Presidential Young Investigator Award: Dynamic Properties of Concentrated Suspensions (REU Supplement)

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Narrative: The use of concentrated suspensions of non-colloidal particles in such diverse applications as the flow of blood in arteries and artificial organs and of coal-oil slurries in mixed fuel furnaces makes the detailed understanding of their properties imperative. Several aspects of the behavior exhibited by these types of suspensions which are, as yet, poorly understood are being investigated. SHEAR INDUCED MIGRATION: It has been shown that when concentrated suspensions of non- colloidal particles are sheared, the particles will drift across streamlines due to gradients in concentration. The mechanism proposed as an explanation for this migration suggests that shear induced migration may arise from gradients in shear stress as well. Preliminary experiments provide indirect evidence for this source of particle drift; however, the results to date are far from conclusive. Experiments aimed at proving or disproving the existence of migrations arising from gradients in shear stress are being conducted. THE EFFECT OF SURFACE ROUGHNESS ON SUSPENSION RHEOLOGY: Recent work has demonstrated that the behavior of a concentrated suspension of non-colloidal particles is critically dependent on the surface properties of the particles. This is primarily due to the closeness of the approach of the particle surfaces in a suspension undergoing shear. Preliminary analysis suggests that the most significant non-Newtonian properties manifested by these suspensions result from the presence of a small degree of surface roughness on these particles. Using both experimental and numerical simulation, it is being attempted to quantify the relationship between surface roughness and the rheological properties of concentrated suspensions. FLOW INDUCED MICROSTRUCTURE IN CONCENTRATED SUSPENSIONS: The microstructure of a suspension plays a critical role in determining important rheological properties. While much progress has been made in determining the microstructure in dilute systems, for concentrated suspensions it remains poorly understood. A key question that is as yet unanswered is the degree to which particles aggregate in concentrated suspensions of non-colloidal particles. Numerical simulations suggest that the lubrication forces between interacting particles may lead to the formation of large structures in concentrated suspensions; however, the formation of aggregates should be a strong function of particle surface properties and the existence of large aggregates has yet to be confirmed experimentally. Using a novel experimental technique, it is being attempted to determine the degree of aggregation by measuring both the particle distribution function and particle velocity distribution of a concentrated suspension of non-colloidal spheres undergoing simple shear flow. Eventually, this study will be expanded to include the complete determination of the microstructure of concentrated suspensions as a function of the flow geometry, particle surface properties such as surface roughness, and particle shape.