Homogeneous Nucleation in Condensed Phases

密歇根大学安娜堡分校的劳伦斯·巴特尔得到了实验物理化学计划的支持，他的研究旨在更好地理解过冷凝聚态物质相变过程中分子的集体行为。一种新的实验方法将与计算机模拟相结合来研究成核机制。通过监测相变的时间演变，电子衍射将探测超大的、高度过冷的分子团簇，以研究均匀成核的动力学。实验项目包括研究与卷云条件相对应的水的冻结和耐火材料的研究。大簇的模拟将遵循分子在转变过程中的自发重排，并在分子细节上分析新相演变的关键核的形成。将特别注意原子核内和原子核与旧相之间的扩散界面中分子的平移和取向顺序。达到这种理解对于发展一个现实的成核理论是至关重要的。有关成核速率的实验预期将提供理论再现的基准，以及对大气科学有用的资料。这些模拟有望验证动力学前因子和成核的自由能垒的理论。均相成核是一个纯过冷冷凝体系从一相转变为另一相的过程。该项目将包括纳米颗粒的研究，由于其特殊的、潜在的有价值的特性，纳米颗粒构成了材料科学中一个重要的兴趣物质状态。将要研究的水冰成核是发生在卷云中对气候产生强烈影响的过程中的一个重要现象。

Lawrence Bartell of the University of Michigan, Ann Arbor, is supported by the Experimental Physical Chemistry Program for studies that aim to achieve a better understanding of the collective behavior of molecules during phase changes in supercooled condensed matter. A new experimental approach will be combined with computer simulations to investigate nucleation mechanisms. Very large, highly supercooled molecular clusters will be probed by electron diffraction to study the kinetics of homogeneous nucleation by monitoring the time evolution of phase transitions. Experimental projects include investigating the freezing of water under conditions corresponding to those in cirrus clouds and the study of refractory materials. Simulations of large clusters will follow the spontaneous rearrangements of molecules during transitions and analyze in molecular detail the formation of critical nuclei from which the new phase evolves. Particular attention will be given to the translational and orientational ordering of molecules within the nuclei and in the diffuse interface between the nuclei and the old phase. Attainment of this understanding is critical for the development of a realistic theory of nucleation. The experiments on nucleation rates are expected to provide benchmarks to be reproduced by theory, as well as information useful to atmospheric science. The simulations are expected to test theories of kinetic prefactors and free energy barriers to nucleation. Homogeneous nucleation is the process that initiates the transformation of a pure, supercooled condensed system from one phase to another. This project will include the study of nanoparticles, which constitute a state of matter of a significant interest in the materials sciences because of their special, potentially valuable properties. The nucleation of water ice that will be examined is an important phenomenon in processes taking place in cirrus clouds that exert a strong influence on climate.