Capturing the water floating as countless droplets in the oceans of air surrounding the Earth has always been a tantalizing dream for scientists. This dream is now apparently on the threshold of becoming reality.

Researchers in the US have reported a successful experiment that used billions of microscopic carbon nanotubes to draw water from the air around it. They reported their findings this month on the website of ACS Applied Materials & Interfaces at http://pubs.acs.org/journal/aamick.

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Next step: develop the technology to make a water harvesting device economically viable.

In their experiment, researchers applied two polymer layers to a one centimeter-high "forest" of carbon nanotubes. The top layer attracted water while the bottom layer repelled water. The forest had a top surface area of 0.25 square centimeters.

Researchers said the top layer drew water molecules into the forest without any need for an external power source. Inside the forest, the moisture is contained by the water-repelling or hydrophobic bottom and sides of the structure.

In essence, the carbon nanotube forest becomes a carbon "nanosponge."

This allows one to simply squeeze the carbon nanotube forest like a sponge to release the water trapped inside, after which the material can be reused.

The amount of water vapor the nanosponge can suck-in depends on the humidity. In dry conditions, the 8 mg test forest pulled in more than a quarter of its own weight in water in 11 hours.

Scientists believe that in humid surroundings, however, the efficiency of the nanosponge increases and it can collect 80% of its weight in 13 hours.

They said the nanosponge concept has the potential to become an effective water harvesting device, one especially useful in arid regions in Africa and Asia.

"The bottleneck" is building a big enough carbon nanotube forest cheaply, said researcher Robert Vajtai, a materials scientist at Rice University in Houston, Texas.

He said getting the actual water harvesting technology to work on a larger scale "would not raise great challenges."

Tagswater, Air, harvesting, experiment, Science