(533c) Experiments and Modeling for Understanding Brekage and Restructuring of Colloidal Aggregates in Contrcting Nozzle
AIChE Annual Meeting
Wednesday, November 10, 2010 - 3:55pm to 4:15pm
The size and size distribution of colloidal aggregates is important in many chemical processes such as polymer processing, coagulation processes, etc. The size of a colloidal aggregate in such processes is decided by the initial size and structure of the aggregate, size of primary particle in the cluster, applied flow field, inter-particle forces, and so forth. The experimental and modeling efforts provided so far still fail to predict the fate of a colloidal aggregate, and do not provide summarizing scaling laws to clarify our understanding on the interdependence of such quantities. On one hand experimental efforts lack modeling proofs and generalization of the observed phenomena while on the other hand, theoretical modeling of such processes still lacks experimental proofs. For example, when clusters are subject to shear, two phenomena occur simultaneously, i.e. breakage and restructuring; the former reduces the size of the aggregate whereas the latter leads to densification of the aggregate. The transition between two processes, which is a function of cluster geometry, is an untouched subject of study. Additionally, recent experimental evidence by Pantina and Furst indicate the presence of bending moments due to contact forces between particles in clusters, but there are only a few simulation studies where such tangential contact forces are included. In this work we set out to provide further clarification to these phenomena, by combining both experimental work and computer simulations. Our experimental work focused on pure breakage of Diffusion Limited Colloidal Aggregates (DLCA) in simple shear and elongation flows. Aggregates of a specific size were prepared by fully destabilizing the primary particles and allowing them to grow under stagnant conditions. Different aggregates were produced starting with different primary particle sizes. The aggregates were then exposed to simple shear in a rheometer and elongational flow in a nozzle (Soos et al.). The time evolution of the average fractal dimension and radius of gyration (Rg) was measured by light scattering and image analysis measurements, wherever possible. The obtained data were used to define scaling laws. To model the process of aggregate breakup, in the present work we have used Stokesian Dynamics (Brady and Bossis) method, which accurately estimates the hydrodynamic interactions between particles by approximating the grand resistance matrix. All particles experience Van der Waals interaction and Born repulsions, which are computed in a pair-wise manner, while particles experience also tangential forces, which are modeled as suggested by Becker and Briesen. Many simulations were carried out for aggregates with various sizes, fractal dimension, primary particle sizes, and different flow fields. The dependence of stable aggregate size on these variables was investigated. Starting from original aggregate size the evolution of average radius of gyration and size distribution of formed chains was studied as well. The experimental results were compared qualitatively with the modeling results and scaling laws were found. The modeling efforts helped to understand and distinguish between breakage and restructuring. Systematic study of dependence of resulting aggregate geometries on different flow conditions and initial aggregate properties helped to express lumped parameters which can be used in population balance breakage kernel.