Thixotropy and Rheopexy of TiO<SUB>2</SUB>-Water Suspensions

Abstract

Constant shear-rate tests and cyclic shearing tests were carried out in a coaxial cylinder viscometer for TiO2-water suspensions stabilized with various amounts of sodium pyrophosphate under a wide variety of histories of shear-rate and rest period before measurements. The electrophoretic mobilities and sedimentation behavior were also measured as a means of studying the stability of the suspension and the state of aggregate. The rheological properties of these suspensions were greatly influenced by the previous history of shear-rate and rest period, and both of thixotropic and rheopectic behavior were observed depending on the history. From these results, the structural rebuild-up due to the mechanical force by shear was recognized to be different in nature from that due to the Brownian motion during rest. These were attributed to aggregation of particles at the primary minimum in the potential energy curve, and to aggregation at the secondary minimum, respectively. The mechanism of the rheopexy observed in this study was attributed to the transient behavior resulting from the structural change under shear from the aggregates at the secondary minimum to those at the primary minimum in the potential energy curve. Thixotropic behavior, as well as non-Newtonian behavior in an equilibrium flow curve, was also studied on the basis of the theory of stability of lyophobic colloids.