Effects of vertical confinement on gelation and sedimentation of colloids

TL;DR

This study investigates how vertical confinement influences gelation and sedimentation in colloidal systems, revealing size-dependent behaviors and comparing experimental results with simulations that neglect hydrodynamics.

Contribution

It provides a direct comparison of experiments and simulations under confinement, highlighting size effects and the impact of attraction strength on sedimentation.

Findings

Large systems suppress sedimentation due to gelation

Small systems show enhanced sedimentation compared to non-gelling suspensions

Simulations without hydrodynamics match experimental sedimentation timescales

Abstract

We consider the sedimentation of a colloidal gel under confinement in the direction of gravity. The confinement allows us to compare directly experiments and computer simulations, for the same system size in the vertical direction. The confinement also leads to qualitatively different behaviour compared to bulk systems: in large systems gelation suppresses sedimentation, but for small systems sedimentation is enhanced relative to non-gelling suspensions, although the rate of sedimentation is reduced when the strength of the attraction between the colloids is strong. We map interaction parameters between a model experimental system (observed in real space) and computer simulations. Remarkably, we find that when simulating the system using Brownian dynamics in which hydrodynamic interactions between the particles are neglected, we find that sedimentation occurs on the same timescale as the experiments, however the thickness of the "arms" of the gel is rather larger in the experiments, compared with the simulations. An analysis of local structure in the simulations showed similar behaviour to gelation in the absence of gravity.