Nature shapes superior surfaces

Coatings were originally developed for high end industrial applications, but soon expanded into other markets – in particular, consumer products. However, many are being re-evaluated for use in industrial products as increase usage reduces costs. A large number are based on sol-gel processes, such as the non stick 'Ceramica' ceramic coating from Weilburger Coatings, which is half the thickness, is harder, more durable and has better heat resistance compared to PTFE. It can also be laid down in various colours and metallic finishes and allows higher heat transfer than a straight polymer. Additionally, only water and alcohol are used as solvents. We recently encountered it applied to metal, but sol-gel coatings have been developed for applications such as glass car windscreens and plastic lenses. The basic process – originally invented by Carl Zeiss in the 1930s – is to produce a sol (colloidal suspension) through the hydrolysis and polymerisation reactions of inorganic metal salts or metal alkoxides in water. These form a gel when applied to a substrate by spraying, dipping or spinning. This can then be converted by low temperature stoving to a dense ceramic. Korean company Hespia offers a similar process, saying its coating offers superior weather resistance and excellent brine resistance. This makes the coating ideal for offshore structures, marine radar installations and wind turbines. The coating can be applied to many different substrates, including stainless steel, cast iron, aluminium, plastic, glass or stone. Sol-gel is also one of the methods for making coatings that are superhydrophobic, which means that they cannot be wetted. This means the coatings repel water and entrain dirt, and also allow water based fluids to pass over them with much less resistance than conventional wetted surfaces. The idea comes from nature, where many plants are covered with microscopic hairs, which cause water to ball up into drops and roll off. The mechanism was explained to at the Royal Society summer show by Dr Jörn Dunkel from the Rudolf Peierls Centre for Theoretical Physics at the University of Oxford. Researchers at the University of Oxford and Nottingham Trent University have got together to discover more about the technology and possible novel industrial applications, in addition to those where it is already being applied under the umbrella name Nature's Raincoats. Dr Dunkel explained that the way hairs support a water droplet is similar to the way by which an Indian Fakir can lie on a bed of nails without penetrating his skin, and demonstrated it by pushing a balloon down onto a bed of spikes without it bursting. The highest contact angle exhibited by a drop of water on a hydrophobic surface is water on PTFE, which is 115°. If the material is in the form of an array of protuberances and the diameter of these tips is much less than the diameter of the water drop, even a low contact angle means that contact between the water droplet and the end of the hair has an infinitesimally small area. A large density of such protuberances ensures that the droplet will be supported and roll over them. The effect has already been exploited by companies such as Zeiss to make 'LotuTec' coatings that keep the lenses of its binoculars and rifle scopes free of water, dirt and grease, giving considerable commercial advantage over its competitors. In addition, researchers at the Royal Society event were handing out business cards coated in white paint on the reverse sides. When held under a tap, the back of the card did not get wet. Professor Glen McHale, associate dean – research and graduate studies – at Nottingham Trent University explained that the paint was called Hirec and was developed by NTT Advanced Technology for coating microwave antennae. The paint uses small protruding particles to form the protuberances. Apparently, Hirec not only resists the build up of water films, but also snow. NTT notes that snow will not fall off Hirec treated surfaces unless they are steeply inclined but, ice and snow will not stick to it. Cleaning therefore takes very little time. The company also notes Hirec treated circuit boards submerged in water still receive signals. According to NTT, a cell phone board treated with Hirec will survive being dropped in water. If the paint is applied to a straining mesh, it will allow oil to pass through but not water, and can thus be used to separate the two. Hirec paints can be applied to metal, plastic or glass surfaces by brush, roller or spray. If they are to be used outdoors, they require the use of an undercoat primer, but if to be used indoors, they don't. The paint on the business cards was Hirec 1440, which has now been succeeded by Hirec 100/1100 for outdoor use and Hirec 450 for indoor use. Cytonix from the US produces a similar product, but colleague Dr Neil Shirtcliffe explained: "It is not very stable to rubbing. It is a fluoropolymer with nano-sized Teflon particles in it. There are some non-Teflon based alternatives on the market, such as Sto house paint, which seems to be a standard acrylic exterior paint with particles that are probably hydrophobised silica plus solvent to cause deposition of a porous matrix of particles. Degussa produces an LE range of silica, which is hydrophobised nanoparticles for producing own formulations – it works well dusted onto sticky paint in my experience." Among British products, micro hairs on a Speedo Fastskin swimsuit has undoubtedly been contributing to victories in sport, along with the company's CFD design capability. However, the move now is to improve superhydrophobicity even further, and apply it to new surfaces in new ways, especially on the micro and nano scale where it is likely to make a considerable difference to 'Lab on a chip' devices for medical analyses and fine pharmaceutical preparation. Dr Dunkel said one can imitate the effects of plant hairs by laying down arrays of silicon posts, 20µm across, and Professor McHale observes in one of his published papers that it is possible to achieve similar effects by electrodepositing metal mounds 40µm across, or by etching metal. His researchers have, he says, been investigating structures on even finer scales: sputter coated 200nm copper, 4µm and 800nm sol-gel particles, and 10nm copper oxide nano needles. Coating has a hot future Dealing with matters somewhat hotter, Zircotec – known for its plasma spray coated zirconium oxide thermal coatings used widely in motorsport – has developed a flexible version of its materials on a roll. The material called Zircoflex weighs 460g/m2 and is supplied 0.25mm thick. The ceramic material is sprayed in the form of thousands of platelets about 1.5mm square on the surface of aluminium foil. There are approximately 1mm wide gaps between them to allow the foil to be bent and manipulated. Sales director Peter Whyman said: "The aluminium is a fantastic reflector of heat and the material insulates partly because of the air gaps and partly because of the ceramic." Whyman adds: "Zircotec has the potential to transform the use of ceramic heat protection. For the first time, these coatings can be installed in line to parts and assemblies during manufacture, even in high volume applications, without disrupting production flow. The potential in maintenance and aftermarket applications is huge too. Users can now upgrade parts with Zircotec ceramic protection without needing to dismantle or decommission their equipment."