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, \. {}. These observations disclose the existence of a viscous lift force, F₋, depending on the viscosity 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₋0ex{0ex}=0ex{0ex} \. {} (R^3/h) f (1-v). This relation is valid for vesicles both close to and farther from the substrate.