Serendipity A bit of a snag

IN 1948, George de Mestral went for a quiet walk in the Swiss countryside. Just like millions of people before him, he returned home to find his trousers covered in burrs, but, unlike everybody before him, Mestral wanted to find out about the mechanism behind these pesky and persistent seed cases. He put one of the burrs under his microscope, and saw that its surface was covered in tiny hooks; these had been responsible for latching on to the loops of fibre in his trousers.

Mestral decided to replicate the hook and loop mechanism in order to produce a fastener for everyday use. Working with the recently developed nylon, he was able to mass-produce sturdy loops, but he was stymied when it came to making the accompanying hooks.

Eventually, he realised that he could create hooks simply by cutting the loops, and thus Velcro was born. The word is a combination of velvet and crochet (the French word for hook), and although it is commonly used to describe all hook and loop fasteners, it is actually a registered trademark, like Hoover and Sellotape.

Velcro is now ubiquitous. Astronauts use it to keep tools from floating away, and a stainless steel version of Velcro is used for attaching insulation in nuclear power plants, where the temperature is 500C. There is even a special silent Velcro used by the military.

Velcro is so strong that a patch as big as a CD can support the weight of a person, and yet the patch can be easily peeled away between two fingers. This mixture of strong bonding and easy peeling is a result of so-called dimensional reduction. If you try to pull apart a patch of hooks and a patch of loops, then the force is spread over the entire area and is ineffective. But if you try to peel the two patches apart, then the force is focused along a single line. In short, applying a force in two dimensions fails to separate the patches, but concentrating it along one dimension works.

The zip functions in exactly the same way, except it deals in one less dimension. If you grip two pieces of zippered fabric and try to pull them apart, the force is spread along the entire length (one dimension) of the zip, and the two pieces of fabric should hold fast. In contrast, the zipper mechanism that actually unravels the zip focuses all the force at a single point (zero dimensions).

Following Mestral's study of burrs, biologists have recently discovered other natural examples of Velcro. The predatory decorator crab uses hooks on its shell and legs to accumulate sponges and other marine life, thus camouflaging the crab. Conversely, sponges belonging to the family Cladorhizidae feed by using their in-built loops to snag crustaceans. Cells from the sponge then smother and digest the crustacean. .

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