- The Washington Times - Thursday, February 27, 2003

KOCAELI, Turkey, Feb. 27 (UPI) — By chemically mimicking the leaves of the sacred lotus, Turkish scientists said Thursday they have devised a simple, inexpensive super-water-repellant coating that could help protect everything from airport runways to the latest fashions.

The novel technique promises to increase the water repellency of commercial plastics significantly through an easy processing step. One immediate application, the scientists said, would be preventing ice formation at airports, and other uses might begin appearing in the next three to five years, researcher A. Levent Demirel, a physical chemist at Kocaeli University, told United Press International.

"The range of applications is very broad. It really depends on your imagination. Anything where you wouldn't want things wet," expert Jan Ganzer, a chemical engineer at North Carolina State University in Raleigh, commented.

Water repellant or "hydrophobic" surfaces could find use in self-cleaning traffic signs, reduced drag ship hulls and in stain prevention on fabrics. Surfaces could be made even more hydrophobic by roughing up their surfaces, so water drops would float on top of the resulting edges much as mystics can lie on beds of nails.

Artificial super-hydrophobic surfaces currently are made with expensive materials applied via complex, time-consuming machining or etching processes. They work by forcing water droplets not to rest their bottoms completely flat against the surfaces as in domes, but buoyed up instead, resembling sunsets with the "horizons" forming angles of 150 degrees or larger with the lower surfaces of the beads. The higher this "contact angle," the more water repellant a surface is.

Demirel, along with Husnu Yildirim Erbil at Kocaeli University and colleagues, noticed that in nature, the leaves of the sacred lotus are super-water-repellant, with droplets forming contact angles up to 170 degrees. The leaves have surface details only millionths of a meter wide, or roughly the same diameter of cells.

In findings appearing in the Feb. 28 issue of the journal Science, the researchers said they have succeeded in imitating the microscopic structure of the sacred lotus leaf to make an ordinary plastic super-hydrophobic.

"We understand that we mimicked nature to find a simple solution for a difficult technological problem," they wrote.

The team chose a simple plastic called polypropylene, used to make dishwasher-safe containers and the indoor-outdoor carpeting found around swimming pools and miniature golf courses. Normally, water droplets resting on a smooth polypropylene surface lead to a contact angle of about 104 degrees — not enough to be classified super-hydrophobic.

Instead, the researchers tried dissolving the plastic in a variety of heated organic solvents, adding other ingredients to precipitate out the plastic as crystals onto glass slides. The solvents chewed out holes rapidly, resulting in spongy, gel-like porous coatings resembling birds' nests, with features only a few millionths of a meter wide.

"It's this control of the porosity of the surface that gives them control of surface wettability," Genzer explained. The air trapped between the water and the wax crystals on the leaf are what minimize contact.

After tinkering with various solvent concentrations and drying temperatures, they influenced crystal size and distribution, increasing surface roughness and managing to achieve contact angles up to 160 degrees.

"What they've come up here is a simple and practical and lower-cost means to reproduce the texture that gives that super-hydrophobic surface," said materials scientist Paul Armistead of the Office of Naval Research in Arlington, Va.

Genzer agreed, foreseeing applications in stain-resistant clothing. "An interesting application is to use them for de-icing airplanes in winter. If you can prevent water from sitting in a continuous layer, you could prevent ice from forming," he added.

Because the effect depends on trapped air bubbles, Armistead said he is unsure such coatings would help lead to water-resistant ship hulls, which are usually submerged in water. However, he said it could help protect radar arrays to improve performance, "because if you get gunk on antennas, they don't work as well."

The researchers also have succeed in applying this super-water-repellant coating on aluminum foil, stainless steel, Teflon and plastics, and think it can be applied to other surfaces, as long as the solvent mixture does not dissolve the underlying material. Genzer said he does not see why this method could not be used with other commercial plastics.

"You should be able to basically use the same strategy. Different materials would require different solvents and different processing, but the principle should remain the same," he said.

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(Reported by Charles Choi, UPI Science News, in New York)


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