The relationship between particle aggregation and rheology in mixed silica–titania suspensions

Abstract

The rheological behaviour of concentrated suspensions of coarse SiO 2 particles containing various amounts of fine TiO 2 has been studied using a viscometer which prevents settling of the suspended material. At a volume fraction of 0.468 the mixed SiO 2/TiO 2 suspensions exhibit shear thinning behaviour, with the yield stress and viscosity being strongly dependent on pH and the TiO 2 content of the suspension. Four different TiO 2 samples are investigated, each having different inorganic surface coatings. The TiO 2 particles adsorb on the surface of the SiO 2 particles and coat them, resulting in suspensions with isoelectric points corresponding to those of suspensions containing TiO 2 alone. The changes in yield stress as a function of pH are influenced by the TiO 2 content of the suspension, and when the silica:TiO 2 volume ratio is greater than ∼0.03, the yield stress vs pH curve resembles that of a pure TiO 2 suspension. The maximum yield stress is observed at a pH corresponding to the isoelectric point of the TiO 2 component, and at this pH, networks of TiO 2 aggregates can form. The formation and breakdown of these aggregates has been monitored in concentrated suspensions using focused beam reflectance measurement (FBRM). FBRM results clearly demonstrate that increases in the yield stress of the suspensions are accompanied by an increase in aggregate size. At a given pH, changes in plastic viscosity with increasing TiO 2 content in the suspensions appear to result from an increase in the maximum packing density due to the corresponding change in the particle size distribution.