Shear-induced aggregation of anatase dispersions investigated by oscillation and low shear rate viscometry

The evolution of shear-induced network structures of aggregates in concentr ated anatase dispersions was studied in a Couette cell by using long sequen ce times, low constant shear rates, and single-frequency oscillation at var ious shear stresses. The potential was used as a measure of the interaction forces between the particles. Dispersions at the vicinity of the isoelectr ic point were observed to aggregate during the early stages of shearing in a Couette cell. The aggregates formed a network of new weak interaction bon ds. Further shearing caused a breakup of the network structure. The rheolog ical response to this process was a peak in the shear stress (the structura l yield stress) as a function of cumulative shear strain at sufficiently lo w constant shear rate. The observed structural yield stress increased with decreasing shear rate and was an indication of the induction of a network s tructure. The behavior of unstable flocculated dispersions at low shear rat es was noticed to be strongly dependent of the shear rate history. Single-f requency oscillation measurements were performed and repeated after short i ntervals of monotonic shearing. The elastic modulus G' increased rapidly (e xceeding G") after a certain length of shearing, indicating the growth of a structure possessing weak gel-like properties. (C) 2001 Academic Press.