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Turkish chemists have found a simple, inexpensive method for converting polypropylene into a coating whose surface strongly repels water, much like the leaves of the sacred lotus. On the micrometer scale, lotus leaves have a bumpy, waxy surface that is renowned for its ability to repel water and dirt. Many scientists and engineers have tried to mimic this "lotus effect" (superhydrophobicity) in artificial materials by tailoring surface chemistry and surface roughness with varying degrees of success. Generally, such efforts have involved expensive materials and complex, time-consuming processes for applying the materials to surfaces. The Turkish researchers believe they have a simpler way to do it. They start with isotactic polypropylene, a commercially available hydrophobic polymer, and transform it into a highly porous gel coating that is even more hydrophobic. This is accomplished by dissolving the polymer in a solvent such as hot p-xylene, casting the solution on a substrate, and then cooling and evaporating the solvent to precipitate the gel. Alternatively, they add a nonsolvent (precipitator) such as methyl ethyl ketone to the polymer solution before evaporating it.
The resulting coating resembles, on the nanoscale, "a bird's nest made of branched and intermingled sticks and bumps," the researchers write.
Because the gel surface is so rough and studded with microscopic air pockets, water droplets do not spread out on it. Rather, they remain highly spherical, forming a contact angle with the surface as large as 160º
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