Scientists have made the world’s smallest motor. In fact, the motor is so small a few hundred of them could fit in the period at the end of this sentence. This means you won’t want to install it in your humvee, but maybe you’ll fancy it in future nanotechnology that needs to swim around your body killing viruses and such. You can watch the motor run here.
As for the explanation. I never got past high school physics, so I’ll just have to quote a LiveScience article.
The motor works by shuffling atoms between two molten metal droplets in a carbon nanotube.
One droplet is even smaller than the other. When a small electric current is applied to the droplets, atoms slowly eek off the larger droplet and join the smaller one. The small droplet grows – but never gets as big as the other droplet – and eventually bumps into the large droplet. As they touch, the large droplet rapidly sops up the atoms it had previously sloughed off. This quick shift in energy produces a power stroke.
The technique exploits the fact that surface tension — the tendency of atoms or molecules to resist separating — becomes more important at small scales. Surface tension is the same thing that allows some insects to walk on water.