From Hindered to Promoted Settling in Dispersions of Attractive Colloids: Simulation, Modeling, and Application to Macromolecular Characterization

TL;DR

This study combines simulations, experiments, and modeling to understand how short-range attractions in colloids can promote settling, leading to a new empirical model that extends traditional hindered settling theories and aids in macromolecular characterization.

Contribution

It introduces a comprehensive simulation and empirical modeling framework that captures promoted settling in attractive colloids, extending existing hindered settling models.

Findings

Simulations quantitatively match experimental settling rates.

Attractive forces lead to clustering that enhances settling.

The new empirical model extends the Richardson-Zaki formalism.

Abstract

The settling of colloidal particles with short-ranged attractions is investigated via highly resolved immersed boundary simulations. At modest volume fractions, we show that inter-colloid attractions lead to clustering that reduces the hinderance to settling imposed by fluid back flow. For sufficient attraction strength, increasing the particle concentration grows the particle clusters, which further increases the mean settling rate in a physical mode termed promoted settling. The immersed boundary simulations are compared to recent experimental measurements of the settling rate in nanoparticle dispersions for which particles are driven to aggregate by short-ranged depletion attractions. The simulations are able to quantitatively reproduce the experimental results. We show that a simple, empirical model for the settling rate of adhesive hard sphere dispersions can be derived from a combination of the experimental and computational data as well as analytical results valid in certain asymptotic limits of the concentration and attraction strength. This model naturally extends the Richardson-Zaki formalism used to describe hindered settling of hard, repulsive spheres. Experimental measurements of the collective diffusion coefficient in concentrated solutions of globular proteins are used to illustrate inference of effective interaction parameters for sticky, globular macromolecules using this new empirical model. Finally, application of the simulation methods and empirical model to other colloidal systems are discussed.