Dilatant Flow of Concentrated ZnO-Water Suspensions in a High Shear-Rate Region

Abstract

The dilatant behavior of concentrated ZnO-water suspensions stabilized with various amounts of sodium hexamethaphosphate was studied using a capillary viscometer. The measurements were made in a high shear-rate range under a variety of colloidal stability, solid concentration, history of shearing, and capillary length. It was found that the extent of dilatancy, which was expressed by the power index of the shear stress-rate of shear relation, increased with increasing colloidal stability and solid concentrations. The suspensions deflocculated mechanically at a high shear-rate were relatively stable at rest, but were flocculated by applying shear at lower rates of shear. Flocculated suspensions showed the pseudo-plastic flow in the shear-rate range where the dilatant flow was observed for deflocculated systems. The dilatant flow was also dependent on the length of capillaries used in the measurements. At lower shear-rates pressure drops were linearly correlated to the length to radius ratio L/R of the capillary. With increasing shear-rate, the plots deviated downwards at smaller values of L/R from the linear relation. The entrance effect was corrected by employing Bagley's method, and the Newtonian behavoir, which was almost coincident with that of the second Newtonian region, was obtained. It was postulated that the dilatant behavior observed in this study was attributed to energy dissipation due to an unsteady flow near the entrance of the capillaries.