Glass sponges with an ethanol thirst.

Glass sponges with an ethanol thirst

Many aspects of liquid behavior, even after decades of study, remain poorly understood, and current research continues to produce surprises. David D. Awschalom and his colleagues at the IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y., have been investigating the properties of liquids confined to networks of microscopic, cigar-shaped pores in glass blocks. Their results show that a confined liquid can be cooled as much as 30 percent below its normal bulk freezing point without freezing. At the same time, sound waves traveling through such a supercooled liquid behave as if the liquid were actually solid.

"That's what's so novel," says Awschalom. "It's a liquid that mimics all the acoustical properties of a solid yet clearly is still a liquid." The IBM team reports its findings in the Aug. 22 PHYSICAL REVIEW LETTERS.

By focusing on liquids confined to networks of cylindrical pores, Awschalom and his colleagues can study what is essentially a one-dimensional liquid -- a long, thin, spaghetti-like tube of fluid. That geometry allows the researchers to investigate a situation in which the liquid has a vast surface area in relation to its volume. In a liquid-filled, porous glass block, much of the liquid has direct contact with the glass walls. In contrast, in a three-dimensional situation, such as water in an ordinary flask, the liquid has a much smaller surface area in relation to its volume. "There's a lot of interest in what kind of role the surface plays in liquid behavior," Awschalom says. Simplifying the geometry allows researchers to concentrate on surface effects.

In their experiments, Awschalom and his group allow a liquid such as ethanol or liquid oxygen to soak into glass samples containing uniform pores of a well-defined size and geometry. The pore radius ranges from 10 to 200 angstroms. By shining extremely short pulses of laser light into the transparent, liquid-filled glass sponges, the researchers generate ultrasonic waves within the material, allowing them to monitor the liquid's behavior.

The results show the geometry of the confining pores has a significant effect on the liquid's freezing point. As the pores get smaller, the freezing point declines. "Here we have a supercooled liquid for which you can predict exactly [at what temperature] it will freeze, based on the size of the pores," Awschalom says. That stability contrasts with the behavior of supercooled bulk liquids, which instantly freeze at no particular temperature when they are disturbed by a jolt or a dust particle.

The IBM findings suggest that researchers may be able to control the freezing point of a liquid by confining the liquid to pores of a certain size. In some cases, that would make it possible to study states of materials not otherwise readily accessible. Awschalom and his group, who term this effect "geometric supercooling," have already tried the technique on half a dozen liquids.

The solid-like acoustic behavior of a confined, supercooled liquid appears to stem from the spontanenous appearance and disappearance of tiny plugs of solidified liquid, which randomly grow and collapse, temporarily blocking the cylindrical pores. "Once these plugs exist, the acoustic properties are exactly like a solid," Awschalom says. As the liquid cools, the number of such fluctuating plugs increases, making the liquid more viscous. Finally, the temperature gets low enough that the plugs fill the pores and the liquid freezes.

The IBM group is now interested in seeing what happens when glassy sponges soak up mixtures of two liquids that, like oil and water, normally separate into two phases. The researchers also would like to obtain better evidence for the formation of solid plugs during supercooling.