(256a) On the Shear Rheology of Particle Suspensions in Shear Thinning Polymer Solutions

Although the rheology of nonBrownian particle suspensions in Boger fluids show shear thickening at modest rates and particle concentrations, most experiments in the literature have examined suspensions in shear thinning fluids. The latter are of greater practical application. These generally do not show shear thickening and in fact, generally show more shear thinning (i.e. rates of viscosity decrease with shear rate higher) than the fluid itself. Thus we examine the shear rheology of particle suspensions in shear-thinning polymeric fluids experimentally using parallel plate measurements, and numerically using fully resolved, 3D finite volume simulations with the Giesekus fluid model. We show in our experiments that the steady shear viscosity and first normal stress difference coefficient of the suspension evolve from shear-thickening to substantially shear-thinning as the degree of shear-thinning of the suspending fluid increases. Moreover, in highly shear-thinning fluids, the suspension exhibits greater shear-thinning of the viscosity than the suspending fluid itself. Our dilute body-fitted simulations show that in the absence of hydrodynamic interactions, shear-thinning can arise from the particle induced fluid stress (PIFS) which ceases to grow with the increasing shear rate at low values of β (solvent viscosity ratio) and finite values of α (the Giesekus drag coefficient). In a Giesekus suspending fluid, the polymers surrounding the suspended particle are unable to stretch sufficiently at high Weissenberg numbers (Wi) and the reduced polymer stress results in a lower PIFS. When coupled with the shear- thinning stresslet, this effect creates an overall shear-thinning of the viscosity. We then explore the effects of particle-particle interactions on the suspension rheology using immersed boundary simulations. We show that multi-particle simulations are necessary to obtain the shear-thinning behavior of the per-particle viscosity of suspensions in shear-thinning fluids at moderate values of β. Particle-particle interactions lead to a substantial decrease in the PIFS and an enhancement of the shear-thinning of the stresslet compared to the single particle simulations. This combination leads to the shear-thinning of the per-particle viscosity seen in experiments.

Finally, if time permits, we discuss the effective rheology of thin layers (i.e. layers a few particles in size) of particle suspensions in viscoelastic fluids, as these are important in some advanced manufacturing applications. Thus, we examine how boundaries increase or decrease the effective shear resistance to flow.