Atomic gecko

Dr Karl:

If you've ever been in the tropics, you'll have heard geckos clicking away, and you'll have seen them scuttling up the walls, and even hanging out on the ceilings. Some encyclopaediae reckon that they stick to the ceiling by tiny suction caps on the ends of their feet. Other books claim that the tiny hairs on the ends of geckos' feet adhere to microscopic rough spots on the surface they're sticking to. But don't believe everything your read.

In fact, geckos stick to the ceiling by atomic forces.

Geckos are a type of lizard, and there are about 750 different species of them. They tend to be small (3-15 cm long), active at night, and have a short stout body with a large head. They can survive anywhere from your house to jungles and deserts. Geckos have the most highly developed hearing of any of the lizards. They're the only lizard that makes any kind of noise beside a "hiss".

For a century, people have been trying to work out how geckos really do stick to something as smooth as glass. But it took a big team effort led by Robert Full from the Department of Biology at the University of California at Berkeley to figure it out. He had help from colleagues in his own University from the Department of Electrical Engineering & Computer Science and from the Department of Integrative Biology. He also had outside help from the Department of Mechanical Engineering at Stanford University, and from the Department of Biology at Lewis & Clark University in Oregon. It took many different disciplines to understand something as simple as how a gecko sticks to a smooth surface.

Suction has nothing to do with the gecko's legendary stickiness, because the feet will stick in a vacuum. Friction is not the answer either, because they stick to low-friction surfaces such as silicon. "Microinterlocking" with tiny rough spots is not the answer, because they stick to super-polished super- mooth glass. And electrostatic attraction is not the answer either, because the feet stick when the air is full of charged particles.

A gecko has four feet. Each foot has about half-a-million tiny hairs on the end. These hairs are very small. A human hair is about 70 microns across (a micron is about a millionth of a metre). The hairs on the end of a gecko's foot are about 7 microns across, and about 30-130 microns long.

But if you think that these hairs are really small, have a look with an electron microscope, and you'll see that things get even smaller. Each of these half-a-million microscopic hairs has several hundred smaller hairs coming of it - each one being about 0.2-0.5 microns across. These itty-bitty hairs are called spatulae, and they are roughly the same size as a wavelength of light. They are extremely small indeed.

The team was able to build, and then attach, a microscopic force measuring device to one of the half-million hairs - and then measure the force needed to pull it off a surface.

First, they discovered why the toes on a geckos have a peculiar way of curling as they stick and then uncurling as they peel their feet from a surface. It's a bit like removing sticky tape. The bigger hairs can generate a lot of grip. They're lined up in such a way as to need only a small force to pull them off - so the gecko won't get exhausted as it peels its feet off the ceiling.

Second, they discovered that the force measurements that they took from the single hair on the gecko's toe, suggest that the geckos stick thanks to atomic forces!

These are called "van der Waals Forces", after the 19th century Dutch physicist who first described them. Imagine that you have a nice round atom. There's a central core or nucleus, which is surrounded by a neat symmetrical cloud of electrons. Over a long period of time, say a second or so, the cloud of electrons is perfectly symmetrical. But over a much shorter time, say a trillionth of a second, the cloud of electrons might be lop-sided, with more of the electrons (say) on the left side of the atom. So the left side of that atom is temporarily more negative. This induces a tiny positive charge in another nearby atom. The opposite charges attract - and there's your atomic attraction.

Of course, in real life, van der Waals Forces are more complicated, but that's basically how they work.

And so we now think that the really tiny hairs on a gecko's foot are so small (sometimes smaller than a wavelength of light) that they can get close enough to the surface to stick by direct atomic or molecular forces.

What use is all this high-powered science? After all, our current technology can't make a packed array of a billion microscopic spatulae. But, now that we know how the geckos do it, we might be able to invent the first dry adhesive. Also, geckos have sticky feet that work perfectly well after they run across dirt - so we might be able to invent self-cleaning adhesive surfaces, that have a tremendous grip, and yet which are easy to remove. One day, climbers could use gecko-technology gloves and shoes to stick to rock faces - and Mr. Mission Impossible, Tom Cruise, wouldn't slip off vertical stone walls anymore.