Sedimenting Particulate Suspensions in Viscoelastic Fluids Under Shear

Stanford University, Stanford CA

Investigators

Abstract

1337051 PI: Shaqfeh Suspensions 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.