking of fluorescent objects, that we developed for that purpose. Bacteria studied in that way display consistently large dispersion of the rotational diffusion coefficient, contradicting the standard vision of run-and-tumble dynamics established for an adapted bacterium. The result is interpreted as a consequence of the power law distribution of run times experimentally found for individual flagella, that up to now remained uncoupled with the motility description.We also study the bacterial swimming in polymeric suspensions, as well as in more concentrated active suspensions.In confined flows, upstream migration of E. coli at the edges remains possible at much larger flow rates compared to the motion at the flat surfaces. The bacteria speed at the edges is not influenced by the advective flow. Upstream motion takes place close to the edges in an “edge boundary layer” whose size varies with the applied flow rate. Upstream migration under flow and direction persistence combine during contamination processes. We show that bacteria can contaminate clean regions by upstream swimming in confined environments. A simple model considering the motor rotation statistics describes well the main features of the contamination process, assuming a power law distribution of run times. The model fails to reproduce the qualitative dynamics when the classical run-and-tumble distribution is determined by the motor rotation statistics.