Colloidal Swarms Can Settle Faster than Isolated Particles: Enhanced Sedimentation near Phase Separation

Abstract

By experimenting on model colloids where depletion forces can be carefully tuned and quantified, we show that attractive interactions consistently "promote" particle settling, so much that the sedimentation velocity of a moderately concentrated dispersion can even exceed its single-particle value. At larger particle volume fraction ϕ, however, hydrodynamic hindrance eventually takes over. Hence, v(ϕ) actually displays a nonmonotonic trend that may threaten the stability of the settling front to thermal perturbations. Finally, by discussing a representative case, we show that these results are relevant to the investigation of protein association effects by ultracentrifugation.

From hindered to promoted settling in dispersions of attractive colloids: Simulation, modeling, and application to macromolecular characterization

“…These simulations and the resulting data possess a number of physical and quantitative issues, including being performed at finite but small Péclet and Reynolds numbers, being limited in the number of particles represented in the periodic simulation box, and having made no systematic finite-system-size corrections to the measured sedimentation rates. Inspired by these simulations, Lattuada et al conducted an experimental study designed to corroborate the simulation results [9]. They found a qualitative agreement between the experiments and simulations.…”

“…The experiments of Lattuada et al control interparticle attraction by adding surfactant micelles to solution in order to induce depletion [9]. Therefore, we model the interaction potential, V (r), with a short-ranged attraction described by the Asakura-Oosawa depletion potential [19] plus a hard-core repulsion at interparticle contact, r = 2a:…”

“…Normalized settling velocity as a function of particle volume fraction for B * 2 = −1.08 (red), B * 2 = −0.27 (blue), and B * 2 = 1 (black) from the experimental data of Ref. [9] (stars with lines, denoted LBP2016 in the legend) and computed in the present work (filled circles) and the SRD simulations of Ref. [22] as interpolated in Ref.…”

“…Lattuada et al provide data at three different attraction strengths, characterized by the reduced second virial coefficients: B * 2 = 1.0,−0.27,−1.08 [9]. These values correspond to hard-sphere interactions (no attraction), a modest attraction (ε = 4.84 k B T ), and a strong attraction (ε = 5.18 k B T ) that is near the boundary for liquidliquid phase separation, respectively.…”

“…[22] as interpolated in Ref. [9] (open squares, denoted MLP2010 in the legend). Error bars represent the standard error of the observed measurements.…”