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MIT finds oil-proofing first for aviation: scientists follow water droplet principle with microfibres that protect vulnerable parts from fuel spills and damage.

ENGINEERS at the Massachusetts Institute of Technology have designed a class of material structures that can repel oils.

The discovery has applications in aviation, space travel and hazardous waste cleanup. The repellant materials could be used to help protect parts of aircraft or rockets that are vulnerable to damage from being soaked in fuel, such as rubber gaskets and o-rings.

"These are vulnerable points in many aerospace applications" said Robert Cohen, the St Laurent professor of chemical engineering. "It would be nice if you could spill gasoline on a fabric or other surface and find that instead of spreading, it just rolled off."

Creating a strongly oil-repelling, or oleophobic, material has been a challenge for engineers, and there are no natural examples to work from.

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Conventional wisdom suggested it could not be done on a large scale without very special lithographic processes.

The tendency of oils and other hydrocarbons to spread out over surfaces is due to their very low surface tension (a measure of the attraction between molecules of the same substance). Water, on the other hand, has a very high surface tension and tends to form droplets. For example, beads of water appear on a freshly waxed car. That difference in surface tension also explains why water will roll off the feathers of a duck, but a duck coated in oil must be washed with soap to remove it.

The MIT team overcame the surface-tension problem by designing a type of material composed of specially prepared microfibres that essentially cushion droplets of liquid, allowing them to sit, intact, just above the material's surface.

When oil droplets land on the material, they rest atop the fibres and pockets of air trapped between the fibres. The large contact angle between the droplet and the fibres prevents the liquid from touching the bottom of the surface and wetting it.

The microflbres are a blend of a specially synthesised molecule called fluoroPOSS, which has an extremely low surface energy, and a common polymer. They can be readily deposited on to many types of surfaces, including metal, glass, plastic and even biological surfaces such as plant leaves, using a process known as electrospinning.

The researchers have also developed some dimensionless design parameters that can predict how stable the oleophobicity, or oil-resistance, between a particular liquid and a surface will be. These design equations are based on structural considerations, particularly the reentrant nature (or concavity) of the surface roughness, and on three other factors: the liquid's surface tension, the spacing of the fibres, and the contact angle between the liquid and a flat surface.

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Title Annotation:TECHNOLOGY; Massachusetts Institute of Technology
Publication:Professional Engineering Magazine
Geographic Code:4EUUK
Date:Jan 16, 2008
Words:432
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