Researchers at MIT have developed what they are calling an oleophobic surface, which resists the absorption and spreading of oils and other hydrocarbons. Using two separate processes, electrospinning and lithography, researchers were able to demonstrate the oil-resistant surface as a method more than a material and released their findings in a science paper.
“Sometimes this has been reported as oil-resistant material, but that’s not quite right — it’s an oil-resistant surface, and the geometry and the spacing and all of these things in our design scheme are critical to that,” says Professor Robert Cohen, St. Laurent Professor of Chemical Engineering at MIT.
Oils and other hydrocarbons have a very low surface tension unlike water, which has a very high surface tension. This is why water will more naturally bead where oil will spread. In the development of hydrophobic (water-resistant) materials, the method of production has focused on very low-energy materials like Teflon, but not as much on the methods of producing a surface. Oleophobic materials still rely on low-energy materials, but Cohen and his team of researchers discovered the key feature in producing an oleophobic surface is producing a rough surface, particularly a rough surface with reentrant curvature.
The reentrant curvature of a surface is the area that curves under a fiber as it sits atop a mesh of other fibers. The method of electrospinning creates a non-woven mat of fibers constructed of fluoroPOSS, a low-energy material originally developed by the United States Air Force. By layering a mat of fibers, the surface has a greater reentrant surface curvature, which helps to support the low surface tension of the oil.
The second method is the process of lithography, which cuts into a surface and undercuts the top of the surface, creating microscopic grooves and overhangs called micro-Hoodoos, a reference to the geological formations in the western U.S. These grooves provide a more tunable reentrant curvature than the random process of electrospinning.
“The second method is useful for further research because we have precise control over size, spacing and shape of the features that provide the needed reentrant roughness,” says Cohen. “The first method might be scalable for practical utility,” he says.
These surfaces have potential for aerospace and waste cleanup applications, but Cohen isn't willing to speculate on specific applications. “We show in our paper that these membranes can be tuned to separate very different materials, like oil and water,” he says.
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 Top Left: A colored drop of water beads on a lotus leaf. Inset shows an SEM micrograph of the lotus leaf surface (the scale bar is 5 µm).
Top Right: The wetted surface of the lotus leaf after contact with a droplet of hexadecane spreads and soaks the surface of the lotus leaf.
Bottom Left: Colored drops of water sit on the surface treated with the electrospun fibers of fluorodecyl POSS.
Bottom Right: Colored drops of hexadecane on the surface also treated with the electrospun fibers of fluorodecyl POSS |
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