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

So what is happening on the rough surfaces of those leaves? The self-cleaning effect depends on the relative ‘wettability’ (#litres_trial_promo) of a leaf. Wettability is something we all recognize but scientifically it is something quite specific. On wettable surfaces, water drops are severely flattened and the contact angle that water makes with the surface of the leaf is very low (fig. 2.3). On a highly non-wetting surface, water forms near-spherical drops and the contact angle is very high – almost 180°.

When a surface has many tiny bumps, and these bumps are formed from a water-repellent substance, water drops ‘sit’ on top of the bumps, cushioned by the air in the space beneath them. The area of contact between the water and the surface is dramatically reduced by these bumps. The curious properties of an array of bumps in providing a cushion for an object sitting on them is demonstrated by the ‘magic’ illusion of the Fakir-on-the-Bed-of-Nails (#litres_trial_promo). The mystery of how the fakir can bear to lie on the bed of nails is no mystery at all.

In a standard demonstration of the ‘fakir effect’, about 1,000 nails are punched through a plank big enough to lie on. Not only is it possible for a person to lie on the board, another board can be piled on top to create a sandwich, a breeze block placed on the recumbent’s chest, and the block smashed with a hammer. (The only danger to the victim – and to the block smasher – is flying debris: goggles must always be worn in this experiment.) The weight of the body distributed over the 1,000 nails does not exert enough force at the points to puncture the skin, although we intuitively feel that nails, however many there are, must be painful.

To translate from the large-scale world of the fakir down to the lotus surface: water drops sit on the points of the bumps, with the compression of the air in the cavities giving extra buoyancy. The self-cleaning effect occurs because when dirt lands on the surface it also has few points of contact. When rain falls, the dirt adheres to the water far better than it adheres to the surface and is carried off with the water, which rolls easily over the bumps (fig. 2.4).

In Barthlott’s studies, the self-cleaning effect was most noticeable in the sacred lotus (#litres_trial_promo) (Nelumbo nucifera). The plant had not been easy to cultivate in Germany but when Barthlott became Director of the Bonn Botanic Garden he set about providing himself with good specimens. Around 1988, Barthlott identified the lotus as the best exponent of the art of self-cleaning; it was a magical completion of an ancient story.

Given the mythical status of the lotus it would have been reasonable to assume that the effect was peculiar to the plant, or at least to plant leaves of the lotus type. But Barthlott realized that the effect was a physical one and absolutely generic: any surface with bobbles of the right size, made from a water-repellent substance, would exhibit the same self-cleaning effect.

By 1988, Barthlott knew there was a technical product in view and he set out to interest the big chemical companies: ‘the tribes along the Rhine’, he calls them, ‘those global players’ (these are the major German chemical companies such as Bayer, Hoechst, BASF, Degussa). He had a party trick: he would squeeze some glue onto a leaf and show that it rolled off, leaving no trace behind. The hard-nosed industrialists refused to believe it. At first they assumed his glue was doctored and produced a tube of their own. The result was the same.

Surface-coatings specialists could not accept that they had anything to learn from plants: they said, ‘Oh, it’s something to do with living things.’ After five years of frustration at the lack of industrial interest, Barthlott realized that he needed a technical demonstration of the self-cleaning effect, so he created the ‘honey spoon’, with a home-made micro-rough siliconized surface. When dipped into a honey pot, these spoons shed their entire load when tipped, leaving nothing behind (fig. 2.5). But this was a demonstration, not yet a technical product: ‘It was very difficult to attach the lotus surface in a stable way, so all our home-made technical surfaces were not really intended for use. However, these first surfaces were a breakthrough: as soon as we could show them to industrial partners they were convinced. A living plant with even better properties did not have the same impact.’

Barthlott showed that not only could a botanist become a technical inventor but also that this botanist had fine PR antennae. He felt that the process needed something shorter and pithier to describe it than ‘Self-cleaning Materials with Nanostructured Surfaces’. So, in 1992, Barthlott established the name Lotus-Effect® (#litres_trial_promo) as a label for self-cleaning products. The lotus flower was the best example of the effect so lotus it had to be. Even so, at the time he did not realize quite how apt the name was:

When I gave a talk to Indian students in ‘95 at the Humboldt Institute, they came to me afterwards and said: ‘It’s a symbol of purity in our religion’.

