A new set of design criteria could enable engineers to invent and manufacture surfaces that can repel almost all liquids, even oily fluids long noted for their ability to foul water-repellent surfaces.
Many surfaces — including, say, a duck’s back — repel water. But no known natural surfaces repel oil, says Gareth McKinley, a chemical engineer at MIT. Indeed, dip a duck in oil-tainted waters and its feathers lose their water-repellency, a phenomenon all too often seen in the wake of marine oil spills.
Even though nature hasn’t come up with an oil-repellent material, scientists can combine nature’s design techniques for water-repellency with modern materials to engineer a variety of surfaces that repel both oil and water, McKinley and his colleagues report in an upcoming Proceedings of the National Academy of Sciences.
One of the biggest differences between water and an oily liquid such as octane, a component of petroleum, is surface tension. A droplet of a fluid with high surface tension — such as water — tends to pull itself into a sphere, whereas one with low surface tension, such as oil, tends to spread across a surface more readily, says McKinley.
But whether a drop of liquid beads up on a surface also depends on the nature of that surface. When a material’s surface energy is low, the material tends to better repel fluids and cause them to form droplets, says McKinley. What’s even more important, he and his colleagues have found, is the texture of a surface. For example, the waxy material on the surface of a lotus leaf — one of nature’s most water-repellent surfaces — actually has a slight attraction for water. In large measure, McKinley notes, the lotus leaf’s water-shedding ability comes not from the waxy material but from the microscopic bumps and ridges on the leaf’s surface.
Taking these observations, the team designed and manufactured surfaces that effectively repelled droplets of oily liquids such as methanol, octane and hexadecane. While some of the surfaces were fabrics of highly repellent microfibers, others were forests of microstructures that supported droplets of oil the same way that a bed of nails supports a circus performer, the team reported in 2007 in Science.
After further study, the team has come up with general rules for designing what they call “omniphobic” surfaces that can repel both water and oily liquids. Besides the material characteristics of a surface and the liquid to which it will be subjected, engineers should consider the size, shape and spacing of the surface’s microscopic features, McKinley notes. The set of design criteria that the team has compiled enables engineers to analyze existing surfaces or to conceive new ones — a process that’s much more efficient than trial and error, says Robert Cohen, a chemical engineer at MIT and a coauthor of the new report.
Although hydrophobic surfaces readily shed water, if they become contaminated with oily substances they lose their water repellency, says Marshall Ming, a materials scientist at the University of New Hampshire in Durham. “Successful development of omniphobic surfaces is a very exciting achievement” because such materials have a variety of practical applications, such as self-cleaning paints or coatings for windows, he adds.
The new findings are “an important step towards creating robust omniphobic surfaces,” says Michael Nosonovsky, a materials scientist at Stevens Institute of Technology in Hoboken, N.J. Omniphobic surfaces repel contaminants in an environmentally friendly way, he notes, so they could be used, for instance, as a coating for self-cleaning solar panels.
