Models for Colloidal Aggregation

Abstract

Computer simulations have been used for more than a quarter of a century to develop a better understanding of non equilibrium growth and aggrega­ tion processes. Important early examples include VoId's ballistic deposi­ tion model for aggregation and sedimentation (1, 2), Sutherland's ballistic cluster-cluster aggregation model for floc formation (3), and Eden's surface growth model for the generation of cell colonies (4). Because of limited computer resources these early simulations were carried out on a small scale using simple models. Since these early pioneering efforts computer speed, storage capabilities, and availability have increased enormously and the task of writing computer programs has become much easier and more reliable. These developments have allowed two almost divergent research directions to develop. On the one hand, models have been developed that incorporate as much as possible of what is known about the physics and chemistry of aggregation processes, and on the other hand, a variety of very simple models have been introduced that allow us to generate the very large structures needed to investigate asymptotic (large size limit) geometric scaling relationships. The exploration of very simple aggregation models has been stimulated by the realization (5-10) that real aggregation processes and simple aggregation models frequently lead to structures that can be described in terms of the concepts of fractal geometry (11). In particular, the recent interest in nonequilibrium growth and aggregation models was initiated by the discovery by Witten & Sander (6) that a simple diffusion-limited aggregation (DLA) model in which particles are added, one at a time, to a growing cluster or aggregate of particles leads to