Clusters in sedimentation equilibrium for an experimental hard-sphere-plus-dipolar Brownian colloidal system

TL;DR

This study investigates sedimentation equilibrium in a colloidal system under gravity and electric fields, revealing cluster formation and diffusion behavior changes at specific dipolar strengths.

Contribution

It provides new insights into the structure and dynamics of dipolar colloidal clusters under sedimentation equilibrium conditions.

Findings

Cluster formation occurs at dipolar strength Λ ~ 1.

Diffusion coefficient exhibits a plateau during chain formation.

Hydrodynamic corrections align with hard sphere models.

Abstract

In this work, we use structure and dynamics in sedimentation equilibrium, in the presence of gravity, to examine, $via$ confocal microscopy, a Brownian colloidal system in the presence of an external electric field. The zero field equation of state (EOS) is hard sphere without any re-scaling of particle size, and the hydrodynamic corrections to the long-time self-diffusion coefficient are quantitatively consistent with the expected value for hard spheres. Care is taken to ensure that both the dimensionless gravitational energy, which is equivalent to a Peclet number $Pe_g$, and dipolar strength $\Lambda$ are of order unity. In the presence of an external electric field, anisotropic chain-chain clusters form; this cluster formation manifests itself with the appearance of a plateau in the diffusion coefficient when the dimensionless dipolar strength $\Lambda \sim 1$. The structure and dynamics of this chain-chain cluster state is examined for a monodisperse system for two particle sizes.