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Mailed 6/13/95


Upton, NY - Physicists at the U.S. Department of Energy's Brookhaven National Laboratory have measured for the first time surface-induced layering in liquid metals, using sophisticated x-ray techniques. Their results were reported recently in Physical Review Letters.

In contrast to solid surfaces, very little is known about the surfaces of liquid metals. Surface layering of liquid metals is predicted by theory, but scientists in the past have never been able to observe this phenomenon. The Brookhaven collaboration, which also includes researchers from Harvard University and from Bar-Ilan University in Israel, devised an experiment that succeeded in measuring surface layering in liquid mercury.

Surface Layering

Atoms are randomly arranged within the bulk of a liquid metal. At the surface, however, the atoms form discrete layers.

As Brookhaven physicist Olaf Magnussen explains, surface layering can be visualized in terms of a popular piece of playground equipment - a clear plastic house filled with colored balls.

"Looking from the outside, you can see a layer of balls pressed against the rigid walls," says Dr. Magnussen. "If you could see deeper into the pile of balls, however, you would see that the layering gradually diminishes until the balls are arranged randomly."

Dr. Magnussen's colleague at Brookhaven, physicist Benjamin Ocko, describes how the same phenomenon is found in metal atoms, which are one hundred million times smaller than the colored balls. "In liquid metals," says Dr. Ocko, "the high surface tension serves as the rigid wall." As he explains, high surface tension forces the surface area to be as small as possible. "That's why liquids form drops," he points out. He adds that high surface tension also smoothes out microscopic waves on the surface. Ordinary liquids, such as water, have much lower surface tensions and, consequently, rougher surfaces. This prevents precise measurements of surface layering in ordinary liquids.

Technical Improvements

In technical terms, the mercury experiment is an achievement in measuring x-rays reflected from liquid surfaces. The research team faced two main problems: surface layering is only evident at high angles, where the reflected intensity is weak, and the liquid sample cannot be tilted as in conventional x-ray diffraction experiments. The experiment was successful because it took advantage of the intense, highly collimated x-rays from Brookhaven's National Synchrotron Light Source, as well as specialized optics that tilt the x-ray beam.

The mercury experiment was conducted to confirm basic physics theory. But the researchers are applying what they have learned to related research on using liquid metals as substrates for organic films. According to physicist Moshe Deutsch, from Bar-Ilan University, thin layers of organic materials, often just one molecule thick, are of interest for various technological applications, such as chemical sensors. For example, organic films on mercury can be used to detect ultra-low levels of cadmium, for environmental monitoring. Adds Harvard physicist Peter Pershan, "Conventional fabrication involving soldering, welding and casting is another area where a basic understanding of fundamental properties of liquid metals can be important."

A pioneer in the study of liquid surfaces, Dr. Pershan says, "All liquids are generally difficult to study because there is no obvious structure to probe. We started at Brookhaven's Light Source about 10 years ago with simple fluids. The discovery of surface layering in liquid mercury is a milestone in our fundamental understanding of liquids."

This research on liquid metals is supported by the Office of Energy Research in the Department of Energy (DOE). Brookhaven is managed by Associated Universities, Inc., under contract to DOE.