The Rheology of Complex Suspensions In Viscoelastic Suspending Fluids

悬浮在流体中的颗粒无处不在，存在于人类生活的方方面面，包括在工业加工和制造中以及在人体内发挥着关键作用。这些颗粒悬浮在其中的许多流体都是弹性的--这意味着这些流体在时间上的记忆与与流动相关的时间尺度相比并不短。例如，细胞上的尾状毛发，称为纤毛，排列在人类呼吸道内，敲打着粘液，将其从肺部清除出去。粘液是一种弹性流体，粘液的弹性直接影响纤毛将粘液移出呼吸道的能力。在另一种情况下，许多常见的复合塑料部件是在液态下成型的，因此是流动的弹性流体，并含有刚性颗粒添加剂。此外，石油行业使用的钻井泥浆和水力压裂液通常是含有支撑剂或钻屑的瓜尔胶溶液，而且同样涉及弹性流体中的悬浮液。虽然小分子或牛顿流体(如水或大多数油)中的悬浮科学已经有了很长的发展历史，但粘弹性流体中的颗粒悬浮科学还处于初级阶段。在对这些弹性流体的功能没有基本了解的情况下，悬浮液经常是在给定应用的反复试验的基础上产生的。因此，该项目的目标是开发能够模拟与粘弹性流体中粒子的集体效应相关的复杂物理的计算工具。这些工具将使人们能够直接设计悬浮液的流体性质，并对在工业应用中经济和安全地使用粘弹性流体具有广泛的意义。这项研究将被整合到高中课程中，每年聘请一名K-12教师参加一个动手的暑期研究项目。该项目的总体目标是研究粘弹性流体中一系列日益复杂的颗粒悬浮液的流变性，这些悬浮液对能源工业、先进制造和医疗/生物流体应用具有直接重要性。流变学，或宏观应力-应变关系，是理解这些材料功能的关键特征，因为它控制着它们在外力或内力下的宏观流动。利用大规模并行计算，该项目的第一个目标是检查粘弹性剪切和拉伸流动中的球形颗粒，这些流动与成型应用以及石油应用中的钻井泥浆或支撑剂有关。流变学将在增加颗粒负载时进行测量，然后在适当的流动中进行模拟-宏观关系由微观图像的正确总体平均确定。颗粒运动与弹性流体的三维耦合以及由此产生的应力场(颗粒诱导的流体应力)对流体的工程应用至关重要。该项目的第二个目标是使用相同的方法来研究可定向粒子(不同纵横比的刚性球体)和可变形粒子。对于后者，开发了一种新的计算工具，该工具使用非结构网格弹性流体中柔性颗粒固体的浸没有限元解。第三个目标是研究“活性物质”悬浮液，包括波动悬浮液和变形体悬浮液，如线虫、精子和细菌游泳。后一个目标将首次阐明悬浮液中的弹性如何影响具有真实粒子级别的游泳分辨率的“主动悬浮物”的集体运动。因此，将实现对指导这些应用的物理原理，甚至这些进化的弹性流体悬浮液的生物学的基础性理解的目标。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Particles suspended in fluids are omnipresent in all aspects of human life, including playing key roles in industrial processing and manufacturing as well as within the body itself. Many of the fluids in which these particles are suspended are elastic - meaning the fluids have a memory in time that is not short compared to the time scales associated with the flow. For example, the tail-like hairs on cells, known as cilia, that line human airways beat in mucus to sweep it from the lungs. Mucus is an elastic fluid, and the elasticity of the mucus directly affects the ability of cilia to move mucus out of the airway. In another context, many common composite plastic parts are molded in the liquid state and, thus, are elastic fluids that flow and contain rigid particle additives. Further, drilling muds and fracking fluids, used by the oil industry, are typically guar gum solutions containing proppants or drilled "shavings" and, again, involve suspensions in elastic fluids. While the science of suspensions in small molecule or "Newtonian" fluids (like water or most oils) has a long, well-developed history, the science of particle suspensions in viscoelastic fluids is in its infancy. Without fundamental understanding of the function of these elastic fluids, suspensions are frequently created on a trial-and-error basis for a given application. Accordingly, the goal of the project is to develop computational tools that enable the simulation of the complex physics associated with the collective effects of particles in viscoelastic fluids. These tools will enable one to engineer the fluid properties of a suspension directly and has vast implications for the economical and safe use of viscoelastic fluids in industrial applications. The research will be integrated into high school curriculum by engaging a K-12 teacher each year in a hands-on summer research program.The overall goal of the project is to examine the rheology of a series of increasingly complex particulate suspensions in viscoelastic fluids that are of direct importance to the energy industry, advanced manufacturing, and medical/biofluids applications. The rheology, or macroscopic stress-strain relationship, is the key feature in understanding the function of these materials since it governs their macroscopic flow under applied or internal forces. Employing large-scale, parallel computing, the first objective of the project is to examine spherical particles in viscoelastic shear and extensional flows associated with molding applications as well as drilling muds or proppants in oil applications. The rheology will be measured at increased particle loading and then simulated in the appropriate flow - with the macroscopic relationships determined from the correct ensemble-averaging of the microscopic picture. This 3-D coupling of the particle motions to the elastic fluid and resulting stress field (particle-induced fluid stress) is critical to the engineering application of the fluids. The second objective of the project is to use the same approach to study orientable particles (rigid spheroids of varying aspect ratio) and deformable particles. For the latter, a new computational tool using an Immersed Finite Element solution of flexible particulate solids in elastic fluids with unstructured grids has been developed. The third objective examines "active matter" suspensions, including undulating and amoeboid suspensions such as nematode worms, sperm, and bacterial swimming. The latter objective will, for the first time, shed light on how elasticity in the suspending fluid affects the collective motion of "active suspensions" with realistic particle-level resolution of the swimming. Thus, the goal of foundational understanding of the physical principles governing these applications, and even the biology of these evolved elastic fluid suspensions, will be achieved.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.