流场环境下复杂囊泡的动力学行为

Dynamics of complex vesicles in shear flow

采用非平衡态分子动力学方法研究了二维复杂囊泡在剪切流中的动力学行为.模拟发现了复杂囊泡经典的翻滚(tumbling)、摇摆(trembling)和坦克履(tank-treading)行为,还观察到由坦克履行为向平动行为(translating)的转变.囊泡的平动行为与剪切率大小、复杂囊泡的形状密切相关.当大囊泡均匀嫁接较多数目的小囊泡后,其平动方式消失.该研究有益于加深对囊泡在剪切流场中复杂性行为的理解.

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.