Part I: Micromechanics of Dense Suspensions: Microscopic Interactions to Macroscopic Rheology & Part II: Motion in a Stratified Fluid: Swimmers and Anisotropic Particles

Suspensions of rigid particles in fluid media are ubiquitous in the industry as well as in biological and natural flows. Fresh concrete, uncured solid rocket fuel, and biomass slurries are typical industrial applications of such concentrated suspensions, while silt transport in rivers and blood are examples of naturally occurring suspensions. In these applications, rheological properties and flow behavior are of interest for high-volume fractions of particles. The suspending fluid medium is typically Newtonian in these suspensions; still, these suspensions exhibit a plethora of non-Newtonian properties such as yield stresses, rate-dependent rheology, normal stresses, to name a few. Other than volume fraction, the type of particle material, presence of fluid-particle or particle-particle interactions such as hydrodynamic, Brownian, colloidal, frictional, chemical, and/or electrostatic determine the rheological behavior of suspension. The average inter-particle gaps between the neighboring particles decrease significantly as the suspension volume fraction approaches the maximum dry packing fraction in dense suspensions. As a result, in this regime, the short ranged non-contact interactions of DLVO (Derjaguin and Landau, Verwey and Overbeek) and non-DLVO origins are important. In addition, the particles can come into direct contact due to asperities on their surfaces. The surface asperities are present even in the case of so-called smooth particles, as particles in real suspensions are not perfectly smooth. Hence, contact forces arising from the direct touching of the particles become one of the essential factors to determine the rheology of suspensions. Part I of this thesis investigates the effects of microscopic inter-particle interactions on the rheological properties of dense suspensions of non-Brownian particles by employing discrete particle simulations. Hydrodynamic interactions are calculated using the Ball-Melrose approximation, and the surface roughness is modeled as a hemispherical asperity on the particle surface. We show that increasing the roughness size results in a rise in the relative viscosity and the normal stress difference in the suspensions. Furthermore, we observe that the jamming volume fraction decreases with the particle roughness underlining the pivotal role in dictating the rheological behavior of dense suspensions of rigid particles. Conse quently, for suspensions with volume fractions close to jamming, increasing the asperity size reduces the critical shear rate for shear thickening (ST) transition, resulting in an early onset of discontinuous shear thickening (DST, a sudden jump in the suspension viscosity at the critical shear rate) in terms of volume fraction, and enhances the strength of the ST effect as it leads to an increase in the viscosity of dense suspensions. These findings are in excellent agreement with the recent experimental measurements and provide a deeper understanding of the experimental findings. Finally, we propose a constitutive model to quantify the effect of the roughness size on the rheology of dense ST suspensions to span the entire phase-plane. These equations can predict exact volume fractions and shear stress values for transitions between three regimes on the shear stress-shear rate flow state diagram for different roughness values. Thus, the constitutive model and the experimentally validated numerical framework proposed can guide experiments, where the particle surface roughness is tuned for manipulating the dense suspension rheology according to different applications