(339s) Restructuring and Fragmentation of Agglomerates in Shear Flows

The nanoparticle product performance is strongly affected by the primary particle size distribution. However the morphology and degree of particle agglomeration can influence drastically the product properties also. At the initial reactor stages particles grow by Brownian motion while later on and in particular at larger sizes particles may grow by flow-induced coagulation also. As agglomerates become comparable to the size of fluid eddies, the effect of fragmentation increases. Finally a balance between coagulation and fragmentation is reached.

Spicer et al. [1] used light scattering to monitor size and structural dynamics of polystyrene-alum flocs in a stirred tank. They showed that for the production of fast settling particles for particle removal, e.g. in wastewater treatment, it is beneficial to apply cycled-shear-induced flocculation. Flesch and Pratsinis [2] developed a sectional population balance model to simultaneously trace the coagulation and fragmentation in turbulent shear. They accounted for the agglomerate fractal structure using appropriate kinetic expressions. Good agreement of the evolution of the mass mean agglomerate diameter was found between flocculation experiments of polystyrene-Al(OH)3/ water systems in a stirred tank and the model results.

The stability of nanoclusters made by particle-particle interactions is investigated by simulations focusing on the forces holding together such nanostructured particles made in aerosol reactors. Agglomerates are formed by cluster-cluster agglomeration, the dominant mechanism in flame reactors, and are exposed to shear flow fields. The attainment of asymptotic or self-preserving-like structures and distributions is explored.

1. Spicer, P.T., S.E. Pratsinis, J. Raper, R. Amal, G. Bushell, and G. Meesters, Effect of shear schedule on particle size, density, and structure during flocculation in stirred tanks. Powder Technology, 1998. 97(1): p. 26-34.

2. Flesch, J.C., P.T. Spicer, and S.E. Pratsinis, Laminar and turbulent shear-induced flocculation of fractal aggregates. AIChE Journal, 1999. 45(5): p. 1114-1124.