The fundamental rheology of rigid particles suspended in viscous shear flows is still not well understood, and in particular, the origins of experimentally observed irreversibility in what is nominally a reversible flow are unclear. The main objectives of this collaborative project are to identify the primary causes of the irreversibility, to quantify the irreversibility, and to determine how best to include irreversibility into mesoscopic models of suspension flows. While there exists a general notion of irreversibility being caused by short-range interactions, the exact nature and magnitude of these interactions is unknown and, in many cases, various roughness models or repulsive forces are applied more or less in an ad hoc manner. As long as this fundamental research question remains unanswered, there is no fully satisfactory way to formulate any type of rheological model. The project will attempt to establish that a universal treatment of irreversibility must be scale-dependent. For relatively large particles (greater than, say,100 microns), the major cause of irreversibility in suspension flows is typically surface roughness. As the size of the particles decreases below 100 microns, it is hypothesized that there exists a transition in the major cause of irreversibility from surface roughness to nonhydrodynamic static forces which may include forces such as electrostatic and van der Waals forces. Furthermore, the project will establish that irreversible particle migration scales linearly with a measure of nonlinearity of the shear flow. The establishment of these hypotheses will have a strong impact on rheological models, which currently lack universality.
The proposed research directly affect several key areas of science and engineering, with many implications for everyday life. Suspension flows are important in a wide variety of evolving technologies including advanced materials processing, chromatography, encapsulation, microfluidics, secondary oil recovery by hydraulic fracturing, carbon-dioxide sequestration, and the transport of sediments, contaminants, and slurries, to name a few. Several rheological models have been developed over the past several years to help engineers develop effective processes involving suspensions. Although these models have been relatively successful in determining steady state concentration profiles, they fail to model the transient states. This research will remediate the shortcomings of the existing models. The research effort will be complemented with a strong educational and outreach component.