The raincoats we wear to stay dry during stormy weather work because a special coating on the fabric prevents water from soaking through the fibers. To determine how water-repellent a given surface is, scientists look at the shape of a water droplet on that surface. More precisely, they study the so-called contact--the angle formed by the intersection of imaginary lines drawn tangent to the droplet surface and the fabric surface. The more hydrophobic a substance is, the higher the contact angle and the more spherical the water bead. Human skin, for example, is a hydrophobic surface with a contact angle of about 90 degrees. Bird feathers and lotus leaves are considered superhydrophobic, with contact angles of 150 and 170 degrees, respectively.
The surface of a lotus leaf is not perfectly smooth. Rather it has a porous texture much like that of a sponge or bird's nest, albeit on a micrometer scale. The air trapped in the crevices prevents water from adhering to the solid. Researchers led by H. Yildirim Erbil of Kocaeli University in Turkey re-created this naturally superhydrophobic surface in their laboratory by first dissolving polypropylene, a commercially available simple plastic, in a solvent. They then added a precipitating agent and applied the solution to a glass slide. After evaporating the solvent mixture in a vacuum oven, they had a highly porous gel coating with a contact angle of 160 degrees and water-repelling capabilities comparable with those of the lotus leaf. (The investigators applied their coating to other substrates, too--aluminum foil and stainless steel, among them--and achieved similar results.)
Superhydrophobic coatings already in use often require expensive materials and time-intensive application procedures. The new findings, published in the current issue of the journal Science, show that the water repellency of polypropylene can be significantly increased through an easy processing step. The researchers thank the lotus flower: "We mimicked nature to find a simple solution for a difficult technological problem."