On the difficulty of determining the apparent wall slip of highly concentrated suspensions in pressure driven flows: The accuracy of indirect methods and best practice

Abstract

• Original Mooney analysis failed for concentrated suspensions. • Same analysis performed using dies with a rough internal surface. • Observed radial dependency related to shear-induced migration. • Best-fit approach advised for the slip velocity relation of concentrated suspensions. • Applies to concentrated suspensions with Newtonian and non-Newtonian matrices. 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.