Low-cost surfaces make sprays cleaner

The properties of water-repellent or hydrophobic surfaces have long been of interest to manufacturers in a variety of fields. Now, however, a new process means they have the ability to produce engineered sprays when a jet of liquid is aimed at them at relatively low pressures. The increasing pressure on the use of volatile organic compounds (VOCs) makes this development of significant potential interest to those who make products in sprays cans. They are also of interest to manufacturers of medical products who want people to inhale and retain droplets; makers of agricultural sprayers, who want users not to retain chemical droplets in their lungs; and even manufacturers of automotive fuel injection systems. 42 Technology in St. Ives, Cambridgeshire, has been working with a new superhydrophobic (water-repelling) surface developed in the research group run by Professor Ullrich Steiner in the University of Cambridge's Cavendish Laboratory. The method, however, works with any superhydrophobic surface, including those already in commercial production. Superhydrophobic surfaces are all are based on what is termed the Lotus effect, where the naturally water-repellent nature of the leaf surface is augmented by hydrophobic hairs, whose tips enhance the effect and ensure that less than 1% of droplet surfaces are in contact, meaning that they roll off with entrained dirt. Superhydrophobic materials currently available commercially include the 'Hirec' paint developed by NTT Advanced Technology for its microwave dishes and the plasma-deposited coatings developed by P2i (see box), which also work with oil (oleophobic). The key advantage of the latest Cavendish-developed coating, however, is that it is potentially much cheaper than some of the others. The surface of interest is first coated with primer and polytetrafluorethylene (PTFE). It is then sprayed with a liquid that lays down a mixture of PTFE and polystyrene (PS) spheres. Firing this boils off the PS, leaving a skeleton of protruding structures made of PTFE. A jet of water-based solution directed onto such a surface creates an unstable disc that immediately breaks up into droplets. This was demonstrated with nothing more sophisticated than a piece of Professor Steiner's material and a small syringe to create the jet. "The challenge for many potential applications, including possible self-cleaning car windscreens," according to 42 Technology managing director Howard Biddle, "is to make superhydrophobic materials sufficiently robust and durable." However, this is not a problem with aerosol spray cans, since the tiny piece of material on which the jet impacts is relatively protected from the outside environment, and the cans are one use only. Most research until now has been undertaken in order to improve inkjet printing, agricultural spray adherence to leaves and painting and has been aimed at reducing the use of aerosol sprays. "The key," according to 42 Technology consultant Chris Walters, "is the creation of instability." The dimensionless number that turns out to govern this (known as the Weber number) is calculated from: density x characteristic velocity2 x characteristic length/surface tension. For a good aerosol spray, the Weber number should be 200 or more. For a small device, this means that a pressure of 3 bar will theoretically produce an aerosol spray with droplets about 5µm across, whereas at a pressure of 0.7bar, they will be around 20µm. For a medical inhaler, the ideal particle size is 5 to 10µm, whereas for applications where inhalation is undesirable, such as a hairspray, droplet size should be 15 to 30µm. A conventional spray can uses HFC (hydrofluorocarbon) propellant of less than 3 bar pressure, which boils at the nozzle and can comprise 60% of the spray can contents. Despite the insistence of the makers of HFCs that they are totally non-toxic and friendly to the environment, suggestions that they have major global warming potential has led to a move to using nitrogen or air in the head space, compressed to 10bar. The only problem, apart from the need to contain a higher pressure, is that as the gas is used, the pressure falls off and the spray size becomes coarser. Other ideas include using compressed CO2 as a propellant, while butane, already used in many aerosols, poses a significant fire and explosion hazard. Using the new technology, on the other hand, requires nothing more high-tech than a small spring and piston or a hand action squeeze pump and leaves no residue in the sprayed aerosol, a particular benefit to users of perfumes and cosmetics. Biddle suggests that banning other propellants 'awaits suitable technology', which he believes his company now has. Other potential applications include agricultural sprays and paint. And, although Professor Steiner's material is hydrophobic and not oleophobic, 42 Technology director Dave Wilson suggests it is also possible to make superoleophobic surfaces by an etching process that produces a pattern of micro 'mushrooms'.