Dissertations and Theses

Date of Award


Document Type



Chemical Engineering

First Advisor

Jeffrey F. Morris


scaling, suspension rheology, bidispersity, friction, shear thickening


Shear thickening, which corresponds to an increase of viscosity with shear rate, is ubiquitously observed in many concentrated suspensions and, as such, has implications for many industrial materials and geological phenomena. Controlling shear thickening is of significant importance, and many studies have been devoted to it. Recent works link shear thickening to the shear-induced transition from a lubricated particle interaction at low shear stress to a predominantly frictional interaction at high stress. In this dissertation, we investigate bidispersity and focus on mechanisms contributing to the viscosity evolution in rate-dependent suspensions, something that has not been addressed before. We perform detailed particle-scale simulations over a broad range of shear stresses using a simulation model for smooth spherical particles. This model accounts for short-range lubrication forces, frictional interaction, and repulsion between particles and implements the frictional transition scenario described above.

Using the results of the simulations, we investigate the effect of the size ratio of large to small particle radii and a fraction of the solid volume occupied by large particles on the rheology of suspensions. We demonstrate that the rate-dependent suspension viscosity displays a significant reduction, going from discontinuous shear thickening to continuous shear thickening, as the particle size ratio becomes larger. We find that under low shear stress conditions, the suspension exhibits an unusual rheological behavior of a gradual decrease in viscosity with the increase of large particle fraction, which we attribute to the particles undergoing ordering under shear flow. In the next simulation study, we explore shear thickening by performing a critical scaling analysis on the bidisperse suspensions while also comparing the results to those of nearly monodisperse suspensions. Specifically, we express the suspension rheological properties, such as the viscosity, the second normal stress difference, and the particle pressure in terms of a universal crossover scaling function between the frictionless and the frictional maximum packing fractions. The rheological properties have been shown to exhibit the scaling collapse identifying a distinctive collapse for the particle pressure, as well as the decrease in the exponent of the divergences of each rheological property studied when transitioning from frictionless to frictional regimes. Finally, we identify the dependency of the fraction of frictional contacts on suspension packing parameters, which form differs from the exponential prediction used widely in the field.



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