The indirect quantification of apparent wall slip of highly concentrated suspensions in pressure driven flows is commonly performed using a Mooney analysis. To prevent poor fits and physically impossible results, several modifications to the original analysis have been proposed in the past. The modifications are mostly empirical and, to date, there is no best practice. In this contribution, the origin of the failing original analysis is shown and the accuracy of several modified analyses is compared. Measurements are performed on a high-pressure capillary rheometer using dies with a smooth and rough internal surface and suspensions with liquid phases showing different shear rate dependencies, i.e. having flow indices between 0.20–1.0. For both types of die, a radial dependency is observed, which is related to shear-induced migration of the solids and macromolecules in the suspension. The original Mooney analysis cannot describe the changes in the local rheology and physically impossible results are a direct consequence. To include the radial dependency in the Mooney analysis, a best-fit approach is advised, until the underlying physics of shear-induced migration are better understood.

The rheological characterization of concentrated suspensions is complicated by the heterogeneous nature of their flow. In this contribution, the shear viscosity and wall slip velocity are quantified for highly concentrated suspensions (solid volume fractions of 0.55–0.60, D 4,3 ~ 5 µm). The shear viscosity was determined using a high-pressure capillary rheometer equipped with a 3D-printed die that has a grooved surface of the internal flow channel. The wall slip velocity was then calculated from the difference between the apparent shear rates through a rough and smooth die, at identical wall shear stress. The influence of liquid phase rheology on the wall slip velocity was investigated by using different thickeners, resulting in different degrees of shear rate dependency, i.e. the flow indices varied between 0.20 and 1.00. The wall slip velocity scaled with the flow index of the liquid phase at a solid volume fraction of 0.60 and showed increasingly large deviations with decreasing solid volume fraction. It is hypothesized that these deviations are related to shear-induced migration of solids and macromolecules due to the large shear stress and shear rate gradients.

Wall slip quantification using the classical Mooney slip analysis has produced physically unreasonable results for many complex fluids. Over the past decades, the assumption that the slip velocity is solely dependent on the wall shear stress has therefore been questioned. In this contribution, the influence of the radius of cylindrical dies on the wall slip velocity of highly concentrated non-Brownian suspensions in a high-pressure capillary rheometer is re-examined, by varying the rheology of the liquid phase. Water- and oil-based suspensions (solid volume fraction similar to 0.60, D-4,D-3 similar to 5 mu m) are made using liquid phases that have different flow indices (n = 0.22 - 1.00) and a variation in their thickener concentration (25 g/L - 350 g/L). All classical Mooney plots showed negative y-intercepts and the fit worsened with decreasing flow index. A modification to the Mooney analysis is proposed that includes a geometrical dependency of the slip velocity that scales with the flow index of the liquid phase. Proposed modified Mooney plots not only show positive y-intercepts, but also show a good fit (R-2 > 0.99) over the entire range of shear rates (10 - 640 s(-1)) and corresponding wall shear stresses. The geometrical dependency is thought to arise from the shear stress gradient within the extrusion die.