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Though buried deep

In the slime of the pool,

Unstained and untouched

You come forth to the world

Glorious in beauty,

Pure and serene:

Yet in your innocence

Oft you deceive us

Transforming the dew

On your life-giving leaves Into sparkling gems!

GONNOSKé KOMAI, ‘To the Lotus-Bloom’

‘Nooks and crannies harbour dirt,’ we have always been told: a piece of folk wisdom scientists would not have bothered to dispute until some 15 years ago. But the self-cleaning powers of the sacred lotus plant – recognized and sanctified thousands of years ago in the East – have turned this on its head. The lotus’s secret is that its surface is rough at the micro- and nanolevels. It is almost embarrassing that such an elemental discovery should have waited so long to be made, but it has opened up for human use a new field of self-cleaning surfaces, utilizing the Lotus-Effect^®.

Water skitters off a lotus leaf like drops of mercury – it doesn’t spread and the globules it forms are highly spherical. So water doesn’t last long on a lotus leaf. As for dirt, it seems to have a greater affinity for water than for the leaf so when it rains it is simply washed off.

There is a school of thought that science has still to rediscover the greater wisdom of the Ancients. In the case of the lotus, they are right. In ancient Eastern cultures, the lotus’s immaculate emergence from muddy water was more than noticed: the plant became a symbol of the triumph of enlightenment over the dross of earthly life. So deeply does the lotus pervade Indian, Chinese and Japanese consciousness that the name is a byword for, and a guarantor of, purity. The most famous Buddhist chant, Om mani padme hum, translates as ‘Behold! The jewel in the lotus’, and the classic Buddhist texts are known collectively as the Threefold Lotus Sutra. The quest for spiritual cleanliness that runs through Buddhism derives from the lotus’s example, so much so that images of cleaning recur in the texts:

The Law is like water that washes off dirt. As a well, a pond, a stream, a river, a valley stream, a ditch, or a great sea, each alike effectively washes off all kinds of dirt, so the law-water effectively washes off the dirt of all delusions of living beings.

Innumerable Meaning Sutra

While researching this book, I experienced my own lotus epiphany. I had flown from San Francisco to Seattle, and was en route from the airport to the University of Washington campus. It was a long day, my trip was almost at an end and I was tired and anxious. I had to change buses in the middle of Seattle’s downtown subway system. I emerged in the middle of Chinatown and walked into the nearest café for a bite to eat. In the middle of the counter, staring up at me, were lotus cakes. I ate one – it tasted rather like chestnut – and a Proustian madeleine feeling came over me, although this was not for the recollection of time past but a kind of blessing on the future of my enterprise. I had risen from the underworld of the subway system, in which the route to enlightenment – Washington University campus – was temporarily lost. The notion of sweetness

arising from dross is such a powerful one that once you know of the lotus you cannot help but refer to it: hence its omnipresence in East and South Asian cultures.

In the West, appreciation of the lotus is more aesthetic than spiritual: ‘No more stately plant adorns our gardens than lotuses,’ is a typical statement from an early 20th-century horticultural book on the water lilies.^* Concerning the flowers, the book goes on: ‘These great blossoms are among the noblest products of the vegetable world. They fairly glow in the morning sunlight.’ With flowers 20–30 cm across, some of the leaves sit on the water, as water lily leaves do, and some stand 1 m from the surface. The water that collects on them is tossed back into the lake by the wind. In size they are dwarfed by the largest water lily, the Victoria regia from the Amazon, which was first brought to flower in England by Joseph Paxton in 1849, but the grace conferred by the lotus’s exceptional purity more than compensates for that. (Incidentally, Victoria regia also has a role in the development of bio-inspiration; Paxton, as the engineer of the Crystal Palace in 1851, was much influenced by its structure; see Chapter 9.)

