  We study mixing in Stokesian vesicle suspensions in two dimensions on a
cylindrical Couette apparatus using numerical simulations. The vesicle flow
simulation is done using a boundary integral method and the advection-diffusion
equation for the mixing of the solute is solved using a pseudo-spectral scheme.
We study the effect of the area fraction, the viscosity contrast between the
inside (the vesicles) and the outside (the bulk) fluid, the initial condition
of the solute, and the mixing metric. We compare mixing in the suspension with
mixing in the Couette apparatus without vesicles. On the one hand, the presence
of vesicles in most cases, slightly suppresses mixing. This is because the
solute can be only diffused across the vesicle interface and not advected. On
the other hand, there exist spatial distributions of the solute for which the
unperturbed Couette flow completely fails to mix whereas the presence of
vesicles enables mixing. We derive a simple condition that relates the velocity
and solute and can be used to characterize the cases in which the presence of
vesicles promotes mixing.
