Science News - December 4, 2010

A patch of this light-absorbing forest the size of a quarter still contains tens of millions of nanotubes.)

While scientists don’t fully understand how the thicket of nanotubes swallows all light, the dilute packing of the tubes seems to be crucial. The trees in this particular carbon nanotube forest are so tall, thin and sparsely planted that there’s no real surface for light to strike. “It’s almost like light has a soft landing on the structure,” Lin says.

The carbon nanotube trees are different heights and tangle together at the tips. And while the tubes are a somewhat hefty 10 nanometers across, Lin and his colleagues made them very, very tall, about 500 micrometers. (A similarly proportioned No. 2 pencil would be more than three times as tall as the Statue of Liberty). This extreme skinniness, uneven height and sparse packing transforms the forest into a sponge that soaks up light.

“For light, it is almost like nothing.
It is like the empty sky,” Lin says.
“Why is the empty sky so dark? Because
it almost has nothing. It is so dilute,
nothing ever comes back. Material is
what reflects.”
When light strikes ordinary mate-
rials, it bounces off in a predictable
manner related to the angle at which it
came in. But the nanotube forest doesn’t
care about angles. The small amount of
reflection that does occur is totally angle
independent, says Lin, which makes no
sense optically.

“There is no classical theory to explain this new type of surface,” Lin says. “There is no theory,” he laughs. “That’s my theory.”

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Scientists at the National Institute of Standards and Technology and Stony Brook University in New York have already put this new dark material to use (for good, not evil). They grew a similar nanotube forest as a coating for a contraption that accurately detects the power of lasers shined into it. This dark detector might also help improve measurements of the temperature of the Earth and sun, the team reports in the September Nano Letters.

Voltage applied to artificial muscle spun
from nanotube sheets (illustration, top)
makes the muscle expand (bottom)—in
some cases by more than 200 percent.

Carbon flex

Fire and light are surely captivating, but no big top is complete without feats of strength. Scientists exploiting carbon nanotubes’ stretchy properties recently created giant artificial muscles.

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.

When natural muscles contract and expand, their length typically changes by less than 40 percent. But the team found that the nanotube muscles can change their length by more than 230 percent in a fraction of the time, giving them some serious punch. And since they can still flex their stuff at extremely high and low temperatures, the artificial muscles may be ideal for robots exploring hostile environments, such as Mars.

“Ordinary muscles of course are
wonderful,” Baughman says. “They
are self-repairing, they can last a life-
time. But these artificial muscles based
on carbon nanotubes are much faster
in terms of response than natural
muscle. And they can operate in extreme
environments where no other artificial
muscle will survive.”
The artificial muscles’ massive con-
tractions can now be nicely described
with theory and numbers, but it took
mucking about in the lab to discover
the strange behavior. And so it goes with
science. Eventually, researchers will
probably gain some clarity concerning
the other unusual properties exhibited
by carbon nanotubes. Those new theo-
ries will lead to more experimental work
and then to additional mysteries. In the
scientific volley between theory and
experimentation, surprises can spring
like a sudden backhand.

Strano says he has complete faith that theory will soon explain why a carbon nanotube can behave so strangely. “But observation and discovery will still play a role,” he says. “Making one and being able to manipulate it in the lab and do strange things to it has taught us quite a bit.” 