MIT engineers have designed a class of material structures that can repel oils, a novel discovery that could have applications in aviation, space travel and hazardous waste cleanup. Such materials could be used to help protect parts of airplanes or rockets that are vulnerable to damage from being soaked in fuel, like rubber gaskets and o-rings.
Creating a strongly oil-repelling, or ‘oleophobic’ material, has been challenging for scientists, and there are no natural examples of such a material.
‘Nature has developed a lot of methods for waterproofing, but not so much oil-proofing,’ said Gareth McKinley, MIT School of Engineering Professor of Teaching Innovation in the Department of Mechanical Engineering and a member of the research team. ‘The conventional wisdom was that it couldn’t 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 (however, over a period of time, oil and grease contaminate the surface and the repellency fades). 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, which resembles a thin fabric or tissue paper, 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 microfibres 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 onto many types of surfaces, including metal, glass, plastic and even biological surfaces such as plant leaves, using a process known as electrospinning.
A group of MIT researchers, gathered here around naturally water-repelling lotus leaves, has developed a class of material structures that repel oil and hydrocarbons. From left are Gareth McKinley, Professor of Teaching Innovation in the Department of Mechanical Engineering; Robert Cohen, St. Laurent Professor of Chemical Engineering; Anish Tuteja, postdoc in chemical engineering; and Wonjae Choi, mechanical engineering graduate student
The researchers have also developed some 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 re-entrant 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.
Using these relationships, the researchers can design fibre mats that are optimised to repel different hydrocarbons. They have already created a nonwoven fabric that can separate water and octane (jet fuel), which they believe could be useful for hazardous waste cleanup.
The US Air Force, which funded the research and developed the fluoroPOSS molecules, is interested in using the new materials to protect components of aeroplanes and rockets from jet fuel.