Homogeneous Nucleation in Condensed Phases

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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.