Lateral migration of vesicles with viscosity contrast in simple shear and Poiseuille flows

The ability of soft objects (such as vesicles, drops or blood cells) to adapt their shapes under non-equilibrium conditions allows them to migrate transversally to the flow in a confined situation, even in the Stokes limit. We present an overview of our recent experiments on phospholipidic vesicles placed in two simple flows : simple shear between two sliding walls and Poiseuille flow in a channel. Some of these experiments were run under microgravity conditions in order to get rid of the screening of the lift forces by the vesicle's weight. Quantitative migration laws are exhibited and discussed. In particular, they depend strongly and non monotonously on the reduced volume (or excess area) of vesicles and the viscosity contrast between internal and external fluids.