Smudges on the touch screens of smart phones are annoying. Even high-tech solar panels suffer dirt and grime, lowering their energy gathering capabilities. But a discovery inspired by Christmas candles may spur the development of improved ‘self-cleaning’ glass surfaces for goggles, touch screens, windows and other products.
Unusually, the new coating repels both oil and water. Super water repellency is not so difficult to achieve, but oil loves soot and readily sinks into the soot layer, wetting the coating.
‘Our oil repellent surfaces are more mechanically robust and they are optically transparent. Therefore, they can be used to coat difficult to access windows or solar cells,’ says lead author Doris Vollmer of the Max Planck Institute for Polymer Research in Mainz, Germany.
To make it, the researchers held the flame of a paraffin candle below a glass slide. The candle soot, mostly carbon particles of around 30nm, deposited on the slide and they then coated this with 25nm of silica using chemical vapour deposition (CVD) to stabilise the surface. Finally, they heated the black coating to 600°C, whereupon it became transparent, and doped it with molecules containing fluorine to improve repellency (Science, doi: 10.1126/science.1207115).
A rough surface with fluorosilane as a top layer is sufficient to repel oil, but only if the roughness of the coating is more or less spherical will it repel both oils and water, Vollmer explained. The coating can be applied to a variety of heat- resistant surfaces, including aluminium, copper and stainless steel.
‘If we can improve mechanical stability even more, there are certainly interesting products possible,’ she says, starting with expensive niche products such as the coatings for eyeglasses, solar cells and diving goggles.
Martin Brinkmann, of the Max-Planck Institute for Dynamics and Self-Organization, describes the behaviour as surprising as the coating should maximise the contact to oily liquids and soak them up spontaneously. ‘This peculiar behaviour is probably linked to the particular type of local topography of the aggregated soot particles, which must display large ‘overhangs’ to pin the interface of the oily liquid phase,’ he explains, adding that ‘this new type of coating appears to be much more stable than any of the [oil and water-repellent] coatings considered before.’