State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an 710049, P. R. China.
Soft Matter. 2017 Nov 15;13(44):8281-8292. doi: 10.1039/c7sm01425b.
Owing to their significance in capsule-related engineering and biomedical applications, a number of studies have considered the dynamics of elastic capsules flowing in constricted microchannels. However, these studies have focused on capsules moving along the channel centerline. In the present study, we numerically investigate the transient motion of an elastic capsule in a microfluidic channel with a rectangular constriction, which is initially trapped at the constriction inlet while off the channel centerline (i.e., on the channel bottom-wall). Under the push of the surrounding flow, the capsule can squeeze into the constriction, but only if the capsule deformability or the constriction size is sufficiently large. We find that the critical capillary number leading to the penetration of the capsule into the constriction is larger for off-centerline capsules compared to centered capsules. The centered capsule is stationary at the steady state when it remains stuck at the constriction; in contrast, the off-centerline capsule is not stationary but exhibits a tank-treading motion, i.e., its overall shape maintains a nonspherical shape with a protrusion into the constriction while its membrane exhibits a continuous rotation. Further, we examine the dependence of the capsule motion type, capsule deformation degree and membrane tension distribution on the capillary number (measuring the effects of flow strength and membrane mechanics) and constriction geometries (including the constriction height and width). Finally, we discuss the mechanism governing the capsule motion by analyzing the hydrodynamic forces acting on the capsule. The shear force acting on the capsule top owing to the fluid flow in the gap between the capsule top and the channel top-wall is the main source inducing the membrane tank-treading rotation.
由于弹性胶囊在胶囊相关工程和生物医学应用中的重要性，许多研究都考虑了在狭窄微通道中流动的弹性胶囊的动力学。然而，这些研究都集中在沿通道中心线运动的胶囊上。在本研究中，我们通过数值模拟研究了具有矩形收缩的微流道中弹性胶囊的瞬态运动，该胶囊最初被困在收缩入口处，而不在通道中心线（即通道底壁）上。在周围流动的推动下，胶囊可以挤入收缩处，但前提是胶囊的可变形性或收缩尺寸足够大。我们发现，对于不在中心线的胶囊，导致胶囊穿透收缩处的临界毛细管数大于中心线胶囊。当胶囊卡在收缩处时，中心线胶囊在稳态下处于静止状态；相比之下，不在中心线的胶囊不是静止的，而是表现出罐滚运动，即其整体形状保持非球形，同时有一个突起进入收缩处，而其膜则连续旋转。此外，我们还研究了胶囊运动类型、胶囊变形程度和膜张力分布对毛细管数（测量流动强度和膜力学的影响）和收缩几何形状（包括收缩高度和宽度）的依赖性。最后，我们通过分析作用在胶囊上的流体动力来讨论控制胶囊运动的机制。由于胶囊顶部和通道顶部壁之间的间隙中的流体流动而作用在胶囊顶部的剪切力是引起膜罐滚旋转的主要来源。
