The sun is the most abundant source of renewable energy. But all the technologies that capitalize on sunlight, including photovoltaics and biofuels, require intermediate steps and infrastructure to turn the sun's rays into something that can be used to perform work in a machine. Researchers at the University of California, Berkeley, are using carbon nanotubes to build small, simple waterborne machines propelled directly by sunlight. In theory, they say, these machines could be scaled up to make energy-generating pumps directly powered by the sun.

The sun-powered machines rely on water's surface tension. Water molecules are strongly attracted to one another. These high-energy interactions can, under the right conditions, pull objects across the water. The Berkeley machines are pieces of clear plastic, about a centimeter on their longest edge, embedded with strips of vertically aligned carbon nanotubes. When light from the sun or from a laser is focused on the machine floating on a pool of water, the nanotubes heat up and heat the water around them. This causes a decrease in surface tension localized to one region of the machine, which is in turn propelled forward away from the low-tension part of the surface.

Other similar systems break surface tension using electrical pulses, but this requires a power source such as a battery or a solar cell. "This is better because you eliminate the middleman and get a lot of work out," says Alex Zettl, a professor of condensed-matter physics at Berkeley who led the research team with Jean M.J. Fréchet, a professor of chemistry and chemical engineering. "We think we're on to something because surface tension is very powerful," says Zettl.

So far, the Berkeley team has demonstrated two basic sun-powered machines. The first, a plastic rectangular boat with a nanotube strip at its back, performs linear motion. By directing laser light or using a lens to focus sunlight either at the center of the nanotube strip or at its corners, the boat can be directed straight forward or in circles. When light is focused onto it, a boat about a centimeter long can travel as fast as eight centimeters per second. The second machine is a simple rotor with one nanotube strip on one side of each of its four fins. When exposed to direct sunlight, it spins at about 70 rotations per minute. Both machines have only been tested in containers in the lab.