BED OF NAILS: A dip-coated "omniphobic" surface repels both oil (red) and water (blue). Each droplet is approximately 0.12 inch (three millimeters) across. Image: Photo courtesy of Anish Tuteja & Wonjae Choi
Nature has conjured up numerous defenses to water, imbuing duck feathers, lotus leaves and even butterfly wings with the ability to repel that ubiquitous liquid of life. But it hasn't had much time to come up with a way to protect its constituents against a newcomer like gasoline.
So a group of researchers at the Massachusetts Institute of Technology (M.I.T.) and the Air Force Research Laboratory sought to correct that shortcoming by engineering so-called omniphobic surfaces that repel not only water—then they would be just hydrophobic—but also oil-based liquids and alcohols. The team's results were published online yesterday by the Proceedings of the National Academy of Sciences USA.
Getting such a liquid to bead up is much more difficult than doing the same to water, as gasoline—like alcohol—exhibits a lower surface tension than water does. "Things like the duck feather, for example, are able to repel water very well because of their surface texture," says Anish Tuteja, a postdoctoral associate in chemical engineering at M.I.T. and the lead author of the study. "But as soon as you put things like oil on them, [the liquids] immediately wet the surface."
Tuteja and his collaborators worked on a few paths to omniphobes. They developed two families of omniphobic surfaces and a dip-coating process that works on reentrant surfaces, or surfaces whose curvature forms pockets underneath.
The first surface, and likely the most practicable, is a mat of randomly deposited fibers whose reentrant texture and chemical composition, including a new class of molecule known as fluoroPOSS that helps repel oils, prevent wetting by forming a robust interface between the liquid, the solid and the air. This interface means that virtually any liquid meets the surface at a high contact angle, forming a spherelike bead rather than spreading over the surface. The fibers could be used as an omniphobic coating, as the researchers demonstrated by covering a lotus leaf to repel low-surface-tension liquids such as wood alcohol and octane.
The second type of omniphobic structure described in the paper is known as a microhoodoo, a replica of geologic rock formations that resemble mushrooms—slender at the stem with broader caps. When placed on a silicon wafer and treated with a compound known as a silane, these microhoodoos also formed a robust barrier against high- and low-surface-tension liquids. (Tuteja says that these structures "were critical in helping us understand the basic physics of the problem and in deriving the design parameters," but that they fall short of commercial viability given current manufacturing processes.)
The dip-coating process is the most straightforward path to omniphobicity and might find use in ultrastainproof fabrics in the near term, say the researchers. After they dip-coated a duck feather—no longer attached to a duck—in a fluoroPOSS solution, the feather was able to repel rapeseed oil, which has less than half the surface tension of water. In other words, the dip-coating process granted the feather greater resistance to oil, or oleophobicity.
"The simplest [application] might as well be textiles," Tuteja says, "so that if you spill wine, for example, it would just roll off and would not stain the shirt or trousers." A truly omniphobic fabric would even repel nastier liquids, such as gasoline, better than current treatments can. Later generations of surfaces, on the other hand, could someday be used to create smudge-resistant computer screens or windows. "If you wanted fingerprint-resistant screens, the reason that the fingerprints are up there is because of all these natural oils," Tuteja says, adding that some of the potential applications have appeared out of left field. The team has heard from hearing-aid manufacturers, for instance, that want oleophobic materials to deter earwax buildup, he says.
"The M.I.T. group's route to textured surfaces," says chemist Edward Samulski of the University of North Carolina at Chapel Hill, who did not contribute to the study, circumvents the necessity of reproducing "natural structures like that found on the superhydrophobic lotus leaf." As an example of such research, he points to research he and his colleagues published in 2006 on reverse engineering the lotus leaf's water-repellent surface.
Omniphobic technologies, even if they never reach the consumer level, could have other useful applications. "Our funding comes from the Air Force, and they were very interested in making membranes and gaskets that would repel oils or jet fuels, basically," Tuteja says. "And before all of these, there were not many materials that would actually repel such low-surface-tension liquids."