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Rheology and transport of active bacterial suspensions
Assemblies of microscopic swimmers dispersed in a fluid display emergent properties that differ strongly from passive suspensions. The momentum and energy transfer balances as well the constitutive transport properties are deeply modified by the active momentum sources distributed in the bulk. Anomalous constitutive properties have already been identified, such as active diffusivity, anomalous viscous response, active transport and mixing, as well as the possibility to use the fluctuations to extract work. In this presentation I will discuss recent work done in the laboratory with wild-type E-Coli bacteria suspensions. At first, a zero flux system is studied in order to explore the effect of complex swimming activity on the Brownian motion problem. We show that, with respect to thermal Brownian motion, the tracer diffusivity is enhanced by the presence of active swimmers and increases linearly with the activity of the fluid which is defined as the product of the fraction of active swimmers by their mean velocity. Second, I will present a measurement using an original microfluidic device of active viscosity. I will show that the low shear rate viscosity of the active suspension can be lower that the viscosity of the suspending fluid. Finally, I will present results on bacterial flow in a microfluidic device with a funnel-like constriction. An effect of flow induced symmetry breaking in the bacteria distribution along the flow axis is discussed. This effect is long range and increases with the flow velocity. The resulting anomalous dispersion is related to elementary mechanisms of interaction between the bacteria and the solid boundaries.
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Contact : mathilde.reyssat@espci.fr