Shear-induced aggregation of colloidal particles - A comparison between two different approaches to the modelling of colloidal interactions

Several methods have been proposed to investigate the dynamics of processes including aggregation and breakup of colloidal particles. Most approaches resort to Population Balance Equations, frequently solved in a stochastic way (Monte Carlo simulations). These methods have a relatively low computational cost, but they are not completely predictive, since they need proper models for the description of the rates of aggregation and breakup. On the contrary, highly accurate and fully predictive description of single aggregation or breakup events can be obtained by Discrete Element Methods (DEMs), in which the motion of each primary particle of an aggregate is tracked by solving its equation of motion. However, so far, the high computational cost of DEMs has restricted their use to the simulation of short sequences of events, thus hindering their application to representative samples of a population of aggregates. The present work aims to investigate the shear-induced aggregation of a large population of aggregates suspended in an aqueous medium. We developed a method which combines a Monte Carlo approach to determine the sequence of aggregation and breakup events and a Discrete Element Method, built in the framework of Stokesian Dynamics, to accurately reproduce them. The DEM is able, in fact, to rigorously evaluate the fluid-dynamic stresses acting on each monomer and to model properly the colloidal interactions. Two different models were used to describe the colloidal interaction between primary particles and their effect on the aggregation behavior was analysed. Results highlighted that aggregation kinetics, size distributions and typical morphology are significantly influenced by the approach used to model colloidal interactions. This study demonstrates that the combination of a Monte Carlo approach with a DEM model represents a powerful tool for the study of the dynamics of a large population of colloidal aggregates.