We propose a unifying mechanism based on the Stribeck curve for the coefficient of friction between the particles to capture the shear thinning - Newtonian plateau - shear thickening - shear thinning rheological behavior at low - intermediate - beyond critical - high shear rates, respectively, for a typical dense non-Brownian suspension. We establish the accuracy of the proposed model by comparing the numerical results with experimental data. The presence of non-DLVO (Derjaguin and Landau, Verwey and Overbeek) forces and a coefficient of friction reducing with asperity deformation explain the existence of the Newtonian plateau at the intermediate shear rates and the second shear thinning regime at high shear rates, respectively. 

A constitutive model for sheared dense suspensions of rough particles (Journal of Rheology, 2020)

We propose a constitutive model to quantify the effect of increasing the roughness size on the rheology of dense shear thickening suspensions as well as on the critical shear rate for ST and the critical volume fraction for discontinuous shear thickening. We fit this model to our simulation data for stress controlled shear flow of dense rough particle suspensions. These equations are used to 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. The results of this study can be used to tune the particle surface roughness for manipulating the dense suspension rheology according to different applications.

Roughness induced shear thickening in frictional non-Brownian suspensions: A numerical study (Journal of Rheology, special issue PDS, 2020)

Increasing roughness leads to early onset of shear thickening, especially discontinuous shear thickening, in terms of both the critical shear rate and the critical volume fraction. In addition, roughness enhances the strength of the shear thickening effect as it leads to increase in the viscosity of dense suspensions. Increasing roughness leads to denser contact networks with high contact stresses and reduction in the jamming fraction. These results are consistent with recent experimental studies, which indicates that the computational framework developed can be utilized to predict and tune suspension behavior for specific applications.

Effect of roughness on the rheology of concentrated non-Brownian suspensions: A numerical study (Journal of Rheology, 2019)

The relative viscosity and the normal stress difference increase with the roughness of the particles. These findings show satisfactory agreement with recent experiments. We also fit our data to the Maron–Pierce law to predict the relative viscosity with varying volume fractions and roughness. The jamming volume fraction decreases with the particle roughness owing to the increase in effective particle radii and the average coefficient of friction with roughness. The jamming fraction is also dependent on the stress and increases with stress. This directly leads to an increase in the relative viscosity with roughness in suspensions of rough non-Brownian particles.

Improved solar cell performance with rough silver electrode suspensions (Advanced energy and sustainability research, 2021)

Investigating the settling dynamics of rigid objects in a fluid has historically been an important area of research. In this study, we find that both the oblate and prolate spheroids reorient to the edge-wise and partially edge-wise orientations, respectively, as they settle in a stratified fluid that is completely different from the steady-state broad-side on orientation observed in a homogeneous fluid. We observe that the reorientation instability sets in if the velocity magnitude of the spheroids falls below a particular threshold. We analyze the vorticity generation due to the non-alignment of density gradient with the direction of gravity or the baroclinic vorticity generation in the vorticity equation to explain the reorientation instability. The asymmetry in the distribution of this vorticity generation term around the spheroids explains the onset of reorientation instability in the settling motion of spheroids in a stratified fluid. 

Orientation instability of settling spheroids in a linearly density stratified fluid (Journal of Fluid Mechanics, 2021)

Investigating the settling dynamics of rigid objects in a fluid has historically been an important area of research. In this study, we find that both the oblate and prolate spheroids reorient to the edge-wise and partially edge-wise orientations, respectively, as they settle in a stratified fluid that is completely different from the steady-state broad-side on orientation observed in a homogeneous fluid. We observe that the reorientation instability sets in if the velocity magnitude of the spheroids falls below a particular threshold. We analyze the vorticity generation due to the non-alignment of density gradient with the direction of gravity or the baroclinic vorticity generation in the vorticity equation to explain the reorientation instability. The asymmetry in the distribution of this vorticity generation term around the spheroids explains the onset of reorientation instability in the settling motion of spheroids in a stratified fluid. 

Hydrodynamic Interactions between swimming microorganisms in a linearly density stratified fluid (Physical Review E, 2021)

Oceans and lakes sustain intense biological activity due to the motion of marine organisms which has significant ecological and environmental impacts. Motion of individual organisms and their interactions with each other play a significant role in the collective motion of swimming organisms. However, vertical density stratification is ubiquitous in these aquatic environments which significantly alters the swimmer interactions as compared to in a homogeneous fluid. Furthermore, organisms have sizes varying over a wide range which results in a finite inertia. To this end, we investigate the interactions between a pair of model swimming organisms using DNS of Navier Stokes equations. 

Motion of an inertial squirmer in a density stratified fluid (Journal of Fluid Mechanics, 2020)

Stratification reduces the swimming speed of pusher and puller organisms. In addition, it also stabilizes the flow around a puller keeping it axisymmetric even at high Re, thus, leading to stability which is otherwise absent in a homogeneous fluid for Re greater than O(10). On the contrary, a strong stratification leads to instability in the motion of pushers by making the flow around them unsteady and three-dimensional, which is otherwise steady and axisymmetric in a homogeneous fluid. A pusher is a more efficient swimmer than a puller owing to efficient convection of vorticity along its surface and downstream.

Mixing dynamics in double-diffusive convective stratified fluid layers (Current Science, 2018)