The Rheology of Complex Suspensions In Viscoelastic Suspending Fluids

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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.