Stretched matter goes to unusual extremes

Like a stretched rubber band, most materials become narrower in cross section when extended. A few materials, however, unexpectedly get fatter when stretched.

Researchers have now discovered an unusual combination of these effects. Certain stretched materials expand in one direction of their cross sections and contract in the other, while maintaining a constant volume. Theory and experimental observation suggest that such behavior could occur naturally in several forms of matter with extremely high or low densities.

Ray H. Baughman of the Honeywell Technology Center in Morristown, N.J., and his coworkers describe their findings in the June 16 Science.

This research provides “an intriguing glimpse into the strange properties of some unusual materials,” comments Roderic Lakes of the University of Wisconsin-Madison. The results may prove important both in fundamental studies of matter under extreme conditions and for applications in sensors or devices for controlling light, he adds.

Poisson’s ratio measures how much a stretched material’s cross section decreases for a given increase in length. If a material’s cross section expands upon stretching, its Poisson’s ratio is negative (SN: 3/14/87, p. 166).

Uniform materials with a negative Poisson’s ratio increase in volume when stretched. But if a material has nonuniform properties, its Poisson’s ratio might be negative in one direction and positive in another. In some instances, the effects would balance, leaving the stretched material’s volume unchanged.

Astrophysicists have proposed that the crusts of neutron stars and the cores of white dwarfs consist of ultradense metallic crystals. At high pressures, such a solid consists of well-separated atomic nuclei surrounded by a sea of free electrons. Such a material would be highly resistant to changes in volume.

Calculations by Baughman and his colleagues predict that these solids would have a large negative Poisson’s ratio in certain directions balanced by a large positive ratio in others. They note that these properties might be used to determine the structure of neutron stars and white dwarfs from their vibrations.

Baughman and his colleagues suggest that similar behavior can occur in colloidal crystals, which consist of widely separated, regularly spaced microscopic particles suspended in a liquid; in assemblages of dust particles in electrically charged gases (SN: 8/6/94, p. 84); and in laser-cooled ion crystals (SN: 1/31/98, p. 69).

The researchers measured changes in the spacing of beryllium ions confined in a laser trap at millikelvin temperatures. They found that applying a force in the appropriate direction increased the distance between ion layers in a different direction, confirming that an ion crystal can exhibit a negative Poisson’s ratio without a change in volume. By adjusting the setup, the researchers could also alter the value of Poisson’s ratio.

The ability to adjust Poisson’s ratio is potentially useful for employing sparse crystals as diffraction gratings for optical devices, Baughman notes.

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