How Peclet number affects microstructure and transient cluster aggregation in sedimenting colloidal suspensions

TL;DR

This study investigates how the Péclet number influences sedimentation, microstructure, and transient clustering in colloidal suspensions, revealing a transition from equilibrium-like to non-equilibrium behavior with increasing Pe.

Contribution

It provides new insights into how varying Pe affects cluster dynamics and sedimentation velocity, highlighting the crossover from equilibrium to non-equilibrium regimes in colloidal suspensions.

Findings

Sedimentation velocity dependence shifts from non-monotonic to monotonic with Pe.

Higher Pe leads to fewer and less stable transient clusters.

Cluster lifetime remains roughly constant across Pe range.

Abstract

We study how varying the P \'eclet number (Pe) affects the steady state sedimentation of colloidal particles that interact through short-ranged attractions. By employing a hybrid molecular dynamics simulation method we demonstrate that the average sedimentation velocity changes from a non- monotonic dependence on packing fraction {\phi} at low Pe numbers, to a monotonic decrease with {\phi} at higher Pe numbers. At low Pe number the pair correlation functions are close to their equilibrium values, but as the Pe number increases, important deviations from equilibrium forms are observed. Although the attractive forces we employ are not strong enough to form permanent clusters, they do induce transient clusters whose behaviour is also affected by Pe number. In particular, clusters are more likely to fragment and less likely to aggregate at larger Pe numbers, and the probability of finding larger clusters decreases with increasing Pe number. Interestingly, the life-time of the clusters is more or less independent of Pe number in the range we study. Instead, the change in cluster distribution occurs because larger clusters are less likely to form with increasing Pe number. These results illustrate some of the subtleties that occur in the crossover from equilibrium like to purely non-equilibrium behaviour as the balance between convective and thermal forces changes.