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