Structure and kinetics in the freezing of nearly hard spheres

TL;DR

This study combines experiments and simulations to analyze the structure and crystallization kinetics of nearly hard colloidal spheres, revealing the role of specific local structures in the crystallization process.

Contribution

It introduces a detailed comparison between experimental and simulated crystallization, incorporating a novel structural analysis method to identify key local structures influencing crystallization.

Findings

Good agreement between experiment and simulation in structure and dynamics.

Crystallization occurs at lower supersaturations in experiments due to larger system size.

Dominant metastable structures are five-fold symmetric clusters with 10 particles.

Abstract

We consider homogeneous crystallisation rates in confocal microscopy experiments on colloidal nearly hard spheres at the single particle level. These we compare with Brownian dynamics simuations by carefully modelling the softness in the interactions with a Yukawa potential, which takes account of the electrostatic charges present in the experimental system. Both structure and dynamics of the colloidal fluid are very well matched between experiment and simulation, so we have confidence that the system simulated is close to that in the experiment. In the regimes we can access, we find reasonable agreement in crystallisation rates between experiment and simulations, noting that the larger system size in experiments enables the formation of critical nuclei and hence crystallisation at lower supersaturations than the simulations. We further examine the structure of the metastable fluid with a novel structural analysis, the topological cluster classification. We find that at densities where the hard sphere fluid becomes metastable, the dominant structure is a cluster of m=10 particles with five-fold symmetry. At a particle level, we find three regimes for the crystallisation process: metastable fluid (dominated by m=10 clusters), crystal and a transition region of frequent hopping between crystal-like environments and other (m\neq10) structures