The Effects of Inter-particle Attractions on Colloidal Sedimentation

TL;DR

A mesoscopic simulation technique is used to study the effect of short-ranged interparticle attractions on the steady-state sedimentation of colloidal suspensions, which shows a nonmonotonic dependence on the packing fraction phi with a maximum velocity at intermediate phi.

Abstract

We use a mesoscopic simulation technique to study the effect of short-ranged inter-particle attraction on the steady-state sedimentation of colloidal suspensions. Attractions increase the average sedimentation velocity $v_s$ compared to the pure hard-sphere case, and for strong enough attractions, a non-monotonic dependence on the packing fraction $φ$ with a maximum velocity at intermediate $φ$ is observed. Attractions also strongly enhance hydrodynamic velocity fluctuations, which show a pronounced maximum size as a function of $φ$. These results are linked to a complex interplay between hydrodynamics and the formation and break-up of transient many-particle clusters.

Figures (4)

• Figure 1: Average sedimentation velocity, $v_s$ as a function of the volume fraction $\phi = \frac{4}{3} \rho a^3$ for four different inter-particle attractions and for hard spheres. The dashed lines are Batchelor's predictions (\ref{['eq:eq1']}) for the dilute limit. The dotted line is a prediction known to be accurate for hard spheres Haya95.
• Figure 2: Correlation length $\xi_{||}$ parallel to the sedimentation direction for velocity swirls, as a function of packing fraction $\phi$ for different inter-particle attractions and for pure HS. Attractions strongly enhance the size of the velocity swirls.
• Figure 3: (a) The probability of finding a transient cluster of size $i$ and (b) the average cluster life-time (normalized by the Brownian time $\tau_B=a^2/D_{col}$) as a function of $i$ for hard spheres and for four different inter-particle attractions. The particle volume fraction is $\phi=0.0233$.
• Figure 4: The plots show (a) the probability of finding a transient cluster of size $i$ and (b) the average cluster life-time (normalized by the Brownian time $\tau_B$) as a function of $i$ for three different particle volume fractions, $\phi$. The normalized second virial coefficient is $B_2^*=-0.507$.