I was not sure whether I had ever seen a lotus before I became interested in the Lotus-Effect: water lilies of course, but had some of these been lotuses? I went to the Botanic Gardens at Kew, London, to find out for myself. Lotus plants die down every year and in cultivation are replanted from the runners that spread from the rhizomes rooted in the mud. At Kew in April they had plants of a variety of the American lotus, ‘Perry’s Giant Sunburst’, growing in tanks next to water lilies. Although it had a name redolent of out-of-town garden centres, nevertheless it was a real lotus: the leaves had that bluish bloom you see on some cabbage leaves. Dropping water on the lotus leaves was like dropping mercury on the table. The water drops gleamed with internal reflection and skittered around like quicksilver (fig. 2.1).

The Lotus-Effect’s discoverer, Professor Wilhelm Barthlott, Director of the Nees-Institute for Biodiversity at Bonn, Germany, is unusual in pursuing parallel careers as a research botanist and as a patent-holding industrial inventor working closely with many industrial partners. ‘Technology transfer’ is a buzz phrase in universities these days, as governments try to kickstart economic growth by applying university expertise to the commercial world. The Lotus-Effect is a model of how it should be done.

Wilhelm Barthlott had no intention of becoming a technologist. He is a benign, avuncular and energetic man with bristling bottlebrush hair and a moustache that perhaps evoke some of plants he encounters. He has made a particular study of cacti and his interest in biodiversity stemmed from visits to Madagascar, where many of the plants are unique to the island. As often happens in life, Barthlott found the Lotus-Effect when he was looking for something else. Evolution was his obsession and in those days – before the emergence of molecular biology in the early 1960s – evolutionary

relationships were studied purely by comparing the anatomy of creatures, especially their micro-anatomy: pollen grains for example. So Barthlott spent a lot of time at the microscope.

But then the scanning electron microscope (SEM) arrived that was to transform his work and would ultimately lead to his discovery of the Lotus-Effect. The SEM, which came onto the market in 1965, uses television-style scanning to produce richly contoured images with the appearance of 3-D.

With the SEM, a wonderland of fine structure, as detailed as any architect’s fantasy, came into view. The surface of plants is a strange other-worldly terrain. The outer surface does not consist of living cells but a non-living shell, the cuticle, covered in layers of waxes of varied composition. Sometimes the waxes are deposited on the surface in bizarre shapes (fig. 2.2). Through the microscope these structures often look more like animals than plants: Virola surinamensis seems to have miniature starfish nestling on a bed of waxy bobbles; the surface of Colletia cruciata resembles nothing so much as Anthony Gormley clay figurines, lolling about on the leaf; and Williamodendron quadrilocellatum has little piles of wax rings that could be a new form of pasta. Then there are miraculous architectural sweeps – the seed coat of Lychnis viscaria has plates that lock together like the tessellations of an Escher drawing. (Chapter 9 explores how structures like this have become important sources of inspiration for contemporary architects.) But most plants have bobbles like miniature topiary yew trees, with a frosting of waxy crystals on top.^*

For a while, Barthlott was engrossed in the sheer beauty of these structures, but then something unexpected emerged. Specimens must be cleaned to be looked at in detail – at very high levels of magnification, contaminants can ruin the picture. But, in 1974, Barthlott realized that certain plants never seemed to need cleaning and that these, under the microscope, were always the ones with the roughest surfaces.

This was the beginning of a trail that was to take Barthlott far from his comparative studies of the structure of plants (although he is still highly productive in this field), into the world of technical production of a new invention. The full impact of the self-cleaning effect crept up on Barthlott over a long period: the early work, he says, was ‘purely descriptive, without measurements’. He believed he had discovered something important in botany but ‘it never occurred to me that it could be something new to physicists and materials scientists’.

So what is happening on the rough surfaces of those leaves? The self-cleaning effect depends on the relative ‘wettability’ 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. 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 (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® 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 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 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.

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^oC) 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 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.

