Rheology of suspensions of vesicles and red blood cells.

We investigate the rheology of dilute suspensions of lipid vesicles and red blood cells (RBC) as a function of the viscosity ratio between the internal and external fluids. Experiments on RBC and vesicles, as well as the result of theoretical investigations on vesicles exhibit a minimum of the intrinsic viscosity when the viscosity ratio is close to the value at which at the miscoscopic scale, a transition from tank-treading to tumbling occurs for individual objects in simple shear flow. This reveals a qualitative change due to the link between microscopic and macroscopic dynamics.