Viscometric functions of concentrated non-colloidal suspensions of spheres in a viscoelastic matrix

We have measured the viscometric functions (eta, N-1, and N-2) for a suspension of noncolloidal spherical particles in a Boger fluid matrix. The polymethyl methacrylate particles were nominally 40 mu m in size, and the matrix fluid was similar to that used by Zarraga et al. [J. Rheol. 45, 1065-1084 (2001)]. The volume concentrations (phi) ranged from 5% to 40%, and a combination of the open trough and parallel-plate methods was used. The viscosity did not shear-thin for higher concentrations, unlike the Newtonian matrix case; for the larger concentrations we saw a mild shear-thickening. This is in agreement with the work of Zarraga et al. [J. Rheol. 45, 1065-1084 (2001)] and Scirocco et al. [J. Rheol. 49, 551-567 (2005)]. N-2 was always negative, and its magnitude increased with concentration. N-1 was initially positive as in the matrix fluid, but for phi > 0.3, it became negative, similar to the result of Aral and Kalyon [J. Rheol. 41, 599-620 (1997)]; also, we found, at a constant shear stress, that vertical bar N-2 = N-1 vertical bar was greater for larger concentrations. The results suggest that the normal stresses, rather than the viscous stresses, dominate the interparticle forces so that the channel surface shapes never look like the sharply ridged Newtonian-matrix shapes [S. C. Dai et al., J. Rheol. 57(2), 493-510 (2013)], where viscosity dominates. Attempts to correlate the observed behaviour with theory gave useful results for lower concentrations. (C) 2014 The Society of Rheology.