Yield stress and microstructure of washed oxide suspensions at the isoelectric point: experimental and model fractal structure

The yield stress and microstructure of washed, relatively monodisperse spherical zirconia (ZrO₂) and titania (TiO₂) suspensions at the isoelectric point (pI) were characterised. The yield stress was found to be dependent upon the particle size. At a given solid concentration, the finer suspensions produced a larger yield stress due to the higher particle concentration and hence, a greater density of attractive interaction. At pI, only the van der Waals force is in play. Vitrified fractal microstructures of these suspensions at pI were captured by cryo-SEM. A power law relationship described the (maximum) yield stress–volume fraction data for both oxides which is consistent with the prediction of scaling theory. An exponent value of ∼3 was obtained for both oxides. The fractal dimension (D _f) extracted from this exponent value of the scaling law for large aggregate cluster interaction in the slow flow regime was ∼2.3. The theoretical fractal structure with the same D _f constructed from monodisperse spherical particles based on the off-lattice variable-D _f model showed strong resemblance to the cryo-SEM imagery of the vitrified structure for both oxides.