New Water-Repellent Material Mimics Lotus Leaves

John Roach
for National Geographic News

February 27, 2003

The lotus leaf, better known as the water lily, is considered sacred in Asian religions for its ability to stay dry and clean. When water drops on the leaf, it beads up and rolls off the waxy surface, washing away dirt as it goes.

In religious circles, this characteristic makes the lotus leaf a symbol of purity. Scientists, too, have long praised the plant for its water-resistant and self-cleaning properties. For years they have tried to mimic its structure.

Many have met success in this endeavor using complex processes and expensive materials. A team of Turkish researchers report in the February 28 issue of the journal Science that they were successful in mimicking the lotus leaf on the cheap.

The procedure, accomplished by Levent Demirel, a professor of chemistry at Koç University in Istanbul, and Yildirim Erbil, Yonca Avci, and Olcay Mert, of Kocaeli University in Izmit, could make highly waterproof materials available for common applications.

One such use, explain the Turkish researchers, would be a coating for radio car antennas that would keep them ice- and snow-free, and thus functional, for the commute into work on a cold winter morning.

Such a highly-waterproof material could potentially be applied to airplane wings (to keep them from icing up), the hulls of ships (to help them ply waters more easily), and clothing (to keep them dry).

"We mimicked nature to find a simple solution for a difficult technological problem," the researchers conclude in their paper.

Wilhelm Barthlott, a botanist at the University of Bonn in Germany who discovered how lotus leaves repel water and are self-cleaning in 1997, said: "The results of the science paper are marvelous."

Angles, Fears, and Roughness

Setting aside all scientific and journalistic caution for the moment, if the Turkish researchers walked into a bar in Toughtown, U.S.A. and tried to explain their work, one speculates they might say something along the lines of:

"Basically, we made a really, really waterproof substance by tossing a material just like your dirty, synthetic long underwear into a vat of chemicals, adding heat, and cooking it until it dissolved. Then we poured it on some glass where it hardened into a material that is just as waterproof as a pretty little water lily."

Talking amongst their scientific colleagues, however, the Turkish team explains their breakthrough in language punctuated phobias (fears), angles, and roughness.

To begin with, scientists classify how waterproof a material is in terms of just how "afraid" of water it is. A material that is waterproof enough to stay dry during a light drizzle is called hydrophobic, or "afraid of water." A material that stays dry during a hurricane is considered super-hydrophobic, or "highly afraid of water."

Whether a water-repellent material is "afraid" of water or "really afraid" of water depends on how a drop of water rests on the material when the material is flat. Water rests on the most waterproof materials in a perfect sphere and is said to have contact angle of 180°. To be considered at least a little afraid of water, a material must have contact angle of more than 90°.

The roughness of a material's surface also plays a key role. When Barthlott, the German botanist who discovered the "lotus effect" in 1997, examined a lotus leaf under a high-powered microscope, he discovered that it did not have the waxy, smooth surface that appeared to the naked eye. Rather, it was covered in microscopic bumps—a characteristic that aids in water repellency.

When water droplets fall on the lotus leaf they touch the surface at only a few points, resting on these microscopic "bumps." A slight tilt to a leaf enables water droplets to roll off under their own weight.

Since Barthlott's discovery, several researchers have taken expensive materials that were already hydrophobic and put them through a complex process that roughed the materials up to the point that they became super-hydrophobic.

Cheap Waterproofing

What makes the feat of the Turkish researchers stand out is that they made a super-hydrophobic material on the cheap.

Most lotus-like materials manufactured to date have been created by etching and machining more expensive water-resistant materials, such as fluorinated polymers like Teflon.

The Turkish researchers used polypropylene, a common water-repellent plastic used to make things such as long underwear, socks, and winter jackets. Air-filled pores in polypropylene make it water repellent in the same way that bumps make the lotus leaf water repellent.

The researchers were able to increase the percentage of pores by dissolving polypropylene in a vat of organic solvents and then dropping the solution onto a flat surface. "As the solvent evaporates at room temperature, porous structure is formed," said Demirel.

The result is a super-hydrophobic coating that has a contact angle of 160°. (By comparison, a lotus leaf has a contact angle of 170°.)

"Our starting point was to understand the effect of surface roughness on water repellency and thus to mimic the lotus leaf," he said.

According to Barthlott, the Turkish team was successful: "The properties of their surface are close to those of the lotus leaf," he said. "The material definitely has super-hydrophobic abilities, but the self-cleaning properties should be tested."