The success of the Lotus-Effect and Activ glass has stimulated much research and the story is far from over. The range of possible applications of the Lotus-Effect is in inverse proportion to its elemental simplicity. If the Lotus-Effect were a plant it would be seen as a rampant ecological invader. ‘Superhydrophobicity’ (super-non-wettability) and ‘superhydrophilicity’ (super-wettability) are buzzwords in many research departments.

Although the Lotus-Effect could work with any number of materials, in practice the early versions all used silicones, contemporary technology’s favourite water-repellents. These are very effective but tend to be expensive. In 2003, a Turkish team of researchers found a way to make Lotus-Effect coatings from poly-propylene (the stuff kitchen bowls are made of). An advantage of this simple technique is that these lotus-style polypropylene coatings can be applied to almost any material: glass, aluminium, steel, Teflon^® and polypropylene itself. The only limitation is that the material the coating is applied to must not be attacked by the solvent used. Commercial exploitation of this technique is under way.

For some people, the exterior walls of their house are only slightly less remote than Alpha Centauri: self-cleaning walls are fine but if this idea is so good, can’t it be used to make self-cleaning clothes? Is there any hope that in the future, accidents with red wine and coffee could be less ruinous? Yes, there is.

A self-cleaning fabric known as Nano-Care® has been developed by the American serial chemical inventor and entrepreneur David Soane (he has about 100 patents to his name and so far has started seven companies) and marketed by his firm Nanotex. Stain-resistant jeans and khakis using Nano-Care have been available in the USA from firms like Gap, Eddie Bauer and Lee Jeans since 2001 and shirts arrived a short while later. The fabrics first appeared in the UK in September 2004 with the Rocola Shirt Tec range from Morrison McConnell, a Derby-based firm and part of the Van Heusen group.

There have been many claims for stain-free clothes over the years and scepticism is understandable. The London Evening Standard tested them on the eve of launch by throwing lager, coffee and a particularly deep ruby red wine at the shirts. They passed: not quite every drop of the coffee was repelled but in all but the most extreme cases the shirt did what it said on the label.

The lotus leaf of Nano-Care is the peach. Peaches have a soft fuzz of hairs on the surface that function like the bobbles on a lotus leaf. They trap air and make water sit on top of the hairs. But this is very much an analogy only. If you put a peach under the tap you will see that water does run off at first, but the downy hairs are soon swamped and the surface wetted. Nano-Care whiskers are made of stronger stuff.

Nano-Care uses the lotus principle but the hairs are very tiny, less than a thousandth of the height of the lotus bumps. Compared to them, the cotton thread they stick to is an enormous tree trunk. The hairs are chemically bonded to the fibre and do not come off in the wash. And because they are so tiny, they do not change the feel of the cotton fabric appreciably.

Nanotex is a 21st-century textile company. It licenses the technology to chemical companies and buys back the nanofibre polymers to sell to textile companies which must then use the Nano-Care^® trademark on the product. Nano-Care is an environmentally friendly technology in more ways than one. It makes traditional, organic cotton into a hi-tech fabric with better properties than synthetics; the process in which the nanowhiskers are attached is a normal textile process using watery solutions, and in everyday use these fabrics require fewer cleaning materials.

Whatever the technique, there will always be a need to make self-cleaning effects last longer. As Pilkington’s Kevin Sanderson says:

I think that’s something that the hydrophobics have got to solve: if someone comes along and puts their fingerprint on it, it’s not going to be superhydrophobic again until someone removes that smudge. The lotus leaf repairs itself because it has tiny wax crystals that grow back; if you have a surface that mimics the effect it can’t do that. The Lotus-Effect is a very nice idea and it clearly works but these kinds of questions need to be answered.

The great thing about titanium dioxide is that it is self-renewing. Sunlight, air and water are all it needs. Lotus-Effect paint has no such renewing power. Like all normal material surfaces, it gradually loses its powers.

