Giving good vibrations but far from rocking

Nanotubes: The world's tiniest guitar string doesn't make a sound but may help revolutionize electronic devices like cell phones.

Medicine & Science

September 20, 2004|By Michael Stroh | Michael Stroh,SUN STAFF

Now playing in the lab: the world's smallest guitar string.

A Cornell University team reports in the current issue of Nature that it has created a twangable carbon nanotube roughly one-millionth the size of a standard steel guitar string.

Rock stars take note: The carbon tube, invisible to the naked eye at just 0.000059 inches long, isn't likely to be the source of blistering blues riffs anytime soon. "There's not much to hear," says physicist Paul L. McEuen, who led the team.

To be precise, there's nothing to hear. The Cornell nanotube vibrates from 10 million to 200 million cycles a second - sounding notes as high as 18 octaves above middle "C" on a piano.

The human ear generally can't detect anything beyond 20,000 cycles a second. So what good is a tiny guitar string that can't be seen or heard?

For musicians, not much, admits McEuen. But physicists and engineers are a different story.

A guitar string is an example of a "mechanical resonator." Andrew Cleland, a physicist at the University of California at Santa Barbara, notes that resonators show up in a number of popular electronic devices, including radio receivers and cell phones.

If researchers could shrink mechanical resonators down to atomic size, it might open the door to smaller high-tech toys that consume less power than current devices.

A vibrating nanotube, says McEuen, might also have important scientific applications. For example, some researchers are trying to turn resonators into tiny scales capable of measuring the mass of a single molecule or atom.

The Cornell effort is part of a wider scientific race to construct exquisitely small machines and materials - hence the nickname "nano." One nanometer equals a billionth of a meter, or about 0.00000003937 inches.

Discovered in 1991, carbon nanotubes have become a hot topic among scientists because of their astonishing strength and other odd properties. But until now vibration has not been one of them.

To make its nanotube sing, the Cornell team clamped it between two gold electrodes and then applied an electrical field to the tube, causing it to vibrate.

By varying the field's strength, scientists found they could increase tension on the nanotube and tune it. When a guitar string is pulled tighter, its pitch rises. The Cornell team was able to alter the nanotube's pitch by as much as an octave.

This isn't the first time Cornell scientists have fiddled with atomic-scale musical instruments. In years past, other groups have created a nanodrum and two guitars, including one unveiled last fall that was the size of a blood cell.

Carved from a slab of silicon, the guitar (or "Nanocaster" as some jokingly called it with a nod to the famous Fender Stratocaster) was even playable. The strings vibrated when researchers plucked them with a laser beam.

McEuen says that the vibrating tube he and his team produced is smaller than the strings on the previous Cornell guitars. He said he has no plans to thread them into an instrument, however. "We're doing this out of curiosity, driving to push things ever smaller," he says.

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