The Gecko’s Foot: How Scientists are Taking a Leaf from Nature's Book. Peter Forbes
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.
What Skill is in the frame of Insects shown?
How fine the Threds, in their small Textures spun?
RICHARD LEIGH, ‘Greatness in Little ’
The astounding properties of spider silk have been recognized for decades. In the force needed to break it when pulled, spider silk is about half as strong as mild steel, so the oft-quoted ‘spider silk is stronger than steel’ is not strictly true. Steel, however, is nearly eight times denser than spider silk so weight-for-weight spider silk is about six times as strong as steel. Spider silk is much more stretchy than steel, extending by 30–40% before it breaks; it is about twice as stretchy as nylon and eight times more stretchy than Kevlar®. What is special about spider silk is that it is both stretchy and tough: a rubber band will stretch more than spider silk but its breaking strength is very low. Spider silk is the only material with exceptional stretchiness and good breaking strength.
Spider silk has been brought to a pitch of perfection by millions of years of evolution. And this optimization means that there isn’t just one generic spider silk: a single spider can make up to seven different kinds of silk, each tailored towards a specific task: the dragline from which the web is hung is the strongest, the capture threads have the greatest extensibility, and so on. Spider silk’s great resilience has long suggested human applications. The web has to catch a heavy insect at speed, and bring it to a standstill without snapping and without flinging it back out again in recoil, a process reminiscent of the arrester wires used to bring jets landing on aircraft carriers to a halt.
Spiders have been working their magic for over 400 million years – that’s pre-dinosaur time. The oldest existing strand of spider silk was reported in 2003, preserved in Lebanese amber. It dates from the Early Cretaceous Period, more than 120 million years ago and what is fascinating about this specimen is that the small globules of ‘glue’ that are strung along the capture threads are still clearly visible, as they are on spider webs today.
We think of spider webs as delicate filigree structures, best seen with dew or frost accentuating their patterns. The garden spider (Araneus diadematus) is one of the best web spinners (fig. 3.1). But tropical spider webs can be very large: the queen of spinners is the golden orb-weaving spider (Nephila claviceps), which can be 5–8 cm long and 20 cm in total span: her webs are up to 2 m in diameter – big enough in fact to be useful economically. In Papua New Guinea, they have been draped across bamboo poles and looped at the end to make fishing nets. Early Western explorers also encountered such webs: in 1725, Sir Hans Sloane reported the nets were “so strong as to give a man inveigled in them trouble for some time