Physicists Studying The Melting Process Find New Liquid Phase

BUFFALO, N.Y. -- To anyone who's ever left a chocolate bar on the dashboard of a car on a hot summer's day, the melting process is fairly straightforward: Heat hits solid, solid turns to mush.

Believe it or not, scientists who study the process haven't been able to shed much more light on it.

But now a paper published in Science by University at Buffalo physicists, in collaboration with researchers at other institutions, has revealed some surprising details.

The findings show that when a solid melts, it undergoes not just one intermediate phase change -- as had been predicted previously -- but two.

The research is the first to show ample evidence of an unusual liquid phase that has some local characteristics of a solid, but which is still technically a liquid.

This novel liquid phase is preceded by the hexatic phase, the existence of which now confirms earlier theoretical predictions.

The new findings should help subsequent efforts to understand the melting process in real systems.

"This work provides evidence that melting does not have to be a catastrophic event," explained John Ho, Ph.D., co-author, UB Distinguished Service Professor in the Department of Physics and associate dean of the Faculty of Natural Sciences and Mathematics. "Things don't melt all of a sudden. The process starts at the surface and then propagates through the material."

The researchers' ability to do the experiment with thin liquid-crystal films suspended in air was a key factor in their ability to isolate what was happening at the surface.

"Melting can be a very hard process to keep track of in real systems," said Ho.

For that reason, scientists have been trying to capture the changes a substance, such as liquid crystals, goes through when it melts in two-dimensional systems, a somewhat simpler process.

In this research, the scientists used two-dimensional films suspended in air because use of a substrate can affect the experiment.

"It's technically difficult to make measurements on such thin films," said Ho.

To determine what kinds of transitions were occurring, the scientists had to overcome some serious technical problems, utilizing two of the world's only scientific instruments designed to measure structural and thermal properties of films suspended in air.

To understand the phase transitions as the temperature rose, the UB scientists studied the molecular structure of the films using a special electron-diffraction microscope at Roswell Park Cancer Institute that was adapted to sustain the liquid-crystal films.

"A solid is a very regular system," explained Ho. "Its molecules sit in fixed positions with regular distances between them, with orientational order in certain directions."

By contrast, he said, a liquid has neither positional nor orientational (directional) order.

The two phases found by the scientists lie between solid and liquid.

The first phase, which was predicted by theorists, is the hexatic phase.

In this phase, there is no positional order, but the molecular arrangement is not completely random, either, and there is some orientational (directional) order.

"Our two-dimensional, liquid-crystal film provides the first unequivocal proof of the hexatic phase because its properties were revealed in the absence of a substrate," Ho said.

"Now between this hexatic phase and the liquid phase, we have found another one, which has a slightly more ordered molecular environment than a liquid."

In this new phase, which has not yet been named, the immediate "neighborhood" of each molecule is more regular and, therefore, closer to a solid.

"The molecular arrangement is more regular locally, but it does not continue or propagate throughout the material and, therefore, it is still a liquid," said Ho.

In the experiments, the material repeatedly demonstrated distinct changes in its temperature and density as a result of the application of heat, strongly suggesting a distinct phase change.

"This still doesn't constitute incontrovertible proof of this phase," said Ho, "but the data are extremely convincing."

Co-authors on the paper were Chia-Fu Chou of Princeton University, Anjun J. Jin of Applied Materials, S.W. Hui of Roswell Park Cancer Institute and C.C. Huang of the University of Minnesota.

The research was funded by the National Science Foundation.