Investigating the causes of shear-thinning in non-colloidal suspensions: Experiments and simulations

Experiments and computations were carried out to explore the origins of shear-thinning in non-colloidal suspensions. Two grades of polydimethylsiloxane (silicone oil) and a glycerine/water mixture were used as matrices for the suspensions. The particles were 40µm diameter polystyrene (PS) and polymethyl methacrylate (PMMA) spheres. We concentrated on 40% volume fraction suspensions where shear-thinning was clear. The silicone oil matrices were nearly Newtonian: at 24°C the viscosity of the 1.15 Pa s sample showed a 2% drop in viscosity a shear rate of about 3000 s-1, the 13.2 Pa s sample showed a drop of 2% at a shear rate of approximately 100 s-1, and the glycerine/water sample appeared to be Newtonian at least up to 104 s-1. Mild shear-thinning was seen with all suspensions, beginning at shear rates of order 0.1-1 s-1, followed by a rapid reduction of torque in the parallel-plate system at shear rates of 14, 150 and 1000 s-1 respectively with the three matrices. These rapid reductions are ascribed to edge effects. Matching smoothed particle hydrodynamics (SPH) simulations were made. The silicone matrix viscosities were modelled by a Carreau-Yasuda (CY) fit up to shear rates of order 107 s-1. The agreement between computations and experiments is generally good for 40% volume fraction suspensions up to the shear rate where edge effects intervene in the experiments--there are no edge effects in the simulations. This confirms the suggestion by Vázquez-Quesada et al. [1] that 'hidden' high shear rates between particles, where the non-Newtonian matrix viscosity comes into play, can result in shear-thinning at the macroscopic level. For the glycerine/water matrix at low shear rates this mechanism does not apply and a separate mechanism based on variable interparticle friction is suggested; the two mechanisms can co-exist.