Toy car powered by evaporating water engine

A group of scientists from Columbia University have created machines that harness the power of evaporating water, tapping in to an area largely ignored.

A floating, piston-driven engine that generates electricity to make a light flash and a rotary engine that drives a miniature car both use the process. Researchers at Columbia University in New York said that evaporating water could one day produce electricity from giant floating power generators that sit in bays and reservoirs or from huge rotating machines, similar to above-the-water wind turbines.  

Ozgur Sahin, an associate professor of biological sciences and physics at Columbia and the paper's lead author, said: “Evaporation is a fundamental force of nature. It's everywhere and it's more powerful than other forces like wind and waves.”

Last year, Sahin observed that when bacteria shrink and swell with changing humidity, they can push and pull other objects together, and now he and his team have built functioning devices that draw energy from water.

To make one of the machines, the floating piston engine, the researchers glued a line of spores to both sides of a thin, plastic tape similar to that used in cassette tapes. The dashed line of spores was arranged so that those on one side overlapped with spores on the other.

When dry air shrinks the spores, the tape retracts like a spring. This transforms the tape from straight to wavy, making it shorter. If one or both ends of the tape are anchored, the tape tugs on whatever it's attached to. In moist air, the tape extends, releasing the force. The result is a new type of artificial muscle that is controlled by changing humidity. They have been named hygroscopically driven artificial muscles, or hydra.

Sahin and Xi Chen, a postdoctoral fellow in his lab, using dozens of hydra, created a stronger artificial muscle which was put inside a plastic case that has little shutter overhead. Inside the case, evaporating water made the air humid. The humidity caused the muscle to elongate, opening the shutters and allowing the air to dry out. When the humidity escaped, the spores shrank and the tapes contracted, pulling the shutters closed and allowing humidity to build again.

“When we placed water beneath the device, it suddenly came to life, moving on its own,” Chen said.

The spore-covered artificial muscles function as an evaporation-driven piston. Coupling that piston to a generator produced enough electricity to make an LED bulb flash on and off. “We turned evaporation from a pool of water into light,” Sahin said. Based on the current electricity it can produce, the engine could supply floating lights or sensors at the bottom of the ocean.

The other device, called the moisture mill, has a plastic wheel with protruding tabs of tape covered on one side with spores. Half of the wheel is kept in dry air, which causes the tabs to curve, and the other half sits in a humid environment, where the tabs straighten. Left to its own devices, the wheel spins around, effectively acting as a rotary engine.

The team built a small toy car powered by the moisture mill that successfully rolled on its own, again powered just by evaporation. Sahin hopes that in the future engines will use the mechanical energy stored in spores to propel a real car, which would require neither fuel nor electricity to work.

A real-life version of the moisture mill could produce electricity, Sahin said, if the wheel were to sit above a large body of water. It would evaporate saltwater to make the wheel rotate and could produce as much electricity as a wind turbine.

Details are published in the journal Nature Communications.

The devices are put together piece by piece by hand. Shown here is the rotating part of the moisture mill. Credit: Joe Turner Lin

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