Tense encounters drive a nanomotor.

Walking on water is a miracle for people, but it's no sweat for some insects. The key for the bugs is surface tension, the cohesive forces of the liquid's surface molecules that are strong enough to support tiny weights. Now, researchers in California have found a way to exploit the powerful force of surface tension on small scales to make an extraordinarily hardworking nanomotor.

The motor is one of the oddest engines around. Its primary components are two minuscule molten blobs of the metal indium, kept at 400[degrees]C to 500[degrees]C, clinging like dewdrops to a kind of carbon wire known as a nanotube (SN: 9/18/04, p. 180). Roughly 200 nanometers across, the entire construction is about the size of a large virus.

When physics researcher B. Christopher Regan of the University of California, Berkeley and his colleagues send electricity through the wire, indium atoms flow from the larger blob to the smaller one. The electric current also keeps the metal hot. Gradually, the smaller blob swells and the larger one shrinks, until the two touch. At that moment, surface tension kicks in with a sudden ferocity: In a mere 200-trillionths of a second, the larger blob gulps down the smaller one. To view an animation of the sequence, go to physics.berkeley.edu/research/zettl/ projects/Relax_pics.html.

The little gulp constitutes the power stroke of the nanoengine, and on this scale it's a mighty one, notes Regan. In a size-adjusted comparison with a Toyota Camry engine, he says, the power produced during consumption of the smaller droplet is 100 million times as great.

Reagan, Alex K. Zettl, also of UC Berkeley, and their colleagues stumbled upon the new-style motor in experiments involving indium-coated carbon nanotubes.

The new engine may prove useful for propelling nanoscale vehicles, says Regan. "Its strong suit is that it's extremely powerful for its size," he says.--P.W.