(155d) Assessment of Gel Formation Conditions in Turbulent Jets

Control of aggregation and gelation of nanoparticles is an important issue in post-processing of dispersed systems, for example in the polymer and biotech industries. Aggregation of nanoparticles into clusters with a desired size and structure can be achieved in continuous processing, assuming that the colloidal stability of the primary particles can be manipulated by mixing with an appropriate coagulant, while at the same time gelation of the bulk dispersion is avoided. On the other hand, gelation might be desired in order to obtain gelled domains of appropriate size, while keeping the resulting solids in a suspended state. Since at the small particle sizes and large concentrations typical of industrial conditions, timescales of mixing, aggregation and gelation are comparable, one needs to account for the interplay between these processes in order to understand the effect of mixing on gel formation in nanoparticle dispersions. Appearance of gelation can be limited by breakage of large aggregates before they interconnect to fill the space or by sufficient dilution of the original dispersion. Onset of gel formation in turbulent jets was studied using a combination of computational fluid dynamics with population balance equations. We include the effect of the solid phase on the fluid flow through an effective viscosity for the mixture. Several different geometries for mixing dispersion of nanoparticles with coagulant were considered. The results are represented in the parameter space of the solid volume fraction and the primary particle diameter. Three distinct regions can be observed: (1) gelation occurs due to Brownian aggregation (small particles, high solid volume fraction), (2) gelation occurs due to shear aggregation (moderate particle size, moderate solid volume fraction) and (3) no gelation occurs before mixing of dispersion and coagulant streams is completed (larger primary particles, lower solid volume fraction).