Thawing the Phase-Transition Puzzle

Jordan has also explored another aspect of how water clusters differ from bulk systems: their transitions from solid to liquid. "The interest in phase transitions," Jordan says, "is derived primarily from intrinsic interest in the clusters themselves."

While there has been extensive study of phase transitions in bulk systems, it is only recently that researchers began to examine them in small clusters. For bulk systems, the freezing and melting point coincides, that is both events occur at the same temperature. When the temperature is slightly below 32 degrees F., all the water is ice. Slightly above 32 degrees F., it is all liquid water. "In clusters, on the other hand," says Jordan, "within a range of temperatures, solid-like and liquid-like structures coexist.

"We have shown that even a water cluster containing as few as eight molecules undergoes a phase transition. At low temperatures, the eight-water cluster is dominated by cubic 'solid-like' structures, and at high temperatures, it is dominated by non-cubic 'liquid-like' structures. In the coexistence region, both types of structures are important. Over time, a single cluster in the transition region switches back and forth between solid-like and liquid-like structures."

Jordan plans to extend his investigation of water clusters to clusters that include a benzene molecule. Developing detailed data on the vibrational frequencies of these structures will assist experiments that attempt to separate and distinguish clusters of different size by ionizing them with a laser beam. To meet the computational challenges posed by these benzene-water clusters, Jordan expects to employ Pittsburgh's new, massively parallel CRAY T3D system.

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