  The effect of membrane viscosity on the dynamics of vesicles in shear flow is
studied. We present a new simulation technique, which combines
three-dimensional multi-particle collision dynamics for the solvent with a
dynamically-triangulated membrane model. Vesicles are found to transit from
steady tank-treading to unsteady tumbling motion with increasing membrane
viscosity. Depending on the reduced volume and membrane viscosity, shear can
induce both discocyte-to-prolate and prolate-to-discocyte transformations. This
dynamical behavior can be understood from a simplified model.
