Aggregation, rheology and electrophoretic packing structure of aqueous A12O3 nanoparticle suspensions Original Research Article

The particle-packing structure and rheological behavior of aqueous alumina suspensions have been investigated using γ-phase Al2O3 nanoparticles (average particle size ∼37 nm) dispersed in pure water. The pH value of the suspensions varied from 2 to 11, and the solids concentration (φ) from φ=0.01–0.16. The suspensions generally exhibited a transition from pseudoplastic to dilatant flow as shear rate exceeded a certain critical level. This was particularly pronounced for well dispersed suspensions (mostly those with pH 2) over shear-rate range examined (1–1000 s−1). The onset shear-rate for dilatancy to occur appeared to be solids concentration and pH dependent. A particulate network was formed in suspensions when the solids loading exceeded 0.1 at pH 11. This network formation resulted in a flocculated structure so that the suspension did not settle even after prolonged periods without disturbance (24 h). The network formed was most likely of a fractal nature; for which an attractive van der Waals force dominates the interparticle potential over the entire cell volume investigated. An estimated fractal dimension (Df) of 2.27 was determined experimentally. This Df suggests operation of a reaction-limited cluster–cluster aggregation (RCLA) in the suspensions or the possibility that the aggregate underwent a substantial flow-induced rearrangement upon shearing.