Dynamics of fluid vesicles in shear flow: effect of membrane viscosity and thermal fluctuations.

The dynamical behavior of vesicles is investigated in simple shear flow. A simulation technique is presented that combines a three-dimensional particle-based mesoscopic model (multiparticle collision dynamics) for the solvent with a dynamically triangulated surface model for the membrane. In this model, thermal fluctuations of the solvent and of the membrane are consistently taken into account. The membrane viscosity can be varied by changing the bond-flip rate of the dynamically triangulated surface. Vesicles are found to transit from steady tank-treading to unsteady tumbling motion with increasing membrane viscosity. At small reduced volumes, the shear induces a transformation from a discocyte to a prolate shape at low membrane viscosity. On the other hand, at high membrane viscosity, the shear induces a transformation from prolate to discocyte, or tumbling motion accompanied by shape oscillations between these two states. Thermal fluctuations induce intermittent tumbling and smooth out the transitions. This effect can be understood from a simplified stochastic model.