The Gecko’s Foot: How Scientists are Taking a Leaf from Nature's Book. Peter Forbes

leaves from dirt and muck, completely clean.

      The Lotus-Effect officially entered the canon of Western inventions in July 1994 when Barthlott applied for a patent. Then, in 1997, came the classic summing up of the Lotus-Effect itself: ‘Purity of the sacred lotus, or escape from contamination in biological surfaces.’ This paper disclosed the Lotus-Effect in full: the biology, the physics, the implications for plant ecology and the technical possibilities. Even at this point there was resistance from some physicists to the idea of the Lotus-Effect. According to Barthlott, several journals rejected the article on the grounds that ‘the so-called Lotus-Effect exists only in the imagination of the authors’. His paper concluded: ‘We assume that this effect can be transferred to artificial surfaces (eg, cars, facades, foils) and thus find innumerable technical applications.’

      Of course, this remark was slightly tongue-in-cheek because by now work on commercial applications was advanced; the requirements and timetables of the patent system and product development are very different to the protocols of academic publication, and anyone wishing to work in both areas simultaneously has to tread a fine line between disclosure and protecting intellectual property.

      Working with Barthlott, Ispo, a paints-and-surface-coatings company, was developing a product for the exteriors of houses which, unlike existing coatings, would stay fresh and clean during its lifetime (fig. 2.6). Barthlott’s patent was granted in Europe in 1998 and Ispo’s paint for the exterior of buildings, Lotusan™, was launched in 1999.^* It had taken 25 years from Barthlott’s initial discovery to commercial exploitation. When applied, Lotusan looks like any other exterior paint. The roughness of the surface is on a scale invisible to the eye and the water-repellent silicone leaves no visible trace.

      The manufacturers produce a neat demo box to demonstrate Lotusan. Half of the plates in the box are coated with Lotusan and half with a standard exterior finish of the same appearance. A bottle of distilled water and a vial of standardized fine grey ash complete the kit. The difference in properties, if not appearance, between the two surfaces is dramatic and instantly demonstrates the effect of highly non-wettable surfaces. Drops of distilled water on the Lotusan and non-Lotusan surfaces take on entirely different appearances. It isn’t only that the former is almost spherical, with its 160° contact angle, while the other is flattened; visually, they are very different: the globule on the Lotusan surface gleams like a gem.

      I opened the demo box in the company of Noah, my partner’s eight-year-old grandson. When I put a drop of water on the Lotusan plate, Noah said, ‘It looks like it’s got sparkling water inside it’ – an echo of the Japanese poet Komai’s reaction in the epigraph to this chapter: ‘Transforming the dew/On your life-giving leaves/Into sparkling gems!’

      The other globule was dull inside because the contact angle is reversed. Multiple drops fuse instantly on the Lotusan surface; on the non-Lotusan surface, two touching globules refuse to join perfectly, a projecting pouch remaining. If you tip up the two plates, the Lotusan globule rolls off almost instantly; the other needs a slope of more than 45° to roll. The trail after the Lotusan drop is dry; a snail trail remains on the other one.

      So, the water repellency is easy to demonstrate but it is the self-cleaning effect that is the commercial raison d’être of Lotusan. When powdered ash is scattered on both plates, a water globule cuts a swathe through the dirt on the Lotusan surface, carrying it off completely, leaving neither dirt nor water behind. On the non-Lotusan plates, the water merely smears the dirt down the plate, leaving a muddy trail.

      The Lotus-Effect throws normal ideas about cleaning into disarray. You should not use detergents on Lotusan surfaces; although they do not destroy the effect, they do weaken it. The more that self-cleaning surfaces become the norm, the less cleaning agents will be used, with obvious ecological advantages. (Some of Barthlott’s research documents the disastrous effect detergents can have on plant leaves, weakening their self-cleaning surfaces and laying them open to attack from moulds.)

      The Lotus-Effect is the most highly developed bio-inspired technique of recent years (the all-time front-runner is the Velcro^® hook-and-loop fastener – see Chapter 4 – but that had a 50-year head start). Lotusan is the only contemporary bio-inspired product to

      have made serious profits and to have achieved the distinction of being mentioned in glowing terms in company annual reports. The most difficult hurdle for bio-inspired products is not the technical development, protracted though that can be, but the crunch of coming to market and surviving the harsh reality of commercial conditions.

      From its launch in 1999, Lotusan, which comes with a five-year no-cleaning guarantee, has been very successful. A measure of its success is that it is mentioned in travel guides: for example, the Nikolai-Viertel in Berlin received this write-up on www.nationmaster.com:

      The small area is famous for its traditional German restaurants and bars. Between 1997 and 1999 all houses were reconditioned (Lotusan with Lotus-Effect) giving this area an unmistakable touch.

      Lotusan was launched at an unpropitious time for the German economy. Ispo was soon acquired by Sto, a world company with roots in Germany and America. In such a climate, even the bio-inspired paint endorsed by the purity of the sacred lotus must get its hands dirty in the commercial world. Barthlott says: ‘I got the message more or less overnight that Ispo had been taken over by one of the competitors, Sto. I immediately phoned up one of our patent attorneys. He said there are two possibilities: either they want to keep it in a drawer, or they’re interested in it.’ They were interested in it.

      The initial enthusiasm of German companies for the process has now spread beyond the country’s borders – the American firm Ferro is making Lotus-Effect coatings for glass and working on coatings for metals. In Germany itself, the ‘global players along the Rhine’ are no longer aloof. In 2000, Barthlott took out a second patent for spray-on temporary Lotus-Effect formulations, which the chemical giant Degussa is developing.

      At this point, the self-cleaning story takes an intriguing turn. There is another method of producing self-cleaning surfaces that is a mirror-image of the Lotus-Effect. Pilkington, the British company that invented the float-glass process by which most sheet glass is made, and which is licensed to every major glassmaker in the world, has developed a self-cleaning glass, Pilkington Activ™ glass, that uses a sort of anti-Lotus-Effect to achieve the same end. Instead of increasing the contact angle of water and making the surface less wettable, it decreases the contact angle and makes the surface more wettable.

      The development of Activ glass is exciting and heartening for many reasons, not least for the fact that it comes not from a university department or DTI-funded start-up but from a traditional North of England manufacturing company. St Helens, Merseyside, is one of the few remaining northern towns for whom a single industry is still its calling card. You can’t ignore glass and Pilkington in St Helens because, unlike so many other ‘heat-and-beat’ heavy industrial companies, the firm has stayed ahead of the game technically and organizationally.

      The modern Pilkington stems from the 1952 invention of the float-glass process by Sir Alastair Pilkington (oddly, not a member of the founding family). The process is production-line technology par excellence. Glass used to be rather irregular-shaped stuff made in small quantities in unreliable furnaces. A modern float-glass factory such as the Greengate plant at St Helens can now run continuously for up to 15 years, with sand, soda ash, limestone, dolomite, sodium sulphate and recycled glass (known as cullet) feeding into a 1,600°C gas furnace at one end, a continuous ribbon of glass forming and floating on a bed of molten tin, and sheets of glass cut and stacked at the other end. The molten tin surface confers perfect flatness and the machine can be tuned to produce any desired thickness up to 20 mm.

      This process produces standard raw glass, but Pilkington has now perfected a technology for depositing thin coatings on the glass from vaporized substances as it is being made; these coatings confer additional properties, as in the very common heat-insulating glass Pilkington K glass™. Activ glass