The Effects of Fluid-Particle and Particle-Particle Interactions on the Structure and Flow Properties of Suspensions of Fibers and Disks

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AbstractCTS-0332902D. Koch, Cornell UniversityIntellectual Merit: A major challenge for the scientific understanding and engineering design of structured materials and complex fluids is to determine the effects of fluid flow on the structure and properties of non-spherical-particle suspensions. A balanced approach of analytical theory, computer simulation and experiment will be applied to two problems on the forefront of this topic: (a) The orientation of slender fibers in high Reynolds number laminar and turbulent flows; and (b) The effects of fluid-mediated particle-particle interactions on the structure and rheology of suspensions of disks. While detailed theoretical models and careful experimental measurements are available for the flow-induced structure in fiber suspensions in viscous fluids, relatively little is known about the rotation of fibers when fluid inertia is important on the fibers length scale. More generally, the proper description of particle-fluid interactions for particles whose size is comparable to or larger than the size of the smallest eddies of a turbulent flow constitutes the most important current challenge in the description of particle-laden turbulent flows. In the proposed project, a novel simulation method will be developed that couples a slender-body description of the force distribution along a fiber with a spectral solution of the Navier-Stokes equations to describe particle-fluid interactions in a turbulent flow when the fiber length is comparable with the size of the eddies of the turbulent flow and inertia influences the fluid velocity disturbances produced by the fiber. This model will be applied to predict the turbulence-induced dispersion of fiber positions and orientations and the rate of sedimentation of fibers in turbulent flows. The most basic question concerning the motion of fibers at finite Reynolds number is how a fiber will rotate in a moderate Reynolds number simple shear flow. In the PIs current NSF sponsored research, analytical predictions have been obtained indicating that inertial effects cause a fibers orientation to drift toward the vorticity axis of a simple shear flow. The proposed study includes an experimental investigation of the motion of single fibers in a Couette cell to validate the predictions for the direction and rate of migration of fiber orientation. The proposed project will also extend the experimental and analytical approaches used to study fluid-mediated fiber-fiber interactions to understand the nature of such interactions in materials filled with disk-shaped particles. Microlithography methods will be used to produce model systems of rigid disks suitable for studies of the rheology and flow-induced orientation of disks in Newtonian fluids over a range of particle concentrations. This experimental study will be complemented by a theoretical analysis of disk-disk interactions to predict the effects of hydrodynamic interactions on the orientation distribution and rheology of the disk suspensions. Broader Impacts: High-aspect ratio fibers and disks are commonly used to enhance the mechanical, thermal and electrical properties of polymeric materials. These properties are highly sensitive to the structure induced by fluid flow when the materials are processed in the molten state. At the present time, commercial software based on the rotation of fibers in a low Reynolds number Newtonian fluid with a rotary diffusion to describe fiber-fiber interactions is widely used to predict structure in fiber composites. The experimental and theoretical studies of the effects of disk-disk interactions on disk orientation will provide the first step toward developing engineering models for the flow-induced structure in polymeric materials filled with platelet shaped particles such as mica flakes and silica clay particles. During the production of paper, pulp fibers suspended in water flow onto a porous conveyer belt to form a fiber network. It is desirable to use a turbulent fluid flow to disperse the fibers uniformly in space with isotropic orientations. In these flows, the Reynolds number based on the fiber length is O (10). The simulation method developed in this project will provide the first rigorous description of fiber motion under these conditions. The project will train a doctoral student and undergraduate researchers with the ability to interface analytical approaches with computer simulation and validate their models experimentally.