No More Thumbprints

Dec. 10, 2007 — -- Researchers have made materials that repel oil and are able to clean themselves without the help of soap and water. What's more, the researchers describe exactly how the materials work, which could help others design similar materials. This could lead to a range of applications, including fingerprint-shedding cell-phone displays. The researchers, from MIT and the Air Force Research Laboratory at Edwards Air Force Base, in CA, describe their results in the current issue of Science.

Making super oil-repellant materials has been one of the great challenges in materials science, says Jeffery Youngblood, a professor of materials engineering at Purdue University, who was not involved in the research. Researchers have been able to make super water-repellant materials that cause water to bead up and form near-spherical droplets that easily roll or even bounce off surfaces. But oil has much lower surface tension than water does, so it has a greater tendency to spread over and cling to surfaces. That makes super oil-repellant materials, also called superoleophobic materials, difficult to create.

"Before now, superoleophobic materials have been a pipe dream," Youngblood says. "As far as I know, no one has seen this type of oleophobicity before." In the past, it's been possible to make oil bead up on a surface, but the oil remained stuck, unable to slide off, Youngblood says.

The MIT and Air Force Research Laboratory researchers overcame the obstacles to super oil-repellant materials by combining two advances. First, the Air Force researchers developed a material that's something like a super Teflon. Fluorine chemical groups in Teflon help make it repellant, says Gareth McKinley, a mechanical-engineering professor at MIT who is involved with the work. The Air Force researchers developed a molecule with a structure that incorporates much more fluorine. Adding this molecule to a material makes it more repellant to liquids.

But the chemistry of the material isn't enough to make it super oil repellant. The researchers at MIT also changed the microscopic structure of the material in a way that traps air near the surface, so that oil on the material is suspended partly on air, which prevents the oil droplets from sticking to the surface, Youngblood says. The resulting material was so oil repellant that oil, which normally clings to surfaces, actually bounced off instead (see this video).