I said, ‘I know.’

‘Do you know why?’ they said. I had thought it was something esoteric – because Buddha hid under the leaves to protect himself, something like that – but no: you can find Chinese and Sanskrit poems describing the lotus, how it unfolds its leaves from dirt and muck, completely clean.

The Lotus-Effect officially entered the canon of Western inventions (#litres_trial_promo) in July 1994 when Barthlott applied for a patent. Then, in 1997, came the classic summing up of the Lotus-Effect (#litres_trial_promo) 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.

(#litres_trial_promo) 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 (#litres_trial_promo), 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 is also made by this process.

When I went to see for myself, I quickly learned how important such technical advances can be to a community. In my B&B, I found that Activ glass is already famous locally and that Pilkington’s share price (it had doubled in the past year) is as much a staple of conversation as the weather.

Pilkington Activ™ glass was developed at the Pilkington research centre in Lathom, 12 miles from St Helens – a green glassy haven set in parkland. Lathom is a pleasant corporate industrial environment of a kind that is increasingly rare in Britain: the calm reception area is festooned with good-employer plaques and mission statements. Simon Hurst, Pilkington Senior Technologist, wears a shirt mono-grammed with both Pilkington and his own name.

Simon Hurst and Dr Kevin Sanderson, Activ’s co-inventor, took me through the development process. Activ glass exploits the surprising properties of titanium dioxide, best known as the white pigment in brilliant white paints. But titanium dioxide also has unusual electro-optical properties (#litres_trial_promo).

The action of sunlight on titanium dioxide has the effect of charging it electrically. The charged surface then interacts with air and water vapour to create ions that can oxidize organic material. This process is called photocatalysis and it means that a titanium dioxide coating can break down any organic substance deposited on it – it is, like the lotus leaf, self-cleaning. Unlike the lotus leaf, it is strongly water-attracting, which means that water forms sheets rather than droplets on a titanium dioxide surface and if the surface is vertical or at a significant angle, water quickly rolls off, carrying away the organic material that it has degraded.

To compare the two approaches: for rain to carry off dirt particles, the dirt must have a greater affinity for the water than for the surface. This can be achieved either by making the affinity of the surface for dirt very weak – as in the Lotus-Effect – or by making the affinity of dirt for water very strong. The latter sounds less promising as water does not remove dirt easily – that is why we use soap and detergents. But the radicals produced by the action of sunlight on titanium dioxide will oxidize any organic matter (insects, pollen, plant debris, bird droppings and suchlike). Once oxidized, the organic matter dissolves in rainwater and washes away. The power of the material is constantly renewed by sunlight.

The self-cleaning ability of titanium dioxide has been known since the 1960s and in the last 10 years it has been exploited in Japan for a myriad purposes. It is used in self-cleaning tiles for bathrooms and it has medical uses – it has even been used against MRSA, the notorious multiply antibiotic-resistant Staphylococcus bacterium. Ironically, titanium dioxide’s photocatalytic properties were once a problem in its traditional use as a pigment in paints. Paints are organic materials and, of course, under exposure to sunlight the titanium dioxide attacks them. Ultraviolet light is the main cause of paint degradation in any case, but titanium dioxide was accelerating this process. The answer, as far as the paint was concerned, was to coat the titanium dioxide with silica to lock up its photocatalytic powers.

Hurst and Sanderson began to work on Activ glass in the early 1990s, and they developed a technique for coating glass when it was still very hot (about 700

C) after it has been formed on its bed of molten tin. A self-cleaning titanium dioxide layer can be applied in this way, but in any significant thickness titanium dioxide is opaque – it is, after all, a white pigment. The breakthrough came in perfecting this process with an ultra-thin coating, less than 20 nanometres thick. The resulting glass is perfectly transparent; next to a pane of ordinary glass it appears slightly more reflective and blue, but to all intents and purposes it is ordinary glass.

