Beetle Sluice

Taking cues from a small desert beetle, MIT researchers have created a surface that can collect water droplets from the air. With more development, it could be used to harvest drinking water in deserts or direct microscopic amounts of fluid in chemical reactions, the researchers say.

Scientists have long admired the Stenocara beetle’s means of survival in Africa’s Namib Desert, one of the driest places on earth. Water comes to the area just a handful of times a month, in a fine sea mist that blows over the desert. To exploit this opportunity, the beetle “performs headstands,” says Andrew Parker, a zoologist at Oxford University. On misty mornings, the beetle perches on a sand dune, lowers its head, and tilts its back into the wind.

The beetle’s back is covered with bumps about a half-millimeter in diame­ter. As Parker and colleagues discovered in 2001, the bumps are made of a material that attracts water, while the waxy channels between them repel it. As water droplets blow by, they stick to the bumps; eventually they coalesce into bigger drops that break off and roll down the beetle’s back, into its mouth.

Inspired by Parker’s description in the November 1, 2001, issue of Nature, MIT chemical-engineering professor Robert Cohen and materials science professor Michael Rubner developed a surface that mimics the beetle’s back. It’s built in layers on a glass slide. The first layer is a polymer that makes the surface porous; the researchers sprinkle that with silica nanoparticles. Next, they coat the whole slide with a fluorinated chemical, making it water repellent. Finally, to replicate the beetle’s bumps, they drip on water-attracting acid molecules, which attach to the nanoparticles and stick up above the surface. The result is highly water-attracting spots on a highly water-repellent surface. The new surface collects water more effectively than the beetle’s shell, Cohen says. He and Rubner describe their work in the June 2006 Nano Letters.

The researchers are now applying the surface to flexible materials. They think that patterned cloth sails, if hung from houses or poles in extremely arid regions like the Namib, could collect mists too fine to be captured today. In wet regions, such sails might also harvest water more efficiently than the mist nets used now. But durability is still a problem; the surface rubs off of the cloth over time.

A British company called QinetiQ, one of whose scientists coauthored Parker’s Nature paper, already makes a similar material, which it is testing in devices that trap and save water in water-based air-conditioning units.

The question for the MIT researchers and QinetiQ, says Parker, is whether the spotty coating can be manufactured inexpensively on a large scale. Cohen thinks that it can be done by employing ink jets to print the acid spots. “If there was a driver for doing so, I can imagine the process could be scaled up appropriately,” he says.

He adds that the material could have uses that wouldn’t require large-scale manufacturing. Slides printed with virtually any pattern could be cut into small pieces and used to build microsensors, he says. Or slides designed to hold, then mix, tiny droplets of chemicals could be used to carry out reactions at the microscale.

Cohen and Rubner’s work is being funded by the U.S. Defense Advanced Research Projects Agency, which hopes to develop a biocidal and self-­cleaning coating for military equipment. ­Stenocara-­­inspired technology might be used to collect and carry away deactivated biological contaminants.