Falling ball viscosimetry of giant vesicle membranes: Finite-size effects

We study the general problem of the friction felt by a spherical solid part icle which moves parallel to the membrane of a spherical vesicle. Experimen ts are carried out with SOPC vesicles at room temperature, with different p article and vesicle sizes. Experimental data show considerable finite-size effects whenever the particle is not very small compared to the vesicle, Th ese effects are found consistent with the hydrodynamical theory of the vesi cle-particle problem. This agreement allows for a "robust" determination of membrane viscosity, independently of particle and vesicle sizes.