A welcome mat composed of billions of tiny carbon tubes could one day suck water out of thin air, even in the driest places on Earth. The advance, which takes a cue from the wings of the Namib Desert beetle, could help bring much-needed water to arid regions.
To survive in its arid environment, the Namib Desert beetle has developed a pattern of water-attracting and water-repelling molecules on its hardened wings, forming a series of peaks and troughs. By angling its body into the wind, the beetle can use this pattern to gather water molecules from the morning fog, forming droplets that are directed down into its mouth.
Following the Namib beetle’s lead, the researchers applied two polymer layers—a water-loving one on top and a water-repelling one on the bottom—to a 1-centimeter-high forest of thin cylinders of carbon atoms, called carbon nanotubes. Once finished, the top layer draws water molecules into the forest, without any need for an external power source, the team reports online this month in ACS Applied Materials & Interfaces. Having been drawn inside the forest, the moisture is contained by the water-repelling bottom and sides of the structure—although some can be lost to evaporation over time. Like a sponge, the forest can simply be squeezed to release the water it has collected, after which the material can be reused.
The volume of water vapor that the nanoforest can gather is dependent on the humidity of the air, the team found. In dry conditions, the 8-milligram test forest—which had a top surface area of 0.25 square centimeters—pulled in more than a quarter of its own weight in water in 11 hours; in humid surroundings, however, its efficiency increased and it collected 80% of its weight in 13 hours.
According to the researchers, the concept has the potential to become an effective water-harvesting device, one which would be especially useful in regions that do not receive enough rainfall to adequately support the local population—although production costs will need to be reduced before the forests can be economically produced on a large scale. “The bottleneck” is building a big enough carbon nanotube forest cheaply, explains author Robert Vajtai, a material scientist at Rice University in Houston, Texas, but getting the actual water-harvesting technology to work at a larger scale “would not raise great challenges.”
“This is an interesting approach and adds to a number of existing alternatives” for water harvesting in dry environments, says Jas Pal Badyal, a chemist at Durham University in the United Kingdom who was not involved in this research. Kenneth Lau, a chemical engineer at Drexel University in Philadelphia, Pennsylvania, agrees, commending in particular the novel plasma-based process that the researchers used to make the forest’s intricate architecture. He questions, however, whether elaborate carbon nanotubes are really the optimal choice for the support structure of such a collector, concluding that, for the sake of simplicity, “perhaps other porous materials can be investigated.”