[1] The miscible displacement of a shear-thinning fluid by another fluid with the same rheological properties is studied experimentally in a transparent fracture by an optical technique that images relative concentration distributions. The fracture walls have complementary self-affine geometries and are shifted laterally in the direction perpendicular to the mean flow velocity U: The flow field is strongly channelized and macrodispersion controls the front structure for Péclet numbers above a few units. The global front width increases therefore linearly with time and reflects the velocity distribution between the different channels. In contrast, at the local scale, front spreading is similar to Taylor dispersion between plane-parallel surfaces. Both dispersion mechanisms depend strongly on the fluid rheology, which shifts from Newtonian to shear thinning when the flow rate increases. In the latter domain, increasing the concentration enhances the global front width but reduces both Taylor dispersion (due to the flattening of the velocity profile in the gap of the fracture) and the size of medium scale front structures.

This paper describes our findings regarding the accumulation of motile bacteria at the rear of a confined obstacle and the physical description of the mechanisms at play. We found that the modification of flow due to the presence of the obstacle produces vorticity that favor the diffusion of bacteria towards the downstream stagnation point. By testing different flow rates, we determined the range in which bacteria accumulate. More interestingly, we observe that hydrodynamic interaction between the bacteria and the top and bottom surface of the microfluidic chip maintain the bacteria in the region where the flow velocity is lower than their own velocity. In the case of non-motile bacteria, this effect is not observed because bacteria follow the streamlines as passive tracers do.

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[1] Dispersion experiments are compared for two model fractures with identical complementary rough walls but with a relative shear displacement d parallel (d k U) or perpendicular (d ? U) to the flow velocity U. The flowing fluid is a shear thinning polymer solution with a Newtonian behavior at low shear rates. For d ? U, the mixing fronts display large structures well reproduced by assuming parallel channels of conductance deduced from the aperture field. This model also explains the amplification of the structures in the shear thinning regime and the distribution of the local transit times t(x, y). For d k U, the front is much flatter. The local thickness of the front is characterized by a dispersivity a(x, y): its distribution is narrow enough to define an effective value a(Pe) only for d k U, and, in this case, a(Pe) has a Taylor-Aris-like variation with Pe.

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