Surface tension drives nanomotor

Researchers from the University of California at Berkeley have found a way to harness surface tension to drive nanomachines.

The researchers' nanoelectromechanical relaxation oscillator is contained in a box that is 200 nanometers on a side and consists of a pair of liquid metal droplets of different sizes on a carbon nanotube. Carbon nanotubes are rolled-up sheets of carbon atoms. A nanometer is one millionth of a millimeter.

When electrical current is run through the nanotube, metal atoms move from the big droplet along the nanotube to the smaller droplet, shrinking the big droplet while the smaller one grows. Eventually the droplets touch above the nanotube and surface tension drives the bulk of the small droplet back to the big one; as soon as this happens the process begins again.

The device could eventually be used as a nanorobot motor that drives crawling, walking, swimming, jumping or flying motions; the physics behind the device should also work at the microscale, according to the researchers. A micron is one thousand nanometers.

This pattern of slow buildup of tension followed by rapid release is found in many phenomena, including earthquakes, heartbeats, drippy faucets, and political upheaval in unstable countries.

The device's peak power output is about 20 microwatts -- 3 million times less than that required by a 60-watt lightbulb. However, the power density achieved during the fast, surface-tension driven relaxation is enormous: 100 million times more power per unit volume than a 225 horsepower, 3.3 liter V-6 car engine, according to the researchers.

The surface tension driven relaxation oscillator could be used in practical microscale devices within two years using existing silicon-based technology; it will take longer, however to implement the technology at the nanoscale, according to the researchers. The work appeared in the March 21, 2005 issue of Applied Physics Letters (Surface-Tension-Driven Nanoelectromechanical Relaxation Oscillator).