Shear-Induced Aggregation of Anatase Dispersions Investigated by Oscillation and Low Shear Rate Viscometry

The evolution of shear-induced network structures of aggregates in concentrated anatase dispersions was studied in a Couette cell by using long sequence times, low constant shear rates, and single-frequency oscillation at various shear stresses. The ζ potential was used as a measure of the interaction forces between the particles. Dispersions at the vicinity of the isoelectric point were observed to aggregate during the early stages of shearing in a Couette cell. The aggregates formed a network of new weak interaction bonds. Further shearing caused a breakup of the network structure. The rheological response to this process was a peak in the shear stress (the structural yield stress) as a function of cumulative shear strain at sufficiently low constant shear rate. The observed structural yield stress increased with decreasing shear rate and was an indication of the induction of a network structure. The behavior of unstable flocculated dispersions at low shear rates was noticed to be strongly dependent of the shear rate history. Single-frequency oscillation measurements were performed and repeated after short intervals of monotonic shearing. The elastic modulus G′ increased rapidly (exceeding G″) after a certain length of shearing, indicating the growth of a structure possessing weak gel-like properties.