Monday, December 10, 2007

No More Thumbprints

New chemistry and microsurfaces have led to super oil-repellent materials that are self-cleaning.

By Kevin Bullis

Good-bye, soap: Oil droplets bead up on a new, microstructured, oil-repellant, self-cleaning material. Credit: Anish Tuteja and Wonjae Choi, MIT

Multimedia •  See the material filter water from fuel. •  Watch how the new material repels oil.

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).

The researchers made the structure in two ways. For the first, they used a process called electrospinning to form microscopic threads of the fluorinated material. The threads formed a fibrous mesh that traps air. In part to study the mechanisms involved, the researchers also etched silicon wafers to form arrays of mushroom-shaped pillars in which air is trapped under the cap of the mushroom shape.

Structuring the surface of just about any type of material in this way will make it more repellant to oil. But if too much force is applied, the air can get pushed out, McKinley says. Adding the fluorine groups cause the material to resist this force much more, making it more practical.

Having created the materials, the researchers then studied the details of what makes them work. As a result, the researchers were able to outline ways to make super oil-repellant structures, describing what sort of chemical properties, combined with what kind of microscopic shapes, are necessary to create them. They described how the liquid interacts with the surface of a material, and how this changes according to the structure of the surface and the presence of air.

Using these rules, the researchers were able to tune the properties of the material. In one example, they were able to make the materials repel water but not oil--something that could be useful for filtering water out of fuel (see this video). By following these rules, researchers may be able to develop even better materials that are more oil repellant, cheaper to make, and even transparent. The last feature could lead to a number of applications, including self-cleaning displays--something that cell-phone manufacturers have been working on for years.