(27e) Shear-Rate Dependence of the Initial Cluster Aggregation Kinetics of Polystyrene Latices Under Turbulent Conditions in Stirred Tank

Ehrl, L., ETH Zurich
Morbidelli, M., Institute of Chemical and Bioengineering, ETH Zurich

When producing or processing sub-micron particles, e.g., in the case of waste-water treatment, emulsion polymerization, or nano-particle precipitation, knowledge of the dependence of the aggregation kinetics on shear rate is required for process design and control. In addition, the correct description of the aggregation kinetics is a key to properly model and simulate such processes, as well as the basis for the development and validation of any breakage model. Therefore, the aggregation kinetics of model colloidal systems consisting of polymer latexes with different primary particle diameter (120nm, 420nm, and 810nm) were investigated under turbulent conditions in stirred tank. It was found that the integral quantities of the cluster population measured by small-angle static light scattering, i.e., the root-mean-square radius of gyration, the zero-angle intensity of scattered light, and obscuration (extinction) all scale with a dimensionless time of the form α×<Gφ×t. The prefactor represents an experimentally obtained aggregation efficiency which according to experimental data follows a power law of the form α∝<G>n, with the value of the exponent, n, being negative and dependent on the primary particle size. The broad range of primary particles sizes allowed investigating the transition from diffusion-affected aggregation to purely shear-induced aggregation. The results of this work together with literature data provide a relation between the experimentally obtained aggregation efficiency and a characteristic dimensionless group, Nf, which is proportional to the ratio of the Hamaker constant normalized by thermal energy over the Péclet number. These findings can help in the practical design of coagulation processes as well as to benchmark models on the aggregation and breakage of colloidal systems under turbulent conditions.