单粒子尺度上直接观测胶体晶体内部单个缺陷处的异质形核熔化

Crystal melting is ubiquitous but its microscopic mechanism is poorly understood, the kinetics and thermodynamics of crystal melting are universally controlled by defects that act as nucleation sites. However, monitoring nucleation process in bulk crystals at single particle level is a big challenge in experiment due to the small spatial and temporal scales involved, especially for nucleation only on a single defect. In this project, we direct observe nucleation on single defects and subsequently growth process during the heterogeneous melting of colloidal crystals at single particle resolution. Under the impact of defects, we will take quantitative and systematic measurements on superheat limit, nucleation precursor, nucleation energy barrier, nucleation induction time, nucleation rate, extra rise in free energy caused by single defects, strain and normal mode evolutions surrounding defects before nucleation, nucleus growth rate, nucleus size and shape evolutions, kinetic pathways for free energy of whole system upon different degrees of superheating. All the results will provide direct experimental input for testing and further refinement of nucleation theory with single defect effect. Meanwhile, we give a clarification of the debate about what kind of defect will preempt superheat; investigate interactions between a given number and species of defects which are nearby, clarify the applicable range of defect-mediated theory; discuss the hierarchy of strength for defects to affect melting, and other related and interesting problems. This project should lead to a better understanding of melting dynamics, and is of central importance and great significance to materials science, metallurgy industry and condensed matter physics.

熔化是自然界的一种普遍现象，但其微观机制和动力学过程尚不清楚。缺陷作为合适的形核位置在熔化中扮演重要角色，然而实验中在单粒子尺度上观测晶体内部的熔化非常困难，尤其是提取出单个缺陷处的异质形核过程。本项目首次在单粒子尺度上直接观测胶体晶体内单个缺陷处的异质形核熔化以及核的生长过程。在不同过热度下，对各类缺陷影响下的形核熔化过程中的各参量、物理量以及其演化路径进行定量系统地测量，为单个缺陷介入下的形核理论的验证、改进和提出提供密切的实验依据。本项目还将澄清各类缺陷是否可以阻止晶体过热疑问；讨论缺陷(包括混合类缺陷)对熔化影响的强弱顺序；研究特定数量和种类的缺陷之间的相互作用和演化，明确缺陷熔化理论的适用范围。以及讨论其它形核动力学过程中的相关且有趣的问题，为晶体熔化理论的完善提供丰富的实验数据，加深人们对熔化内在机制的认识。此项目对于凝聚态物理和材料科学有重要的基础和实际应用价值。

本项目大部分按计划执行，但是因为前期实验中遇到的难点，对研究内容做了微调整和改变，到目前为止，此项目主要有三项研究成果：..第一，利用椭球胶体粒子分析玻璃态转化中过剩熵与扩散系数之间的关系，很好地将热力学量与动力学量联系起来，给出了胶体液体玻璃化过程中的扩散定律，在Communication Physics上发表论文一篇，Communications Physics,1,79(2018)。本次研究首次发现玻璃化过程中的特征点,对于预测玻璃态的转化有重要的实际指导意义。..第二，通过实空间测量第一次给出短程和长程相互作用势系统的二维热力学相图，其中长程相互作用势系统的二维相图与热力学教科书上经典三维相图形成对照。该研究成果可以广泛应用于解释和预测原子分子系统如石墨烯、吸附型纳米晶体、生物系统如蛋白质膜的相行为。相关内容在物理学期刊Phys. Rev. X 9, 031032 （2019）上发表。..第三，对单个缺陷处的熔化的动力学过程，尤其是三维胶体晶体中的晶界进行了定量地实验研究和理论分析，在实验方法上突破了精确测量晶体熔点这一难点，且从实验上验证邻近的缺陷在晶体熔化过程中的相互影响，此项研究成果目前尚在投稿审稿中。晶界以何种机制熔化，以及晶界角度对熔化的影响在材料科学中争论已久，此次实验第一次在单粒子尺度上通过准确测量晶体熔点并排除其他缺陷的影响回答了这些问题，这对材料科学的研究有重要意义。