from Brownian motion ‚ the laws of thermodynamics did not allow it.

The researchers used tiny polymer gears with asymmetric teeth floating in a thin film teeming with Bacillus subtilis, a bacterial species known for its swimming ability. Above a concentration of about 10 billion bacteria per cubic centimeter, the gear would rotate. Dr. Aranson said that unlike molecules in Brownian motion, which reflect off whatever they strike, when the bacteria hit a tooth, they just keep pushing.‚ They slide along the edge of the tooth until they reach the ‚ junction where the next tooth starts. Since one edge of each tooth is longer than the other, more bacteria slide along the long edges, transferring more momentum to them and rotating the gear in one direction.

One of the limitations of the process, Dr. Aranson said, is that the bacteria eventually run out of nutrients. But they can stop pushing even before that.