BacFlow

For a long time, bacteria have been considered as passive colloidal particles assumed to have no influence on the flow beside a viscous loss increasing with concentration. It is now proven that the swimming properties of microorganisms and the interactions between individuals – the cause of collective motion – play an essential role on the macroscopic properties of bacterial transport. The ambition of the project is thus: (i) To create a new scientific dynamics around the hydrodynamics of bacterial fluids transport and, by studying their behavior in disordered and porous media, to determine their transport equations at a macroscopic scale. Such a theme is included in the general scope of “active matter”. (ii) To develop tools specifically dedicated to the manipulation and characterization of such active fluids and assess their impact in terms of potential applications. Our consortium is built on three teams, recognized in their field, with complementary expertizes The first team is specialist of hydrodynamics and complex fluids, the second one has a strong knowledge in the biophysics of bacteria and their chemotactic response and the third one, is expert in macroscopic transport and hydrodynamic dispersion in porous media. A key phenomenon that we wish to study here is explicitly the response of a population of swimming to a flow. That involves to understand the influence of the hydrodynamics on swimming properties – including rheotaxis phenomena, chemotactic orientation changes and couplings still not understood, between the geometry of a bacterium and hydrodynamic shear. By identifying these phenomena at microscopic scales via direct visualization techniques of bacteria motion, we want to determine the weight of the different contributions involved in the bacteria transport and also on the active fluid rheology. At higher bacteria concentrations, we identify out and quantify the impact of collective behavior under shear and its influence on the constitutive relations. Various geometries will be considered, each chosen to study a particular phenomenon: Hele-Shaw cells to understand the fluid exchanges between the bulk flow and the solid surfaces, capillary tubes or microchannels to analyze the influence of rheotaxis on bacteria concentrations or on the flow property. Finally, disordered 2D environments and porous columns to obtain a set of experimental data suited to build a complete theory of bacterial transport. We have at our disposition several bacterial models such as wild-type or mutant E. coli strains or a magnetotactic strain the motility of which is sensitive to the action of external fields such as the magnetic field and the oxygen gradient (aerotaxis). Note that in the framework of this project, specific techniques will be developed, some of them are original in this context. They concern the manipulation of bacteria (micro-channels of finely controlled oxygen gradients, magnetic fields, acoustic traps) or the characterization of their activity (via Lagrangian tracking of the trajectories or the rheology). The approach that we propose combines physics, hydrodynamics and biology and hopefully will bring a fresh unprecedented look into the long-known problems of hydrodynamic dispersion. We believe for example, that practical issues such a bio-contamination could be better understood as they require a new approach in which the motility of microorganisms would be explicitly taken into account.