Simple Models for Particle Aggregation, Deposition and Segregation
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SIMPLE MODELS FOR PARTICLE AGGREGATION, DEPOSITION AND SEGREGATION PAUL MEAKIN E. I. du Pont de Nemours and Company, Central Research and Development Department, Wilmington, DE 19880-0356 ABSTRACT Most conventional and experimental methods for the formation of ceramic objects involve the manipulation of small particles and/or structures comprised of these particles. Consequently, a better understanding of the physics and chemistry of small particles could contribute to the development of better ceramics. Here the results of some recent computer simulation studies of processes such as particle aggregation, deposition and segregation are surveyed. More realistic models are needed to represent accurately the behavior of most real systems. INTRODUCTION Many of the most promising approaches to the manufacture of ceramics with superior mechanical properties involve the manipulation of small particles. Phenomena such as aggregation, segregation, packing, sedimentation, sintering and fragmentation are an integral and unavoidable part of many of these processes. Consequently, the development of improved ceramics can be facilitated by a better understanding of the behavior of large assemblies of small particles. The continual increase in the power, availability and convenience of digital computers has stimulated the development of a variety of simple models for the behavior of small article systems. Despite their unrealistic simplicity, these models frequently lead to structures and kinetic behavior that resemble quite closely those observed in real systems. The objective of this short survey is to describe some of the recent progress in this direction. AGGREGATION The aggregation of small particles to form extended structures is a ubiquitous process that has important implications for ceramic processing. It is an essential part of processes such as sol-gel processing [1] and can be a major impediment to the achievement of high densities in compaction processes. The use of computer models to study particle aggregation has a relatively long history [2,3]. However, most of the recent work in this area appears to have been stimulated by the introduction of the diffusion-limited aggregation (DLA) model by Witten and Sander [4] in 1981. In this model particles are added, one at a time, to a growing cluster or aggregate of particles via random walk trajectories. When a particle contacts the growing cluster, it is joined irreversibly to the cluster at its contact position. This model leads to the formation of random ramified structures that have a fractal dimensionality [5], D, that is distinctly smaller than that of the space or lattice in which the simulation process is embedded. Under some circumstances, this model can be used to describe fluid-fluid displacement during the drying of ceramic powders [6], but it does not provide a basis for understanding most colloidal aggregation processes. During recent years a variety of models have been developed to represent the aggregation of small particles to form tenuous random flocs. Th
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