**Dynamics of complex vesicles in shear flow**

ZY Deng and LX Zhang, ACTA PHYSICA SINICA, 64, 168201 (2015).

DOI: 10.7498/aps.64.168201

Vesicles exposed to shear flow exhibit a remarkably rich dynamics. With the increase of shear rate, one can observe a tumbling-to-tank-treading transition. Besides, a complex oscillating motion, which has alternatively been called trembling, swinging, or vacillating breathing, has also been predicted theoretically and observed experimentally. While in biological systems, vesicles are always decorated by a large number of macromolecules, rendering the dynamics of vesicles in shear flow much more complex. As a powerful supplement to analytical techniques, the dissipative particle dynamics has been proved to be a useful tool in simulating nonequilibrium behaviors under shear. By replacing the conservative force in dissipative particle dynamics with a repulsive Lennard-Jones potential, the density distortion has been overcome and the no-slip boundary condition is achieved. In this article, a nonequilibrium molecular dynamic method is used to study the dynamics of two-dimensional complex vesicles in shear flow. The dynamical behaviors of the complex vesicles are closely related to shear rate and the size of small grafting vesicle. We first consider a vesicle with two small vesicles symmetrically grafted. At a weak flow, the complex vesicle maintains its equilibrium shape and undergoes an unsteady flipping motion, known as tumbling motion. At a moderate shear rate, the tumbling of the vesicle is accompanied with strong shape oscillation, which is consistent with Yazdani's simulation, in which a breathing-with-tumbling type of motion is observed, and is called trembling in this article. As the shear rate further increases, the vesicle is oriented at a fixed angle with respect to the flow direction, while the vesicle membrane circulates around its surface area, exhibiting a "well-known" tank- treading motion. For sufficiently large grafted vesicles and at a high enough shear rate, a transition from tank-treading to translating motion is observed, in which the flipping of the vesicle or the circulating of the vesicle membrane is hampered. A crossover regime, namely, the tank- treading/translating mixture motion is also found, where translating motion alternates with tank-treading chaotically. However, when a sufficient number of small vesicles are uniformly grafted to the vesicle, the newly observed translating motion is eliminated. This study can give a deeper insight into the complexity of vesicle motions in shear flow.

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