Polymeric microfibres may hold the key to the first simple process for manufacturing super oil-repelling (oleophobic) materials, say scientists at the MIT, USA. The material, which could be applied as a flexible surface coating, may have uses in aerospace, space travel and in hazardous waste cleanup.
‘We have provided design equations that take into account the surface tension of the liquid and the surface chemistry of the solid,’ explains Robert Cohen, Professor of Chemical Engineering at MIT. ‘We have also introduced a third consideration, namely the need for roughness on the surface that includes re-entrant curvature [affecting the local contact angle and hysteresis of surfaces].’
Re-entrant curvature has been achieved using non-woven microfibres, made from a blend of specially synthesised fluorodectyl polyhedral oligomeric silesquioxane (fluoroPOSS) molecules. These fibres, say researchers, can be readily deposited, using electrospinning, onto a range of surfaces, including metal, glass, plastic and even biological surfaces such as plant leaves.
The resulting fibre mat, with multiple scales of roughness and high porosity, provides a cushion for oil droplets. They rest on the pockets of air trapped between the fibres, preventing them from touching the bottom of the surface and wetting it.
‘Nature has developed methods for waterproofing, but not oil-proofing,’ says Gareth McKinley, Professor at the MIT School of Engineering. Analysing superhydrophobic surfaces such as the lotus leaf has led to the development of similar synthetic surfaces.
While water has high surface tension and tends to form droplets, oil and other hydrocarbons have extremely low surface tension and spread out over surfaces. This explains why water rolls off the hydrophobic feathers of a duck, but a duck coated in oil must be washed in soap.
‘The conventional wisdom was that oil-proofing could not be done on a large scale without special lithographic processes,’ says McKinley.
The researchers at MIT believe their technique is scaleable and could be used to design fibre mats optimised to repel different hydrocarbons.
Cohen adds, ‘Our ability to tune precisely the wetting characteristics leads us to potential applications in membranes.’ Low POSS concentrations result in fabrics that are superhydrophobic but superoleophilic, enabling water and oil to be separated. The team has already created a membrane that divides water and jet fuel for potential use in hazardous waste cleanup.
‘We hope to continue along these lines for the more challenging hydrocarbon mixtures,’ says Cohen. ‘We will also be pursuing switchability so we can turn oleophobicity on and off on demand.’
Another technique under development for achieving re-entrant curvature is lithographic etching of reverse-curvature objects, enabling precise control over the spacing and shaping of the required surface features.
For now, the US Air Force, which funded the R&D, aims to use the fibre mat to protect components of its aeroplanes and rockets from jet fuel.