From compact to fractal crystalline clusters in concentrated systems of monodisperse hard spheres

TL;DR

This study investigates how the structure of crystalline clusters in monodisperse hard spheres changes from compact to fractal as packing fraction increases, revealing different nucleation regimes and mechanisms through large-scale simulations.

Contribution

It demonstrates the transition from classical to spinodal-like nucleation regimes and links cluster morphology to packing fraction in hard sphere systems.

Findings

Compact clusters form at lower packing fractions ({} ; 0.54).

Fractal, ramified clusters emerge at higher packing fractions ({} ; 0.56).

Gradual crystallization mechanisms are observed above freezing.

Abstract

We address the crystallization of monodisperse hard spheres in terms of the properties of finite- size crystalline clusters. By means of large scale event-driven Molecular Dynamics simulations, we study systems at different packing fractions {\phi} ranging from weakly supersaturated state points to glassy ones, covering different nucleation regimes. We find that such regimes also result in different properties of the crystalline clusters: compact clusters are formed in the classical-nucleation-theory regime ({\phi} \leq 0.54), while a crossover to fractal, ramified clusters is encountered upon increasing packing fraction ({\phi} \geq 0.56), where nucleation is more spinodal-like. We draw an analogy between macroscopic crystallization of our clusters and percolation of attractive systems to provide ideas on how the packing fraction influences the final structure of the macroscopic crystals. In our previous work (Phys. Rev. Lett., 106, 215701, 2011), we have demonstrated how crystallization from a glass (at {\phi} > 0.58) happens via a gradual (many-step) mechanism: in this paper we show how the mechanism of gradual growth seems to hold also in super-saturated systems just above freezing showing that static properties of clusters are not much affected by dynamics.