When Is A Liquid Not A Liquid?

In a finding that may change the way lubricants and thin film coatings are developed, researchers at Northwestern University have for the first time directly observed that molecules of liquid close to a solid surface organize into layered structures much like a solid.

"In a confined geometry, like in a filter, a pore, or between two plates, there has been evidence that liquids don't behave like bulk liquids, and that their properties are different," says Pulak Dutta, professor of physics and astronomy at Northwestern. Dutta led the study, reported in the March 15 issue of the journal Physical Review Letters. "The liquid isn't changing its chemical composition, so it must be changing how the molecules are arranged," he said.

Dutta and his co-workers bounced extremely brilliant X-rays off of a thin film of a liquid known by its chemical-name acronym, TEHOS, which had been applied to a solid surface of silicon. The reflected X-rays formed an "interference pattern" similar to the checkerboard light patterns created by shining light through a grating. With X-rays, such behavior indicates a molecular solid, and analysis of the pattern showed that the TEHOS molecules, which were used for the study because they are nearly spherical, were forming three solid-like layers, each one molecule thick.

The layered TEHOS molecules were in an intermediate physical state between a bulk liquid and a solid, according to graduate student Chungjong Yu, who played a key role in the study. The physical state, he said, can be deduced from the shape of the reflectivity curve.

"A very shallow ripple indicates a liquid, and sharp peaks indicate a solid," Yu said. "These are broad humps."

The findings were made using synchrotron radiation at the Advanced Photon Source at Argonne National Laboratory and the National Synchrotron Light Source at Brookhaven National Laboratory.

Dutta said X-ray reflectivity should be a useful test for evaluating new lubricants and coatings.

"This is a new application of synchrotron radiation," Dutta said. "It's a first step to making rigorous measurements of how liquids organize near solid surfaces." As more studies are completed, he said, "we can use this body of knowledge to learn how to design molecules that would make better lubricants, to understand why lubricants behave the way they do, why they fail and how to prevent them from failing."

The new findings are also of interest from a purely theoretical standpoint, Dutta said.

"There has been an enormous amount of work on the free surface of liquids, the liquid-air interface," Dutta said. "Scientists worked for years to see if there is layering on the free surface. Well, there isn't. If you have a cup of a liquid, on the top layer, the air-liquid interface, it's a liquid."

It was only in the last few years, he said, that physicists began considering liquid next to the cup.

"The question was, if you take a typical, garden-variety liquid, perfectly disordered -- then, when it starts flowing against a surface or being squeezed through pores, is it still a liquid? What we've now shown is no. It's one of these intermediate structures -- the molecules are somewhat ordered -- they're neither perfectly ordered or disordered."

In addition to Dutta and Yu, other authors, all of Northwestern, are graduate students Andrew G. Richter and Mary K. Durbin and post-doctoral researcher Alokmay Datta.

The research was funded by the National Science Foundation.