Sedimenting Particulate Suspensions in Viscoelastic Fluids Under Shear

1337051PI: shaqfeh粘弹性流体中刚性颗粒的悬浮液在许多能源应用，材料设计应用和消费品应用中发挥关键作用。例如，在石油钻井中，所谓的钻井泥浆是一种非常粘稠的粘弹性流体，在钻井过程中被设计成剪切薄，但在停止时变厚，这样岩屑就可以保持悬浮状态。在水力压裂（一种用于提高油气产量的作业）的相关应用中，一种被称为支撑剂的固体悬浮液通过泵入井中来支撑裂缝的打开。支撑剂的高质量性能要求放置在裂缝深处，这就要求在流动过程中支撑固体的多余重量。一种常用的支撑剂运输液体是瓜尔胶聚合物水溶液，它与硼酸盐离子短暂交联，因此具有很高的弹性。众所周知，粘弹性流体中颗粒的沉降可能与牛顿流体中观察到的沉降大不相同，特别是在剪切作用下。例如，在非牛顿流体中，复杂的流变特性导致了沉积和剪切流动之间的非线性耦合，这在牛顿流体中是没有的，这对上述钻井和压裂流体的功能至关重要。在相关材料应用中，从表面清除颗粒物质，特别是在与硅晶圆蚀刻相关的应用中，通常通过称为冲洗的后处理来完成。最近，有研究表明，即使在相同的粘度下，粘弹性溶液也比牛顿溶液成为更有效的漂洗剂。同样，在这个应用中，流动的剪切流体中的弹性似乎以一种未知的方式改变了受重力作用的颗粒在与剪切流正交方向上的力，从而产生了增强的升力，从而清洁了表面。在这个项目中，研究小组将开发粘弹性流体中有限浓度颗粒悬浮液的大规模计算机模拟。重点将放在受物体力（即重力）作用的粒子上，以及在与重力正交的方向上施加剪切流，目的是理解上述应用。计算机模拟将是独一无二的，首次将弹性液体中的非结构化有限体积技术与浸入式边界技术相结合。在作者看来，这样的模拟是理解这些高度非平衡流动应用中的非线性物理的唯一已知方法，从而设计这些流体，而不是根据历史先例开发它们。

1337051PI: ShaqfehSuspensions of rigid particles in viscoelastic fluids play key roles in many energy applications, materials design applications, and consumer product applications. For example, in oil drilling the so-called drilling mud is a very viscous, viscoelastic fluid designed to shear-thin during drilling, but thicken at stoppage so that the cuttings can remain suspended. In a related application known as hydraulic fracturing (an operation used to stimulate petroleum and gas production) suspensions of solids called proppant are used to prop open the fracture by pumping them into the well. Quality performance of the proppant demands placement deep into the fracture requiring that the excess weight of the solids be supported during the flow. A commonly used proppant-transport liquid is an aqueous guar polymer solution, transiently cross-linked with borate ion, such that it is highly elastic. It is well known that the sedimentation of particles in a viscoelastic fluid can be quite different from that which is observed in Newtonian fluids, especially under shear. For example, in a non-Newtonian liquid, the complex rheological properties induce a nonlinear coupling between the sedimentation and shear flow which is not found in Newtonian liquids and which is critical to the function of the aforementioned drilling and fracking fluids. In a related materials application, the cleaning of particulate matter from surfaces, particularly in applications associated with etching of silicon wafers is often done by a post processing known as rinse. Most recently, it has been shown that a viscoelastic solution becomes a far more effective rinse agent than a Newtonian solution, even at the same viscosity. Again, in this application, the elasticity in a flowing, sheared fluid seems to change, in a manner as yet unknown, the forces on particulates subject to gravity in directions orthogonal to the shear flow, creating thereby an enhanced lift force that cleans the surface. In this project, the research team will develop large scale, computer simulations of particle suspensions at finite concentration in viscoelastic fluids. The focus will be on particles acted on by a body force (i.e. gravity) as well as an applied shear flow in a direction orthogonal to gravity with the goal of understanding the applications described above. The computer simulations will be unique, first of their kind, combining unstructured, finite volume technology with immersed boundary techniques in elastic liquids. Such simulations are, in the authors view, the only known way to understand the nonlinear physics in these highly nonequilibrium flow applications and thus to engineer these fluids rather than develop them based on historical precedent.