Tank treading and unbinding of deformable vesicles in shear flow: Determination of the lift force

Deformation and tank-treading motion of flaccid vesicles in a linear shear flow close to a wall are quantitatively studied by light microscopy. Velocities of bounded vesicles obey Goldman's law established for rigid spheres. A progressive tilt and a transition of unbinding of vesicles are evidenced upon increasing the shear rate, (gamma)over dot. These observations disclose the existence of a viscous lift force, F-l, depending on the viscosity eta of the fluid, the radius R of the vesicle, its distance h from the substrate, and a monotonous decreasing function f (1 - v) of the reduced volume v, in the following manner: F-l = eta(gamma)over dot (R-3/h)f(1 - v). This relation is valid for vesicles both close to and farther from the substrate.