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
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
“There is no classical theory to explain
this new type of surface,” Lin says.
“There is no theory,” he laughs. “That’s
s TeM filMs
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.
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.”
s The nanotube site:
december 4, 2010 | science news | 23