Water Repellency Goes Nano: Carpet of carbon nanotubes cleans itself

The amazing water-shedding ability of the lotus leaf has long inspired materials scientists. The leaf’s wax-coated microstructures cause rain droplets to bounce off the surface, carrying away with them dust particles and other contaminants. In trying to reproduce this so-called lotus effect in the lab, chemical engineers have fabricated a similar self-cleaning material out of forests of carbon nanotubes.

Led by Karen Gleason of the Massachusetts Institute of Technology (MIT), the researchers first created arrays of tiny islands of nickel on a surface of silicon. From these islands, the researchers grew vertically aligned carbon nanotubes. “Sort of like a bed of pins” is how coinvestigator Kenneth Lau describes the result.

The minuscule pillars, each measuring 50 nanometers in diameter and standing 2 micrometers, form a rough surface. Because there are air pockets between the nanotubes, when a drop of water lands on the surface, “it’s essentially held up by air,” says Lau.

Using a technique called chemical-vapor deposition, the researchers coated the top of each carbon nanotube with poly(tetrafluoroethylene), otherwise known as Teflon. Not only did this arrangement stabilize the nanotube forest, but it also boosted the surface’s water-repelling properties. When water droplets squeezed from a syringe hit the Teflon-coated nanotube carpet, the droplets bounced from and rolled off the surface.

Because the spacing between nanotubes was less than a micron, the material could repel tiny droplets of water just a few micrometers in diameter. The researchers describe the new material in a forthcoming issue of Nano Letters.

Materials such as these are called superhydrophobic because of their ability to repel water so completely. On more standard hydrophobic surfaces, such as the hood of a car, water droplets form flatter-bottomed spheres that might not roll off.

Researchers throughout the world have been striving to invent superhydrophobic materials for products such as antifouling paint, self-cleaning garments, or coatings on airplanes to prevent ice buildup. In pursuit of that goal, investigators have tried a variety of polymer fibers and spheres (SN: 3/1/03, p. 132: Waterproof Coats: Materials repel water with simplicity, style). So far, however, the resulting materials have been too fragile to be practical.

To join the competition, the MIT group turned to one of nanotechnology’s poster materials, carbon nanotubes. “Coating the nanotubes with [Teflon] is very neat and novel,” says A. Levent Demirel, a chemist at Ko University in Istanbul, Turkey. Without the Teflon, the water droplets would eventually seep through the material.

David Quéré, a chemist at the Collège de France in Paris, says the nanotube route to superhydrophobic materials is attractive, but he isn’t yet convinced it will lead to commercial products. Even though the Teflon coating stabilizes the nanotubes, he says, “if you press your thumb on them, you immediately destroy the structures.”

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