The characterization of the particle normal stresses of concentrated granular suspensions by local rheometry

The normal and shear viscosities of non-Brownian suspensions are measured by optical suspension imaging for particle volume fractions f between 0.3fm and 0.98fm, where fm is the jamming fraction. Two distinct refractive-index-matched suspensions, made with the same polymethyl methacrylate spherical particles dispersed in a mixture of water and Triton X-100, are studied. One is density-matched while the other one is negatively buoyant. They are both sheared in a Couette rheometer where the velocity and particle volume fraction fields are measured. The shear viscosity and the second particle normal stress S p 22 are determined through the study of these profiles in the neutrally buoyant suspension, while the third particle normal stress S p 33 is deduced from the analysis of the vertical f profiles measured in the negatively buoyant suspension. Our results indicate that the shear viscosity decreases with shear stress S12, and that this shear-thinning behaviour can be captured by the variation of fm with S12. We show that S p 33 is proportional to S12, and that S p 33/.0.. is a function of only f/fm(S12). The values of S p 22 deduced from the radial f profiles are consistent with the results of Zarraga et al. (J. Rheol., vol. 44, 2000, pp. 185-220). We conclude by discussing our results in the framework of the mu(J) rheology for viscous numbers J ranging from 2 x 10-4 to 3 x 101. We obtain very good agreement with the results obtained by Boyer et al. for J 10-1 (Phys. Rev. Lett., vol. 107, 2011, 188301) and by Zarraga et al. for J 10-1.

Automated summary

The normal and shear viscosities of non-Brownian suspensions were measured using optical suspension imaging. Two suspensions with different refractive indexes were studied, one density-matched and the other negatively buoyant. The shear viscosity decreases with shear stress and the third particle normal stress is proportional to shear stress. The results are in agreement with previous studies and are discussed in the context of mu(J) rheology.