Abstract
This paper investigates numerically the shear-induced aggregation of mixed populations of colloidal particles leading to the formation of clusters. Suspensions with different amounts of positively and negatively charged colloidal particles are simulated. To resolve the aggregation kinetics and structural properties of the formed clusters, we resort to a mixed deterministic-stochastic simulation method. The method is built on a combination of a Monte Carlo algorithm to sample a statistically expected sequence of encounter events between the suspended particles and a discrete element method built in the framework of Stokesian dynamics to simulate the encounters in a fully predictive manner. Results reveal a strong influence of the composition of the population on both the aggregation kinetics and the aggregate structure. In particular, we observe a size-stabilization phenomenon taking place in the suspension when the relative concentration of the majority particles lies in the range 80–85%; i.e., starting from primary particles, after a short growth period, we observed a cessation of aggregation. Inspection of the aggregate morphology shows that the formed clusters are composed of few minority particles placed in the inner region, while the aggregate surface is covered by majority particles, acting to provide a shielding effect against further growth.
Highlights
Aggregation of colloidal particles is a relevant phenomenon encountered in a wide variety of natural and industrial settings, such as aquatic environments,[1,2] wastewater treatment,[3] drug and material syntheses,[4,5] and food colloids.[6]
We adopted a computational approach based on a combination of a Monte Carlo (MC) algorithm coupled to a discrete element method (DEM);[26] the MC algorithm was used to sample a statistically expected sequence of binary encounter events, whereas the DEM was employed to simulate in detail the encounter between the sampled clusters
The setup of the encounter is comparable to the one depicted in Figure 1: at the beginning of each DEM simulation, the center of mass of the primary particle β was placed on a node of the grid, whereas particle α was placed in the origin of the reference system
Summary
Aggregation of colloidal particles is a relevant phenomenon encountered in a wide variety of natural and industrial settings, such as aquatic environments,[1,2] wastewater treatment,[3] drug and material syntheses,[4,5] and food colloids.[6]. Dispersed particles frequently present a surface charge, which provides an energy barrier against aggregation. There exist a number of different methods to trigger the aggregation of charged particles:[7] high-molecular-weight polymers can be used to promote the formation of bridges between particles;[8,9] the increase in the ionic strength of the dispersing medium can be exploited to compress the electrical double layers and screen the electrostatic repulsion between the particles; and the pH can be adjusted to neutralize the particle surface charge.[10−12] When the increase in the solid content is of no importance to the outcome of the operation, the addition of particles bearing an opposite charge can be a viable option. The resulting aggregation process is often referred to as heteroaggregation in contrast to homoaggregation, i.e., the aggregation of colloidal particles of one single type.[13,14]
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