Tumbling of viscous vesicles in a linear shear field near a wall

The dynamics of lipid vesicles enclosing a viscous polymer solution is studied in a simple shear flow near a wall. The ratio between the internal fluid viscosity and the one of the outer aqueous solution is chosen to be above the tank-treading-totumbling transition for all analyzed vesicles. A clear influence of the presence of the wall on the tumbling motion of vesicles has been detected. In the entire range of applied shear rates, pure tumbling has not been observed due to the high density difference between the internal fluid and the suspending medium, keeping vesicles close to the wall. The strong deformability of their membranes, coupled to the high viscosity ratios, leads to a periodic modulation of the vesicle shape, with a periodicity correlated to the shear rate of the hydrodynamic flow and amplitudes depending on the vesicle's deflation. This shape modulation is coupled to a rolling motion of the vesicles.