重力作用下蠕动体的集体沉降
Collective Sedimentation of Squirmers under Gravity
摘要：
流体动力学相互作用的活性粒子显示出各种各样的涌现的集体现象。我们用蠕动体模拟球形微泳者，并利用多粒子碰撞动力学方法模拟流体流动，探讨了在强引力作用下数千个蠕动体的集体行为。沉积剖面取决于游动速度与沉积速度之比以及蠕动类型。它显示了在底部的紧密堆积的蠕动层和一个高度动态的区域，指数密度依赖于顶部。蠕动的平均垂直方向强烈依赖于高度。当游动速度大于沉降速度时，蠕动体在指数区域表现出较强的对流。我们量化的对流强度和对流细胞的程度由垂直电流密度和电流偶极子，这是大中性蠕动，以及为弱推和拉。
英文摘要：
Active particles, which interact hydrodynamically, display a remarkable variety of emergent collective phenomena. We use squirmers to model spherical microswimmers and explore the collective behavior of thousands of them under the influence of strong gravity using the method of multi-particle collision dynamics for simulating fluid flow. The sedimentation profile depends on the ratio of swimming to sedimentation velocity as well as on the squirmer type. It shows close packed squirmer layers at the bottom and a highly dynamic region with exponential density dependence towards the top. The mean vertical orientation of the squirmers strongly depends on height. For swimming velocities larger than the sedimentation velocity, squirmers show strong convection in the exponential region. We quantify the strength of convection and the extent of convection cells by the vertical current density and its current dipole, which are large for neutral squirmers as well as for weak pushers and pullers.
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