Unifying disparate rate-dependent regimes in non-Brownian suspensions

A non-Brownian suspension undergoes shear thinning (decreasing viscosity) at low shear rates followed by a Newtonian plateau (constant viscosity) at intermediate shear rates which transitions to shear thickening (increasing viscosity) beyond a critical shear rate/stress and finally, a second shear-thinning transition is observed at extremely high shear rates. We quantitatively unify all the disparate non-Newtonian regimes and the corresponding transitions with increasing shear rates. Direct comparison with experimental data from the literature corroborates the validity of the model. Inclusion of traditional hydrodynamic, DLVO, and contact forces with constant friction reproduce the initial thinning and the shear thickening transition. However, to quantitatively capture the intermediate Newtonian plateau and the second shear thinning, an additional non-hydrodynamic interaction of non-DLVO origin and a decreasing coefficient of friction, respectively, are essential; thus, providing the first explanation for the Newtonian plateau along with reproducing the second shear thinning in a single framework. We demonstrate the model versatility in predicting a variety of other rate-dependent behaviors, normal stress differences, and particle tribology effects on suspension rheology.