Freezing line of polydisperse hard spheres via direct-coexistence simulations

TL;DR

This paper introduces a new simulation method to determine the freezing line of polydisperse hard-sphere systems, revealing how increasing polydispersity affects crystallization and interface properties.

Contribution

A novel direct-coexistence simulation approach in the semi-grand canonical ensemble for mapping the freezing line of polydisperse hard spheres.

Findings

Crystallization becomes more size-selective with higher polydispersity.

Crystals at high polydispersity have lower density than the fluid.

Interface properties like surface stress are characterized.

Abstract

In experimental systems, colloidal particles are virtually always at least somewhat polydisperse, which can have profound effects on their ability to crystallize. Unfortunately, accurately predicting the effects of polydispersity on phase behavior using computer simulations remains a challenging task. As a result, our understanding of the equilibrium phase behavior of even the simplest colloidal model system, hard spheres, remains limited. Here, we present a new approach to map out the freezing line of polydisperse systems that draws on direct-coexistence simulations in the semi-grand canonical ensemble. We use this new method to map out the conditions where a hard-sphere fluid with a Gaussian size distribution becomes metastable with respect to partial crystallization into a face-centered-cubic crystal. Consistent with past predictions, we find that as the polydispersity of the fluid increases, the coexisting crystal becomes increasingly size-selective, exhibiting a lower polydispersity and larger mean particle size than the fluid phase. Interestingly, for sufficiently high polydispersity, this leads to a crystal phase with a lower number density than that of the coexisting fluid. Finally, we exploit our direct-coexistence simulations to examine the characteristics of the fluid-crystal interface, including the surface stress and interfacial absorption.