Could titanium dioxide be used with Lotus-Effect coatings to produce a self-renewing capability? On the face of it this is unlikely because a waxy Lotus-Effect coating and titanium dioxide at first seem to be chalk and cheese (or oil and water). They work in opposite directions: Lotus-Effect coatings being super-water-repellent and titanium dioxide super-water-attracting. But it turns out that very small quantities of titanium dioxide can have a significant effect in breaking down organic deposits on a Lotus-Effect coating without significantly weakening its water repellency. It could so easily have been the other way round: there is an element of pure luck in technology.

Not surprisingly, nature has already combined the Lotus-Effect and Activ technology – in the shape of a beetle that lives in the Namib Desert in southern Africa. The purpose here is not self-cleaning but water collection, for this is a harsh, arid, almost rainless environment where the only moisture comes in the form of wind-driven morning fogs. Remembering that Activ glass captures the dew, gives us the clue that creatures in this environment might want to use water-attracting surfaces to harvest what water there is in fog.

This is just what the Namibian Darkling beetle does. The beetle is 2 cm long and its wing covers are warty, with bumps about half a millimetre in diameter. Under the microscope, the area between the bumps is also seen to be bumpy but at a nanoscale; the peaks of the big bumps are water-attracting whilst the rest of the surface is waxy and water-repelling.

The tips of the bumps attract and collect very fine droplets from the mist; they coalesce and grow and then the waxy portions come into play. When the droplets reach a certain size (about 5 mm), they swamp the tip and begin to roll. The other bumps help the drops roll towards the mouth of the beetle. The beetle has a rather comical ‘water-collecting posture’ in which it stands into the wind, face down, to present a sloping back for the water to run down.

The beetle’s trick with the foggy foggy dew came to light, as so often, when researchers were looking for something else. In 2001, Andrew Parker, a young zoologist at Oxford, came across a photograph of beetles eating a locust in the Namib Desert. The desert is probably the hottest on Earth and the locust, which had been blown there by the strong winds typical of the region, would have perished the instant it hit the sand. But the beetles were obviously comfortable.

Parker investigated the beetles, expecting to find sophisticated heat-reflection surfaces. They do indeed have such a capacity but Parker also immediately noticed the bumps on their backs. Parker is a modern researcher with an eye for bio-inspiration; the fog-harvesting ability of these beetles had been noticed back in 1976 but at the time no one looked at the mechanism. Parker immediately suspected that some adaptation of the Lotus-Effect was at work in the water-collection process.

As with the Lotus-Effect proper, you don’t need a beetle, or any kind of living thing to get the effect. Water collection from fog in arid regions is an established technology: it is usually done with large nylon nets. But experiments on coated glass slides with artificial surfaces mimicking those of the beetle and control slides with entirely waxy or water-attracting surfaces quickly showed that the beetle’s structure is the best for the job. Here was an efficient new way of collecting water. Parker is developing the idea with QinetiQ, the hi-tech research company spun off from the Ministry of Defence research department at Farnborough. In 2004, the process was patented and commercial applications are forthcoming.

Stripped of the needs of the beetle, the system boils down to alternating regions of water-attracting and water-repelling surfaces with the latter being the background, as it is with the beetle. The width of the water-attracting regions governs the droplet size. The technical device mimics the beetle’s head-down posture by setting the collecting plates at an angle so that the water collected simply runs off into a trough. Although there is a tendency for the wind to roll the droplets back, if the size of the droplets is tuned to be large enough, they will roll against the wind into the collecting trough.

The desert-beetle water-collection mechanism is so simple and founded on such basic properties of matter it seems astonishing that it should have waited till our technology had reached such a peak of sophistication before being discovered. Water is so ubiquitous that we take it for granted. But nature exploits every possible property of a substance. Having mastered all kinds of complexity, we are now catching up on some tricks that are simplicity themselves, like this new source of water we might call Beetle Juice.

