Chemical Technology March 2015

NANOTECHNOLOGY

are industrial. Think aircraft wings that don’t ice-up (the cause of numerous disasters), turbine bladeswith improved longevity and resistance to abrasion, ship hulls that don’t foul and so improve fuel economy and speed, hospital medical devices which are intrinsically sterile and prevent bacterial growth, solar panels which are uniformly black at any angle and low maintenance. Cornell University and Rensselaer Polytechnic Institute are using the electrochemical process of anodisation to create nanoscale pores. These, they believe, change the electrical charge and surface energy of metal surfaces, which then repels bacterial cells and prevents thin-film formation. In "Alumina surfaces with nanoscale topography reduce at- tachment andbiofilmformationby Escherichia coli and Listeria spp ", Guoping Feng, et al , present their research. Importantly, the approach is low-cost and results in a ‘generally recognised as safe’ material. The pore sizes they achieve are in the range of 15 to 25 nm. “It’s probably one of the lowest-cost possibilities to manu- facture a nanostructure on a metallic surface,” said Carmen Moraru, associate professor of food science and one of the paper’s senior authors. “The food industry makes products with low profit margins. Unless a technology is affordable it doesn’t stand the chance of being practically applied.” The benefit here is also that metal surfaces (rather than coatings) aremore robust and so using them inmarine biofoul- ing environments is possible. Some of these applications are already available. Anodic

They use chemical vapour deposition. The phones are placed in a vacuum chamber under low pressure, and a plasma pulse is used to ‘activate’ the surfaces to be coated (ie, allow them to become charged). The fluoropolymer gas is introduced where it forms covalent bonds with the surface. A pulsed radio frequency plasma polymerises the coating to form the layer. Obviously, such a surface will not stand abrasion, but the parts of a phone you want to waterproof are on the inside. These surfaces are not ‘permanent’ and P2i imagines that their coating system will be incorporated in devices that they hope will become as ubiquitous as a microwave oven. P2i have also used their technique to coat air filter media for the oil industry where Teflon (PTFE, the original fluoropolymer coating) is normally used. Simply put, though, this approach is too expensive. On high- margin goods like phones, fine, but howabout if all youwant to do is get the last drops of premier tomato sauce out the bottle? Professor Kripa Varanasi and his team at MIT have de- veloped a liquid-impregnated surface which can be coated onto the inside of pipes and bottles. Their coating creates a permanent liquid layer against a porous solid coating over the original surface. They have a YouTube video of tomato sauce pouring out of a bottle that has a voodoo-like look. The problem with these sorts of additive coatings is that they have a very short life-span and don’t handle abrasion or temperature variation without degrading. The applications where superhydrophobic surfaces would be of greatest value

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Chemical Technology • March 2015