Coupling of sedimentation and liquid structure: influence on hard sphere nucleation

TL;DR

This study investigates how sedimentation affects the structure and density fluctuations of colloidal hard spheres, revealing that sedimentation alters five fold symmetry and fluctuations but does not fully explain nucleation rate discrepancies.

Contribution

It demonstrates the impact of sedimentation on colloidal structure and fluctuations, and assesses their influence on nucleation rate differences between experiments and simulations.

Findings

Sedimentation reduces five fold symmetry by a factor of two.

Density fluctuations are twice as large in experiments compared to simulations.

Changes in symmetry and fluctuations are insufficient to explain nucleation rate discrepancies.

Abstract

The discrepancy in nucleation rate densities between simulated and experimental hard spheres remains staggering and unexplained. Suggestively, more strongly sedimenting colloidal suspensions of hard spheres nucleate much faster than weakly sedimenting systems. In this work we consider firstly the effect of sedimentation on the structure of colloidal hard spheres, by tuning the density mismatch between solvent and colloidal particles. In particular we investigate the effect on the degree of five fold symmetry present. Secondly we study the size of density fluctuations in these experimental systems in comparison to simulations. The density fluctuations are measured by assigning each particle a local density, which is related to the number of particles within a distance of 3.25 particle diameters. The standard deviation of these local densities gives an indication of the fluctuations present in the system. Five fold symmetry is suppressed by a factor of two when sedimentation is induced in our system. Density fluctuations are also increased by a factor of two in experiments compared to simulations. The change in five fold symmetry makes a difference to the expected nucleation rates, but we demonstrate that it is ultimately too small to resolve the discrepancy between experiment and simulation, while the fluctuations are shown to be an artefact of 3d particle tracking.