Axial Dispersion Due to Shear-induced Diffusion in Suspensions

1234500PI：Stone了解悬浮力学对于新的和开发中的技术的设计和表征至关重要，因为颗粒材料存在于无数流体系统中，例如颗粒增强塑料、用于化学和生物检测的胶体材料、疾病识别和药物输送、细胞悬浮液、乳液和建筑材料。由于颗粒浓度分布与材料流变学的耦合，对常见悬浮流的定量理解是复杂的。特别是，典型的悬浮液是非牛顿的，因为它们表现出对有效粘度和法向应力差的依赖关系，既依赖于颗粒的局部浓度，也依赖于剪切或拉伸速率。此外，由于剪切诱导的迁移现象，流动中的颗粒分布通常在平行于平均流动方向和横向于平均流动方向的方向上演化。输送过程的这一特征既影响颗粒塞的分散，也影响悬浮液的平均运动速度。我们最近在理论上描述稀悬浮液中的轴向弥散方面取得了进展，其中引起弥散的波动是由于剪切引起的颗粒的迁移/扩散而发生的。最重要的是，对于典型的流动条件，当剪切诱导扩散效应显著时，一团颗粒的有效轴向弥散减小。此外，有效轴向弥散系数与平均流速成线性关系，并且作为管道或通道半径的3次方，这两个特性都不同于传统的布朗运动驱动的轴向弥散。该提案的重点集中在三个主要问题上：(I)单分散悬浮体的轴向扩散是由剪切引起的迁移/扩散驱动的波动产生的情况下的轴向扩散概念的推广，(Ii)分析剪切诱导扩散对双分散悬浮流中输运和颗粒分离的影响，以及(Iii)分析具有非均匀截面形状的通道的轴向扩散过程，这是许多微器件的典型特征。在每种情况下，都有必要将颗粒的分布与传输特性(有效扩散系数、有效粘度、法向应力差等)相结合。实验将是对建模的补充。这些结果将适用于涉及悬浮或检测方法的新应用，即向流动中注入一团特殊设计的颗粒。许多常见的材料是悬浮，指的是分布在液体中的固体颗粒(或其他类型的颗粒)。这些材料被用于工业，并在家庭、食品、制药等中普遍使用。通常需要将更多的颗粒混合到悬浮液中，以改变悬浮液的性质(例如，材料流动的难易程度)，或向混合物中添加功能(例如，添加药物)，或添加可能检测到流动中的有害颗粒的物体(例如，可能识别或捕获致病细胞的小型传感器)。在这些情况下，有必要定量和定性地了解这种混合是如何发生的，以及添加剂如何在悬浮液中重新分布。尽管有复杂的理论和数值工具来研究分子溶质的混合，但对宏观粒子的扩散和分布却知之甚少。我们将通过理论、建模和实验来解决这些问题，并制定定量的指导方针来理解这些分散的，即传播的过程。

1234500PI: StoneUnderstanding suspension mechanics is crucial to the design and characterization of new and developing technologies since particle-laden materials occur in a myriad of fluid systems, e.g. particle-reinforced plastics, colloidal materials for chemical and biological detection, disease identification and drug delivery, cellular suspensions, emulsions, and building materials. Quantitative understanding of common suspension flows is complicated by the coupling of the particle concentration distribution to the rheology of the material. In particular, typical suspensions are non-Newtonian as they exhibit dependence of the effective viscosity and normal stress differences on both the local concentration of particles and the shear or extension rate. Furthermore, the particle distribution in a flow typically evolves in directions both parallel and transverse to the mean flow direction due to the phenomenon of shear-induced migration. This feature of the transport process affects both the dispersion of a plug of particles, and the average speed of motion of the suspension. We have recently made progress towards characterizing theoretically axial dispersion in a dilute suspension, where the fluctuations responsible for the dispersion occur as a consequence of shear-induced migration/diffusion of the particles. Most significantly, for typical flow conditions, the effective axial dispersion of a bolus of particles is reduced when shear-induced diffusion effects are significant. In addition, the effective axial dispersivity scales linearly with the average flow speed and as the third power of the pipe or channel radius, both features differ from the traditional Brownian motion-driven axial dispersion. The focus of the proposal centers on three main problems: (i) the generalization of axial dispersion ideas for the case where axial spreading of a monodisperse suspension is produced by fluctuations that are driven by shear-induced migration/diffusion, (ii) analyzing the influence of shear-induced diffusion on transport and particle separation in bidisperse suspension flows, and (iii) analyzing the axial dispersion process for channels with non-uniform cross-sectional shapes as is typical of many microdevices. In each case it is necessary to couple the distribution of particles to the transport properties (effective diffusivity, effective viscosity, normal stress differences, etc.). Experiments will complement the modeling. The results will be applicable to new applications involving suspensions or detection methods whereby a bolus of specially designed particles is injected into a flow.Many common materials are suspensions, which refers to solid particles (or other kinds of particulates) distributed in a liquid. These materials are used industrially and are in common uses in households, food products, pharmaceuticals, etc. Often it is necessary to blend additional particles into a suspension, either to modify the properties of the suspension (e.g. how easily the material flows), or to add a functionality to the mixture (e.g. adding a drug), or to add objects that may detect harmful particles in the flow (e.g. small sensors that might identify or capture pathogenic cells). In these cases it is necessary to understand quantitatively and qualitatively how this mixing takes places, and how the additive is redistributed in the suspension. Although there are sophisticated theoretical and numerical tools for such mixing of molecular solutes, there is much less known about the spreading and distribution of macroscopic particles. We will address these questions using theory, modeling and experiments, and develop quantitative guidelines for understanding these dispersive, i.e. spreading, processes.