TUMBLING OF LIPID VESICLES, ENCLOSING A VISCOUS FLUID, UNDER A SHEAR FLOW

We performed an experimental study of the tumbling motion of deflated lipid vesicles, when the viscosity, rii', of the fluid, enclosed by the vesicle membrane, is sufficiently higher than that, clout, of the outer aqueous solution. Vesicles with reduced volumes (the ratio between the vesicle volume and the volume of a sphere, having the same surface area) in the range 0.74 to 0.96 were subjected to a simple shear flow, and the vesicle tumbling motion was characterized for various viscosity contrasts (r = n.in out ranging from 9.6 to 28.3). Experiments reveal a strong effect of vesicle deformability (high flexibility of the lipid membrane) on the tumbling motion in a hydrodynamic field: the vesicle shape significantly changes during the rotation, influencing the relationship between rotation speed and angle. For less deformable vesicles (higher reduced volume or lower capillary number), the experimental motion is in good agreement with simpler analytical models for fixed shape ellipsoidal vesicles. We compare the experimental measurements with numerical results, obtained by a boundary integral method.