The smallest electric motor in the world, devised by physicists at UC Berkeley, is based on the shuttling of atoms between two metal droplets---one large and one small---residing on the back of a carbon nanotube.

An electric current transmitted through the nanotube causes atoms to move from the big to the small droplet. In effect, potential energy is being stored in the smaller droplet in the form of surface tension. Eventually the smaller drop grows so much that the two droplets touch.

Then the accumulated energy is suddenly discharged as the larger droplet reabsorbs its atoms through the newly created hydrodynamic channel. This device constitutes a "relaxation oscillator" with an adjustable operating frequency. If the oscillator is attached to a mechanical linkage, it acts as a motor and can be used to move a MEMS device in inchworm fashion (movie: physics.berkeley.edu/research/zettl/projects/Relax_pics.html).

The peak pulsed power is 20 microwatts. Considering that the device is less than 200 nm on a side, the power density works out to about 100 million times that of the 225 hp V6 engine in a Toyota Camry.

Chris Regan (bcregan@berkeley.edu), a member of Alex Zettl's group at Berkeley, reported these and related results at the recent APS meeting in Los Angeles and in the 21 March 2005 issue of Applied Physics Letters.