Wall slip of highly concentrated non-Brownian suspensions in pressure driven flows: A geometrical dependency put into a non-Newtonian perspective

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.