When servers in a data center run at full tilt, they get as hot as a forgotten iron. That heat goes to waste, and cooling them eats up even more energy. Engineers in Germany have proposed putting that heat to work: let it drive liquid through thin tubes all by itself, with no motors or pumps.
Their device looks like a flat box with an asymmetric bottom. One half of the bottom is made of a material that conducts heat well; the other half conducts it poorly. When heated from below, the liquid on the bottom surface warms unevenly. In the hot zone, it expands, becomes lighter, and spreads sideways, dragging neighboring layers along. A steady flow arises, like water in a pot when heated only on one side.
This "heat pump" has no moving parts, makes no noise, and consumes no electricity. It can be printed on thin plates and built right into chips or lab-on-a-coin devices. Heat that used to be a nuisance now does useful work.
The effect that makes liquid move due to a temperature difference is known as the Marangoni effect. It's also what makes wine creep up the sides of a glass in "tears".