The Gecko’s Foot: How Scientists are Taking a Leaf from Nature's Book. Peter Forbes
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 700oC) 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’