Department of Mechanical and Aerospace Engineering, The George Washington University, Washington, DC 20052, USA.
Soft Matter. 2021 Sep 29;17(37):8523-8535. doi: 10.1039/d1sm00938a.
Hydrodynamic interactions generate a diffusive motion in particulates in a shear flow, which plays seminal roles in overall particulate rheology and its microstructure. Here we investigate the shear induced diffusion in a red-blood cell (RBC) suspension using a numerical simulation resolving individual motion and deformation of RBCs. The non-spherical resting shape of RBCs gives rise to qualitatively different regimes of cell dynamics in a shear flow such as tank-treading, breathing, tumbling and swinging, depending on the cell flexibility determined by the elastic capillary number. We show that the transition from tumbling to tank-treading causes a reduction in the gradient diffusivity. The diffusivity is computed using a continuum approach from the evolution of a randomly packed cell-layer width with time as well as by the dynamic structure factor of the suspension. Both approaches, although operationally different, match and show that for intermediate capillary numbers RBCs cease tumbling accompanied by a drop in the coefficient of gradient diffusivity. A further increase of capillary number increases the diffusivity due to increased deformation. The effects of bending modulus and viscosity ratio variations are also briefly investigated. The computed shear induced diffusivity was compared with values in the literature. Apart from its effects in margination of cells in blood flow and use in medical diagnostics, the phenomenon broadly offers important insights into suspensions of deformable particles with non-spherical equilibrium shapes, which also could play a critical role in using particle flexibility for applications such as label free separation or material processing.
在切变流中，流体动力相互作用会使颗粒产生扩散运动，这在颗粒整体流变学及其微观结构中起着重要作用。在这里，我们使用一种数值模拟方法来研究红细胞（RBC）悬浮液中的切变诱导扩散，该方法可以解析单个 RBC 的运动和变形。由于 RBC 的非球形静止形状，在切变流中会出现定性不同的细胞动力学状态，例如坦克履带、呼吸、翻滚和摆动，这取决于由弹性毛细管数决定的细胞弹性。我们表明，从翻滚到坦克履带的转变导致梯度扩散率降低。扩散系数是通过随时间变化的随机堆积细胞层宽度的演化以及悬浮液的动态结构因子，从连续体方法计算得出的。尽管操作方式不同，但这两种方法都匹配，并表明对于中等毛细管数，RBC 停止翻滚，梯度扩散系数下降。进一步增加毛细管数会由于变形增加而增加扩散系数。还简要研究了弯曲模量和粘度比变化的影响。计算出的切变诱导扩散系数与文献中的值进行了比较。除了在血流中细胞边缘化和在医学诊断中的应用之外，该现象还广泛提供了关于具有非球形平衡形状的可变形颗粒悬浮液的重要见解，这对于利用颗粒的弹性在无标记分离或材料处理等应用中也可能起着关键作用。
