Nano-nails can repel almost any liquid
A material which repels almost any liquid can have its non-stick properties turned on or off at the flick of a switch, according to its inventors. They say that the material could be used in self-cleaning surfaces, or to control the flow of fluid through miniature ’lab-on-a-chip’ devices.
The new surface, developed by scientists at Bell Laboratories, New Jersey, US, is actually made from rows of tiny, closely-spaced ’nano-nails’. The nano-nails are made from silicon and coated in a fluoropolymer, with each nail head around 400 nanometres in diameter.
But it’s actually the space underneath the nail heads that gives the surface its special properties. Even liquids with a low surface tension, like oil, have to be under tremendous pressure to slip between the heads, because the air trapped beneath the nails keeps droplets of liquid in place.
When the researchers applied a voltage across the material, it lost its ability to repel solvents (known as lyophobicity). Although the exact mechanism is not known, the scientists suggest that the electrostatic charge in the nails can pull a small amount of liquid to wet the underside of each nail head, which then lets more liquid flow through. The research, which was published online in October 2007, appears in the latest edition of Langmuir.
The geometry of the surface is similar to mushroom-shaped ’hoodoos’ described by Robert Cohen and Gareth McKinley at the Massachusetts Institute of Technology, US. In December 2007, they reported that they could tune the shape of the hoodoos to produce tailor-made wetting properties.
The inspiration for the nano-nail surface came from the art world, according to Tom Krupenkin, a member of the research team. ’The actual "Eureka!" moment came as I was looking at "White Field", created by Gunther Uecker from regular nails,’ he told Chemistry World. ’I realised then that nano-nails were what I needed for the superlyophobic system.’
’The authors have for the first time elaborately examined the liquid-solid contact line on the overhang structures,’ said Di Gao of the University of Pittsburgh, US. ’Such technology may find applications in manipulating liquids in small volumes.’
A. Ahuja et al, Langmuir, 2008, 24, 9