"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. Potential applications for UF's ultra-water-repellent surfaces are many. 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. Boat designers might coat hulls with it, making boats faster and more efficient.
"Most people that publish in this field always go for these perfect structures, and we are the first to show that the bad ones are the better ones," Sigmund said. "Of course this is a finding in a lab. This is not something you expect from theory."
To be sure, water-repelling surfaces or treatments are already common, spanning shoe wax to caulk to car windshield treatments. Scientists have also reproduced other biologically inspired water repelling surfaces, including ones patterned after lotus leaves.
But the new surface may be the most or among the most water phobic. Close-up photographs of water droplets on dime-sized plastic squares show that 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 new 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. Theoretically, that means the technique could transform even the most water-sopping materials – say, sponges – into water-shedding ones. It also means that this surfaces need never slough off dangerous chemicals. Provided the surface material itself is made safe, making it water repellent introduces no new risks.
A variation of the surface also repels oil, a first for the industry, though no research has been published yet.
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. More research is also needed to make the surfaces hardy and resistant to damage.
Citation: Shu-Hau Hsu, Wolfgang M. Sigmund, 'Artificial Hairy Surfaces with a Nearly Perfect Hydrophobic Response', Langmuir, February 2010, 26(3), 1504–1506; doi: 10.1021/la903813g