The interplay between hydrodynamic and Brownian fluctuations in sedimenting colloidal suspensions

TL;DR

This study uses hybrid simulations to explore how hydrodynamic and thermal fluctuations influence sedimentation behavior in colloidal suspensions across various Peclet numbers, revealing the dominance of hydrodynamic fluctuations over thermal effects at larger Pe.

Contribution

It introduces a hybrid simulation approach to analyze sedimentation, highlighting the independence of hydrodynamic back-flow effects from Pe and the emergence of lateral patterns consistent with experiments.

Findings

Hydrodynamic back-flow reduces sedimentation velocity, independent of Pe.

Hydrodynamic fluctuations dominate diffusive behavior at modest Pe.

Lateral patterns form in simulations, matching experimental observations.

Abstract

We apply a hybrid Molecular Dynamics and mesoscopic simulation technique to study the steady-state sedimentation of hard sphere particles for Peclet numbers (Pe) ranging from 0.08 to 12. Hydrodynamic back-flow causes a reduction of the average sedimentation velocity relative to the Stokes velocity. We find that this effect is independent of Pe number. Velocity fluctuations show the expected effects of thermal fluctuations at short correlation times. At longer times, non-equilibrium hydrodynamic fluctuations are visible, and their character appears to be independent of the thermal fluctuations. The hydrodynamic fluctuations dominate the diffusive behavior even for modest Pe number, while conversely the short-time fluctuations are dominated by thermal effects for a surprisingly large Pe numbers. Inspired by recent experiments, we also study finite sedimentation in a horizontal planar slit. In our simulations distinct lateral patterns emerge, in agreement with observations in the experiments.