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Waterproof Coats: Materials repel water with simplicity, style

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11:32am, February 26, 2003
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Scientists have long sought new coatings that zealously repel water. This week, publications describe two promising finds. Research from Japan shows that water-repellant materials can also be decorative. In a separate report, Turkish researchers describe a way to convert a plastic into a new type of cheap, easily produced waterproofing.

Although their final coatings are different, both teams took their inspiration from nature–from the wings of a butterfly and the leaves of the lotus plant. The microscopically rough surfaces of these organisms prevent water drops from flattening, so the drops roll off and carry away dirt. Because water beads so well on these surfaces, they're called superhydrophobic.

Using the brilliantly blue Morpho sulkowskyi butterfly as their model, Zhong-Ze Gu of the Kanagawa Academy of Science and Technology in Japan and his coworkers designed a synthetic superhydrophobic coating in a variety of bright colors. The microstructure of the insect's wings not only shuns water but also scatters and diffracts light to create an iridescent color.

Similarly, the microstructure of the new, decorative coating repels water while producing striking colors. Gu's team made the material by permitting 6-nanometer-wide silica particles and several-hundred-micrometer-wide polystyrene spheres to assemble into a film. The researchers then heated the film to remove the polystyrene, leaving the silica particles uniformly spaced with air gaps between them. To this rough surface, the scientists added a layer of fluoroalkylsilane, a commercially available waterproofing compound. The researchers describe the procedure in the Feb. 24 Angewandte Chemie International Edition.

The bumpy topography enhanced the fluoroalkylsilane's water-repelling power, says Gu. By varying the distance between air gaps, the team created materials in colors ranging from red to blue and versions with no apparent color.

"It's a very clever trick," comments Manoj K. Chaudhury of Lehigh University in Bethlehem, Pa.

The new technique "may lead to self-cleaning photonic crystals for decoration and optical circuitry," adds Ray Baughman of the University of Texas at Dallas.

The new material may also provide a colorful, self-cleaning coating for cameras or windows, says Gu. It would be environmentally friendly because no organic dye would be required to create color and no detergent would be needed to clean the surface, he adds.

The Turkish research team set its sights on low-cost coatings that are easy to make and use. In the Feb. 28 Science, A. Levent Demirel of Ko University in Istanbul and researchers at Kocaeli University report that they've created a superhydrophobic coating from a low-cost, widely produced plastic called isotactic polypropylene, or iPP.

Making the coating is "simple, inexpensive, and time-saving," says Demirel. The Turkish group dissolved iPP in organic solvents, dropped the solution onto glass slides, and then evaporated the solvents. This procedure produced a porous plastic film that, when viewed with a microscope, "resembles a bird's nest made of branched and intermingled sticks and bumps," the researchers report. Just as they do on rough lotus leaves, water drops readily bead up on the rough plastic coating.

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