Liquids that tiptoe on the edge of solidity.

Liquids that tiptoe on the edge of solidity

In extremely tight spots, liquids get away with some very unliquid-like behaviors, often masquerading as solids. Some refuse to freeze even when chilled far below their freezing point for weeks at a time. These surprising findings, described this week in separate reports at a meeting of the Materials Research Society in Boston, may help chemists to design better lubricants, electronic engineers to build faster chips, and petroleum hunters to detect hidden oil reserves.

In one set of studies, Jacob N. Israelachvili and his colleagues at the University of California, Santa Barbara, measured physical properties of ultrathin liquid films -- sometimes no more than a single molecule deep -- confined between a pair of supersmooth mica plates. with the plates far enough apart to sandwich at least 10 layers of liquid molecules, the molecules behave like a "normal," bulk liquid in a beaker. But pinching the plates together so that only a few molecular layers can squeeze between them elicits deviant behavior. "We get liquids that behave like solids," Israelachvili says.

These rigid liquid films display the orderly molecular arrangement of a solid. But when the researchers briefly apply a shear force by sliding the plates across each other, the molecules "melt" into the disordered structure of a liquid or a partially ordered, liquid crystalline state before "refreezing" into a solid-like state.

Israelachvili reports that the liquid films under shear force can have viscosities surpassing those of their bulk-liquid counterparts by a factor of 10 billion. That's such a dramatic difference that he now questions whether viscosity -- a fundamental property of bulk liquids and liquid lubricants -- can apply to ultrathin liquid films, which seem to hover on the edge of solidity.

In other studies, a team led by David D. Awschalom of IBM's Thomas J. Watson Research Center in Yorktown Heights, N.Y., confined fluids within tiny compartments -- some no more than 20 atoms across -- in specially prepared glass blocks. Even when chilled as much as 40 percent below their normal freezing temperatures, liquid oxygen and some other fluids can spend weeks in a remarkably stable liquid state, the researchers report. Awschalom suggests chip designers might exploit the effect to cool superdense circuitry with minuscule "integrated refrigerators," consisting of channels micromachined into the chip's silicon base and filled with a supercooled fluid.

The group also found that sound travels through the confined, supercooled fluids at the same speed and dampens over the same distance as it would in a solid. Usually, sound travels more slowly and dampens more quickly in liquids. Awschalom speculates that arctic oil hunters who rely on sound data may mistake supercooled oil for rock.