An immensely powerful yet invisible force pulls water from Earth to the top of the tallest redwood and delivers snow to the tops of the Himalayas. Yet despite the power of evaporating water, its potential to propel self-sufficient devices or produce electricity has remained largely untapped -- until now.

In the June 16 online issue of Nature Communications, Columbia University scientists report the development of two novel devices that derive power directly from evaporation -- a floating, piston-driven engine that generates electricity causing a light to flash, and a rotary engine that drives a miniature car.

When evaporation energy is scaled up, the researchers predict, it could one day produce electricity from giant floating power generators that sit on bays or reservoirs, or from huge rotating machines akin to wind turbines that sit above water, said Ozgur Sahin, Ph.D., an associate professor of biological sciences and physics at Columbia University and the paper's lead author.

"Evaporation is a fundamental force of nature," Sahin said. "It's everywhere, and it's more powerful than other forces like wind and waves."

Last year, Sahin found that when bacterial spores shrink and swell with changing humidity, they can push and pull other objects forcefully. They pack more energy, pound for pound, than other materials used in engineering for moving objects, he reported in a paper published in Nature Nanotechnology, which was based on work Sahin had started as a Scholar in Residence at the Wyss Institute for Biologically Inspired Engineering at Harvard University.

Building on last year's findings, Sahin and his Columbia colleagues sought to build actual devices that could be powered by such energy.

To build a floating, piston-driven engine, the researchers first glued spores to both sides of a thin, double-sided plastic tape akin to that in cassette tapes, creating a dashed line of spores. They did the same on the opposite side of the tape, but offset the line so dashes on one side overlapped with gaps on the other.

When dry air shrinks the spores, the spore-covered dashes curve. This transforms the tape from straight to wavy, shortening the tape. If one or both ends of the tape are anchored, the tape tugs on whatever it's attached to. Conversely, when the air is moist, the tape extends, releasing the force. The result is a new type of artificial muscle that is controlled by changing humidity.