Homogeneous Nucleation in Condensed Phases

密歇根大学安阿伯分校的劳伦斯·巴特尔（Lawrence Bartell）得到了实验物理化学计划的支持，其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结合计算机模拟来研究成核机制。 非常大的，高度过冷的分子团簇将探测电子衍射，通过监测相变的时间演化来研究均匀成核的动力学。 实验项目包括调查在与卷云中的条件相对应的条件下水的冻结情况以及耐火材料的研究。 大型团簇的模拟将遵循过渡期间分子的自发重排，并详细分析新相演变的临界核的形成。 特别注意的是，将给予的平移和取向有序的核内的分子和扩散的核和老阶段之间的界面。 这种理解的实现是一个现实的成核理论的发展是至关重要的。 成核率的实验有望提供理论重现的基准，以及对大气科学有用的信息。 预计模拟测试理论的动力学前因子和自由能障碍成核。均质成核是一个引发纯的过冷冷凝系统从一相转变为另一相的过程。 该项目将包括对纳米颗粒的研究，纳米颗粒由于其特殊的、潜在的有价值的性质而构成了材料科学中一种重要的物质状态。 将要研究的水冰成核现象是卷云中发生的过程中的一个重要现象，对气候有很大影响。

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.