March/April 2010

Hydrophobic interface mimics hairs on spider

Engineering researchers have crafted a flat surface that refuses to get wet. Water droplets skitter across it like ball bearings tossed on ice.

University of Florida engineers have achieved what they label in a new paper as a "nearly perfect hydrophobic interface" by reproducing, on small bits of flat plastic, the shape and patterns of the minute hairs that grow on the bodies of spiders, according to ScienceDaily.

"They have short hairs and longer hairs, and they vary a lot. And that is what we mimic," said Wolfgang Sigmund, a professor of materials science and engineering.

Spiders use their water-repelling hairs to stay dry or avoid drowning, with water spiders capturing air bubbles and toting them underwater to breathe. When water scampers off the surface, it picks up and carries dirt with it, in effect making the surface self-cleaning. As such, it is ideal for some food packaging, or windows, or solar cells that must stay clean to gather sunlight, Sigmund said. Boat designers might coat hulls with it, making boats faster and more efficient.

Sigmund said he began working on the project about five years ago after picking up on the work of a colleague. Sigmund was experimenting with microscopic fibers when he turned to spiders, noted by biologists for at least a century for their water-repelling hairs.

As a scientist and engineer, he said, his natural tendency was to make all his fibers the same size and distance apart. But he learned that spider hairs are both long and short and variously curved and straight, forming a surface that is anything but uniform. He decided to try to mimic this random, chaotic surface using plastic hairs varying in size but averaging about 600 microns, or millionths of a meter.

Water-repelling surfaces or treatments are already common, spanning shoe wax to caulk to car windshield treatments. However, Sigmund said the UF surface may be the most or among the most water phobic. Close-up photographs of water droplets on dime-sized plastic squares show the droplets maintain their spherical shape, whether standing still or moving. Droplets bulge down on most other surfaces, dragging a kind of tail as they move. Sigmund said his surface is the first to shuttle droplets with no tail.

Also, unlike many water-repelling surfaces, the UF one relies entirely on the microscopic shape and patterns of the material-rather than its composition. In other words, physics, not chemistry, is what makes it water repellent.

Sigmund said making the water or oil-repelling surfaces involves applying a hole-filled membrane to a polymer, heating the two, and then peeling off the membrane. Made gooey by the heat, the polymer comes out of the holes in the desired thin, randomly sized fibers. While inexpensive, it is hard to produce successful surfaces with great reliability, and different techniques need to be developed to make the surfaces in commercially available quantities and size, Sigmund said. Also, he said, more research is needed to make the surfaces hardy and resistant to damage.