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."
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