Some of the ongoing Lotus-Effect research has a playful quality in keeping with the purity of this blindingly simple idea. In 2001, two French researchers came up with a Zen-like party trick by coating drops of water so that they can roll on glass without breaking up, or even float on water itself (fig. 2.8). These ‘liquid marbles’ are made with lycopodium^* grains coated with a silicone. This creates a lotuslike surface with almost perfect water repellency: hence their spherical shape and ability to float on water. This ‘non-stick water’ may eventually find applications in the packaging and delivery of fluids, but for now it induces a Buddha-like smile at the quirkiness and eternally surprising nature of the physical world.

As usual, when we think we’ve invented something really far-out, nature seems to have got there first. There are aphids that, in an example of the crazily degraded lifestyles that are so common in the natural world, live all their lives inside plant galls. In fact, the galls – those warty lumps found on the undersides of tree leaves, especially on oaks – are created by the aphids, which interfere with the host plant’s metabolism, thus creating the galls. In choosing, in evolutionary terms, to live like this, these aphids have created a problem for themselves. Aphids feed on the sap of plants and they produce large quantities of a whitish, sugary excrement known as honeydew. Aphids that live on the surface of plants have developed a symbiotic relationship with ants, who feed on the honeydew and protect the aphids. But gall-living aphids have no such means of disposal: they risk drowning in their own excrement unless they can easily evacuate it from the gall. The honeydew is very sticky and once an aphid gets trapped in a ball of honeydew it can’t escape.

To the rescue comes super-non-wettability of an ingenious kind. The aphids produce needles that break off and line the inside of the gall with a rough waxy coating. The drops of honeydew are coated with the wax and become non-wetting honeydew parcels just like the water marbles. There is even a caste of soldier aphids whose job it is to elbow the parcelled-up honeydew balls out of the gall!

The aphid’s secret was revealed in a paper, wittily entitled ‘How aphids lose their marbles’, by the young Indian physicist L Mahadevan and his team. Mahadevan, at Cambridge University when he did this work and now at Harvard, is one of the most dazzling figures in bio-inspiration. He is a mathematical physicist who works with biologists to unravel bio-inspired problems right across the spectrum. His papers have artistic references wherever possible, rigorous mathematics and, above all, they impart a sense of the remarkable creativity, chutzpah even, of nature in devising these solutions.

When I visited Mahadevan at Harvard, his computer desktop was a treasure trove of biological curiosities, involving origami, the draping patterns of clothes, biological springs and ratchets, and those aphids that lose their marbles. Mahadevan admits to having a short attention span, which means that he attacks these problems in a brilliant mercurial way and then passes on to the next. He is a delighted roamer in this new terrain of bio-inspiration, throwing out brilliant suggestions that others can follow up.

So we see that the Lotus-Effect is not just a matter of building maintenance. It sheds light on many strange corners of the natural world as well as adding some radiance to the built environment. Just as the self-cleaning properties of the sacred lotus were of philosophical, spiritual and artistic importance to eastern civilizations, the idea of self-cleaning can be a secular boon to the northern latitudes. In The Poetics of Space, the French philosopher Gaston Bachelard has suggested that cleaning might itself have spiritual/aesthetic value:

And so, when a poet rubs a piece of furniture – even vicariously – when he puts a little fragrant wax on his table with the woollen cloth that lends warmth to everything that it touches, he creates a new object; he increases the object’s human dignity; he registers the object officially as a member of the human household.

Water is one of our prime elements and in our whoring after complex chemistry we have forgotten how many subtle effects nature produces simply by manipulating water in some way. Repelling water is both the mechanism and the purpose of the Lotus-Effect, but at the nanoscale the subtle control of the water-attracting and water-repelling qualities of proteins can produce properties that have nothing to do with cleaning.

Spider silk is composed of such a protein and its strength comes from the way the fibre is spun from a watery solution, using water-attracting and water-repelling regions to create a composite structure that materials scientists would dearly love to mimic. Indeed, spider silk is regarded by many as the holy grail of materials science. The Lotus-Effect still has much scope for development but it has reached a degree of fruition: the spider guards many secrets still.