Computation and experiment in non- colloidal suspension rheology

This paper presents a current view of non-Brownian (non-colloidal) suspension rheology; experimental and computational works are compared where possible. The matrix fluids are either Newtonian or viscoelastic; the rigid particles are compact and close to spherical. Volume fractions of 0.5 and below are considered. It is important to move beyond steady simple shearing, so unsteady shearing and uniaxial extensional flows are discussed. Steady shearing is fairly well understood but the extensional flows are not prominent in the literature. In steady shearing with Newtonian matrices the role of friction and the estimation of the average friction coefficient, which reduces as the macroscopic shear stress (or shear rate) increases, is discussed. The ratio of the two normal stress differences to the shear stress is modelled well provided a reasonable value for the average friction coefficient is assumed. With viscoelastic matrices agreement between experiment and theory is less satisfactory. Unsteady and oscillatory flows are surveyed briefly; the prominent hysteresis in oscillatory flows appears to be a frictional effect. While progress has occurred it appears that satisfying agreement between computation and experiment is rare. More attention to rheological and frictional modelling is needed, and improved computational methods may need to be developed.