Direct observation of liquid nucleus growth in homogeneous melting of colloidal crystals.

Ziren Wang,Yi Peng,Yilong Han,Feng Wang

doi:10.1038/ncomms7942

Abstract

The growth behaviour of liquid nucleus is crucial for crystal melting, but its kinetics is difficult to predict and remains challenging in experiment. Here we directly observed the growth of individual liquid nuclei in homogeneous melting of three-dimensional superheated colloidal crystals with single-particle dynamics by video microscopy. The growth rate of nucleus at weak superheating is well fitted by generalizing the Wilson–Frenkel law of crystallization to melting and including the surface tension effects and non-spherical-shape effects. As the degree of superheating increases, the growth rate is enhanced by nucleus shape fluctuation, nuclei coalescence and multimer attachment. The results provide new guidance for the refinement of nucleation theory, especially for the poorly understood strong-superheating regime. The universal Lindemann parameter observed at the superheat limit and solid–liquid interfaces indicates a connection between homogeneous and heterogeneous melting.

Highlights

The growth behaviour of liquid nucleus is crucial for crystal melting, but its kinetics is difficult to predict and remains challenging in experiment
By fabricating high-quality colloidal crystals with thermally sensitive particles and applying the local optical-heating technique, we were able to resolve the microscopic kinetics of post-critical nucleus growth in homogeneous melting
When the nucleus radius was more than twice the critical radius, the surface tension effect was negligible and the growth rate became a constant at each degree of superheating (Fig. 3a,b)

Summary

Introduction

The growth behaviour of liquid nucleus is crucial for crystal melting, but its kinetics is difficult to predict and remains challenging in experiment. As the degree of superheating increases, the growth rate is enhanced by nucleus shape fluctuation, nuclei coalescence and multimer attachment. The nucleation process of melting can be divided into three stages: (I) The incubation stage in which the superheated crystal remains metastable without forming critical liquid nuclei. Nucleation precursors such as defects[11,12] or particleswapping loops[13,14] may form and trigger the formation of liquid nuclei. The nucleation process can hardly be observed at the singleparticle level in atomic systems due to their small spatial and time scales, especially the nucleation process inside the bulk

Methods

Results

Conclusion