Abstract
The aggregation kinetics of sedimenting colloidal particles under fully destabilized conditions has been investigated over a wide range of particle volume fractions (Φ) and Péclet numbers (Pe) using the recent PSE algorithm implementing the Rotne-Prager-Yamakawa (RPY) approximation for long-range Hydrodynamic Interactions (HI). Fast Lubrication Dynamics (FLD) and simple Brownian Dynamics (BD) methods have also been employed to assess the importance of long range hydrodynamic interactions on the resulting dynamics. It has been observed that long-range hydrodynamic interactions are essential to capture the fast aggregation rates induced by the increase in sedimentation rate of clusters with increasing mass, which manifests with an explosive-like cluster growth after a given induction time. On the contrary, simulations employing only short-range hydrodynamic interactions (such as FLD) and BD (which neglects completely hydrodynamic interactions) are incapable of predicting this very rapid kinetics, because sedimentation simply leads to all particles and clusters moving vertically with identical velocity. It has been observed that at high volume fractions and low Pe values, a gel point can be formed and a phase diagram predicting when gelation is reached has been obtained. It was also observed that, as Pe increases, the anisotropy of the resulting clusters decreases, suggesting that denser clusters with spherical-like morphology are formed due to cluster breakage and restructuring. We can conclude that long-range hydrodynamic effects are of crucial importance in understanding the aggregation dynamics of settling clusters, revealing important features of the complex interplay between sedimentation, and colloidal interactions.
Highlights
The dynamics of colloidal aggregation is of fundamental importance in materials science and in a wide range of industrial applications, including paints preparation, wastewater treatment, formulation of pharmaceutical products, and food processing.[1]
Most of the work to date has been concerned with the fundamental understanding of the physical mechanism governing aggregation kinetics in aqueous suspensions under both diffusion-limited cluster aggregation (DLCA) regime, where particles stick immediately as they encounter each other’s and the aggregation rate is solely governed by diffusion, and reaction-limited cluster aggregation (RLCA) conditions,[7] which is a regime governed by interparticle repulsive interactions
We performed large-scale Brownian Dynamics (BD)-Hydrodynamic Interactions (HI) simulations with the Positively Split Ewald (PSE) algorithm to study the effect of short-range attractive forces and hydrodynamic interactions on the aggregation process of sedimenting clusters
Summary
The dynamics of colloidal aggregation is of fundamental importance in materials science and in a wide range of industrial applications, including paints preparation, wastewater treatment, formulation of pharmaceutical products, and food processing.[1]. Extensive experimental and theoretical studies pioneered by the Weitz group[7,8] showed that the fractal dimension df, which describes the internal structure of the aggregates, depends on the mechanism of aggregation, but is independent of the chemical and physical properties of the system (i.e., the colloidal particle type). This suggested a universal nature of the aggregation processes for clusters formed in both the DCLA and RCLA regimes. Aggregates formed in the DLCA regime are rather open with fractal dimensions
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