The interplay between bulk flow and boundary conditions on the distribution of micro-swimmers in channel flow

Data for the JFM paper "The interplay between bulk flow and boundary conditions on the distribution of micro-swimmers in channel flow" by Smitha Maretvadakethope, Andrew L. Hazel, Bakhti Vasiev, and Rachel Bearon. Abstract: While previous experimental and numerical studies of dilute micro-swimmer suspensions have focused on the behaviours of swimmers in the bulk flow and near boundaries, models typically do not account for the interplay between bulk flow and the choice of boundary conditions imposed in continuum models. In our work, we highlight the effect of boundary conditions on the bulk flow distributions, such as through the development of boundary layers or secondary peaks of cell accumulation in bulk-flow swimmer dynamics. For the case of a dilute swimmer suspension in Poiseuille flow, we compare the distribution (in physical and orientation space) obtained from individual based stochastic models with those from continuum models, and identify under what conditions it is mathematically sensible to use specific continuum boundary conditions to capture different physical scenarios (i.e. specular reflection, uniform random reflection and absorbing boundaries). We identify that the spread of preferred cell orientations is dependent on the interplay between rotation driven by the shear flow (Jeffery orbits) and rotational diffusion. We find that in the absence of hydrodynamic wall-interactions, swimmers preferentially approach the walls perpendicular to the surface in the presence of high rotational diffusion, and that the preferential approach of swimmers to the walls is shape-dependent at low rotational diffusion (when suspensions tend towards a fully deterministic case). In the latter case, the preferred orientations are nearly parallel to the surface for elongated swimmers and nearly perpendicular to the surface for near-spherical swimmers. Furthermore, we highlight the effects of swimmer geometries and shear throughout the bulk-flow on swimmer trajectories and show how the full history of bulk-flow dynamics affects the orientation distributions of micro-swimmer wall incidence.

本数据集对应发表于《流体力学杂志》（Journal of Fluid Mechanics，简称JFM）的论文《槽道流中微泳动体（micro-swimmers）分布的本体流场与边界条件耦合效应》（The interplay between bulk flow and boundary conditions on the distribution of micro-swimmers in channel flow），作者为Smitha Maretvadakethope、Andrew L. Hazel、Bakhti Vasiev及Rachel Bearon。摘要：过往针对稀疏微泳动悬浮体系（dilute micro-swimmer suspensions）的实验与数值研究，多聚焦于本体流场及近壁区域内泳动体的运动行为，但现有连续介质模型（continuum models）通常未考虑本体流场与连续介质模型中所采用边界条件之间的耦合效应。本研究着重探讨了边界条件对本体流场分布的影响，例如在本体流场泳动体动力学过程中，边界层的形成以及细胞聚集次级峰的出现等现象。针对泊肃叶流（Poiseuille flow）中的稀疏泳动体悬浮体系，我们将基于个体的随机模型（individual based stochastic models）所得到的物理空间与取向空间内的分布结果，与连续介质模型的对应结果进行对比，并明确了在何种数学条件下，采用特定连续介质边界条件可以复现不同物理场景——即镜面反射（specular reflection）、均匀随机反射（uniform random reflection）与吸收边界（absorbing boundaries）。我们发现，细胞偏好取向的分散程度，取决于剪切流驱动的旋转（杰弗里轨道（Jeffery orbits））与旋转扩散之间的耦合作用。我们发现，在无流体力学壁面相互作用（hydrodynamic wall-interactions）的情况下，当旋转扩散程度较高时，泳动体更倾向于以垂直于壁面的方向趋近壁面；而在旋转扩散程度较低（此时悬浮体系趋近于完全确定性场景）时，泳动体的壁面趋近偏好取向则与自身形状相关。在后一种场景下，对于细长型泳动体，其偏好取向几乎平行于壁面；而对于近球形泳动体，其偏好取向则几乎垂直于壁面。此外，我们还阐明了全本体流场内的泳动体几何形状与剪切作用对泳动体运动轨迹的影响，并揭示了本体流场动力学的完整历程如何影响微泳动体壁面入射的取向分布。