Rheological response and direct visualization of collective patterns formation leading to shear-bands formation in a suspension of active E.coli at the super-fluidity transition*

Suspensions of motile Escherichia coli were found to display `negative viscosity increment' at low shear rate viz., adding such bacteria lowers the shear viscosity of the system (H.M.Lopez , J. Gachelin, C. Douarche, H.Auradou, E. Clément, Phys. Rev. Lett. 115, 028301 (2015)). Furthermore at higher concentrations a regime of zero viscosity, akin to a "superfluidity" transition, can be reached. Here, for a strain of very active bacteria, we report a full exploration of the rheological response changing confinement, shear rate and concentration, both in a low-shear Couette rheometer and in a cone-plane rheometer. The last allows a direct visualization of the collective organization under shear and the exploration of the different regimes leading the “superfluidity transition” in relation with the dynamics of shear-band formation.