Abstract
The particle-packing structure and rheological behavior of aqueous alumina suspensions have been investigated using γ-phase Al 2O 3 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 ( D f) of 2.27 was determined experimentally. This D f 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.
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