KINETICS OF AGGREGATION AND DISAGGREGATION OF TITANIUM-DIOXIDE PARTICLES AND GLASS-BEADS IN A SHEARED FLUID SUSPENSION

Colloidal (0.2 mu m) titanium dioxide (rutile) particles aggregated wi th and disaggregated from much larger (26 mu M) glass beads in a conti nuously sheared suspension with constant pH and ionic strength chosen to yield relatively slow (hours to days) kinetics. Measured rates of a ggregation and disaggregation varied in response to systematic perturb ations in the fluid shear and in the concentration of titanium oxide p articles in suspension. The observed kinetics indicate that aggregatio n and disaggregation occurred simultaneously, the net direction of exc hange being determined by the relative rates of attachment and detachm ent. Aggregated TiO2 particles were either weakly attached and exchang ed relatively rapidly or strongly attached and exchanged relatively sl owly, probably reflecting the influence of TiO2 particle clusters and the geometric and electrostatic heterogeneity of the glass bead surfac e. The rate of fluid shear did not appear to affect significantly the kinetics of attachment and detachment, and the associated steady-state partitioning of TiO2 between the free and attached states. However, T iO2 particles that were effectively strongly attached under weak shear were observed to detach when the shear increased. Collisions between the glass beads had little effect on the aggregation and disaggregatio n rates.