Saturday, December 4, 2010

Tiny Tubes, Big Riddles - Science News

Tiny Tubes, Big Riddles - Science News: Many materials when stretched one way, will contract in another way, says Ray Baughman, director of the NanoTech Institute at the University of Texas at Dallas. Think of yanking on a rubber band — as it lengthens, its width shrinks. The relationship between the amount of stretching and contraction is known as Poisson’s ratio. Rubber, for example, has a very high Poisson’s ratio, nearly 0.5. Stretch it one way, and it contracts in the other direction by quite a bit. Cork, on the other hand, doesn’t bulge out much when pushed. It has a Poisson’s ratio near zero, making it easy to wedge back into a wine bottle.

While exploring the push-and-pull of various materials in the lab, Baughman and his colleagues spun carbon nanotubes into airy sheets. These sheets “represent a strange state of matter,” he says, with fantastic elastic properties that correspond to Poisson’s ratios as high as 15.

Taking advantage of these bizarrely large Poisson’s ratios, Baughman, colleague Ali Aliev and others turned their sheets into giant muscles that contract like crazy when pulled just a tiny bit. Stretch these sheets just 1 percent in one direction and their volume shrinks by 23.5 percent, the team reported in Science in 2009.

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