At Lathom, you feel that the world is getting better and brighter through industry. Pilkington Activ glass is the embodiment of an ancient dream: our smeary dirty world just got a little cleaner thanks to human ingenuity. And with its many cleaning properties it is a kind of miracle product.

Kevin Sanderson says, ‘Activ has caught people’s imagination but for many people glass is glass; we have to educate them into thinking that glass can do other things as well.’ In fact, although glass may once have been taken for granted as a generic, low-profile building product, this is no longer the case. Simon Hurst says: ‘Glass grows faster than GDP and has done for the last twenty or thirty years – on average four to five per cent globally every year. You’ve only got to look at trends in architecture – glass usage has never been higher. The new Swiss Re Tower in the City of London is entirely clad with glass.’

The final stage in the development of a technical innovation is its emergence into the real world, where it is hoped it will find a niche among ‘real’ people: people who have habits, customs and practices that do not respect the tidy protocols of research. Products need to be robust and easy to use to be able to claim a place ‘as a dear and genuine inmate of the household of man’, as Wordsworth put it. They need to be humanized. While researching this book, a building project at my home suggested a chance to try Pilkington Activ™ glass. The small conservatory at the back of the house needed a new roof. It has a shallow 10° slope and every year it collects algae and grit that has to be laboriously cleaned off to preserve any kind of acceptable appearance. A classic potential use for Activ.

You can see the difference with Activ instantly. The conservatory roof is next to a 45° sloping glass roof at the end of the kitchen, glazed before Activ came on the market. The Activ coating gives it added reflectance that shines out against the duller standard glass. When it rains, a myriad separate drops form on the 45° standard roof but a continuous sheet quickly forms on the Activ (fig. 2.7). When there is a dew, Activ attracts it, so the water needed to do the trick is harvested from the air.

The conservatory roof sits beneath a birch tree that drops a fair amount of debris. Dry debris cannot be magically spirited away. On a roof pitched as gently as this, it needs a fairly brisk rainstorm to shift it; and fairly brisk rainstorms tend to bring down more debris. So, self-cleaning doesn’t mean always clean but Activ is always at work and there is always new dirt falling. Because of the way it dries, Activ reduces spotting but doesn’t entirely eliminate it. When an Activ surface does need a helping human hand, sluicing with water does the trick because nothing really sticks to this surface.

The Consumers’ Association’s Which? magazine gave Activ glass a brief write-up (#litres_trial_promo) in June 2003. Tested against standard glass for two months, they commented:

We struggled to find the odd smeary trace on the Activ glass. The technology doesn’t work instantly, nor does it completely do away with window cleaning – you’ll still need to clean the inside – and it won’t deal with some marks such as paint. But it does make life simpler.

Activ glass has been on test at Pilkington since 1997 but they have simulated weathering cycles lasting much longer than that.

The prime use of Activ glass is facing out, but the omnivorous appetite of titanium dioxide for pollutants means that the technology has a potential application facing in; a case in point would be in structures suffering from ‘sick-building syndrome’, those large offices in which the internal atmosphere causes a sense of malaise in workers. It will help remove the oily pollutants of the kitchen, and Simon Hurst says: ‘It can remove ozone – we have to be careful how we say this because of the ozone-layer, but ozone is a ground-level contaminant and Activ converts it back to oxygen.’ In fact, there is a whole range of applications in the pipeline.

In many respects, the Lotus-Effect and Activ glass are equal and opposite solutions to the same problem: the road to self-cleaning can go either the super-non-wettable or super-wettable routes. The world and its materials with which we are familiar inhabit a murky zone between the two, a world in which, as the poet Philip Larkin says, ‘nothing’s made/As new or washed quite clean’. By discovering the extremes, we have opened up enormous possibilities: it is like extending our vision by means of infra-red and ultraviolet, radio and X-rays – all the forms of radiation beyond the tiny band of the visible spectrum.