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

The rheological behaviour of concentrated suspensions of coarse SiO2 particles containing various amounts of fine TiO2 has been studied using a viscometer which prevents settling of the suspended material. At a volume fraction of 0.468 the mixed SiO2/TiO2 suspensions exhibit shear thinning behaviour, with the yield stress and viscosity being strongly dependent on pH and the TiO2 content of the suspension. Four different TiO2 samples are investigated, each having different inorganic surface coatings. The TiO2 particles adsorb on the surface of the SiO2 particles and coat them, resulting in suspensions with isoelectric points corresponding to those of suspensions containing TiO2 alone. The changes in yield stress as a function of pH are influenced by the TiO2 content of the suspension, and when the silica:TiO2 volume ratio is greater than ∼0.03, the yield stress vs pH curve resembles that of a pure TiO2 suspension. The maximum yield stress is observed at a pH corresponding to the isoelectric point of the TiO2 component, and at this pH, networks of TiO2 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 TiO2 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.