The particle-packing structure and rheological behavior of aqueous alumina suspensions have been investigated using gamma-phase Al2O3 nanoparticles (average particle size similar to37 nm) dispersed in pure water. The pH value of the suspensions varied from 2 to 11, and the solids concentration (phi) from phi=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 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. (C) 2002 Published by Elsevier Science Ltd on behalf of Acta Materialia Inc.
