Sunday, 21 November 2010

Sticky Feet. I've Got Those Sticky Feet.

Walking on walls, clambering up trees, hanging about on the ceiling. All great stuff. Great stuff we can't really do, not easily anyway. Most people who CAN do all that seem to end up fighting crime or something, whereas I would much prefer to just get a bit more living space. In any case, there are a whole host of animals with sticky feet and some can even walk about on glass. Flies can fly AND walk on glass. Greedy. And I don't think they are effective at crime reduction either. No, it's all about themselves, isn't it. Pah! Eating my food. Flying through my air. Walking on my windows. Giving absolutely nothing back. Something has got to be done. You know what? I hereby announce my candidacy... wait. What? 

Copyright © 2010, Alan D. Wilson
Why not let's begin with good ol' fashioned claws. They obviously work by simply gripping and hooking onto bumps and sticky-out bits. Squirrels can climb up and down trees with equal ease because not only do they have claws, they also have special ankles on their back legs. They have swivel joints so that their feet can make a full 1-80 and face the other way round. Impressive, also a bit creepy.

Image: א
Treecreepers are cute little birds that are related to pretty much every other cute little bird in the world. What marks them out is the fact they hop up tree trunks in search of insects and spiders to eat. To enable this they have long, curved claws at the end of their long toes. Their feet look quite vicious actually, but in a cute little way.

Image: Stanislav Gorb
Insects also have claws. Of course, they're tiny so they can grip onto minute pits and dimples on surfaces that look quite smooth to us. Some insects also have two pads on each foot which are covered in tiny hairs. This may help by working like both a suction cup and velcro on a microscopic scale, but they also secrete a glue to simply stick to the floor. It seems the claws are now important for actually peeling the sticky foot off the floor! Or the ceiling of course. Being small and light and ensuring that several feet touch the surface at all times ensures the fly can walk upside down. Cockroaches can do this too. It's possible that all insects have these pads, but perhaps some are more effective than others.

Slugs 'n' Snails
Slugs and snails produce a sticky mucus on that big foot of theirs. It's usually gooey and nasty, it's mucus after all, but when pressure is applied to it the stuff becomes a slippery, non-sticky liquid. So the snail's foot ripples as some parts push down and slide over the liquid, and other parts relax and stick to the surface. This mucus is what leaves those trails all over the place when it dries up and it also allows snails to slide over razor blades and (more likely) sharp stones without getting hurt.

Image: steve_lodefink
Octopus 'n' Squid
Cephalopod suckers might seem pretty obvious, they sort of... suck, right? They are actually remarkably complicated and full of three different kinds of muscles that all work in creating suction. Basically there's a funnel thing and a cup shaped thing that have water in them. The edge of the sucker is spongy and forms a seal against any surface it's pressed against. Contracting various muscles causes the cup shaped thing to get bigger, but no more water can get in. So the water that is already inside has more space to move around in and the pressure falls. This creates suction. Relaxing those muscles stops the suction so the octopus can let go whenever it wants. Squid also have a piston structure inside. When something pulls and tries to get away, the piston is lifted and increases the volume even more, lowering the pressure even more and making the suction even more powerful. Scary stuff.

Image source
Frogs 'n' Toads
Some frogs and toads also have sticky feet. They have pads that secrete a thin layer of mucus, in some it's sticky, in others it's just wet. They use wet adhesion, like how wet tissue paper sticks to glass. You can try this at home, simply:

1. Do it once and never again. If you must.

This is good since most amphibians need to be moist at all times, so it's a natural solution. Minute canals ensure the mucus gets all over the foot and any excess can drain away. There are also bumps tipped with microscopic columns known as nanopillars. These are soft and can come into intimate contact with the surface. They then have to peel their feet off the surface, but they are pretty good at doing that really quickly by now, what with all the practise they've had.

Image: eien no dreamer via Flickr
Starfish and the Like
Most echinoderms actually have sea water flowing through their veins. It's this that is used for breathing, feeding and of course, walking. They have lots and lots of tiny tube feet. Pumping water into them pushes them out and up against the surface they're on. Chemicals are then released to stick on and other chemicals released to stop sticking on. This is done in a wave so that they can slowly move in whatever direction they feel like. Those feet are also used for grasping onto prey and eating it, so the whole walking thing almost seems like little more than a side effect. No wonder they don't care how fast they go!

Image source
No list of sticky feet, and I'm sure there are thousands, can be complete without the gecko. And the gecko has a remarkable way of being sticky. They make use of the van der Waals force. This is a force that exists between molecules that comes about from electrons orbiting around the nucleus of atoms. Wait, molecules? Atoms? Electrons? How big are these feet?

Of course gecko feet are suitably sized for the gecko,  what they have are millions of tiny hairs called setae on their toes. There are about 14, 000 of them per square millimetre. Each seta is about 5 micrometres in diameter, 5 millionths of a metre. Clunky human hairs by comparison range between 18 and 180 micrometres in diameter.

But wait, there's more! Each seta has at its tip between 100 and 1,000 spatulae. So the tiny hairs end in hundreds more tinier hairs. These are about 0.2 micrometres long, just under the wavelength of visible light. Yeh, if they were electromagnetic waves they would be ultraviolet.

This stickiness is dry, leaves no slimy trails and would work in space. Furthermore, any bits of dirt that gets stuck on the feet comes off by simply walking a couple steps, which is really useful since these feet were precisely made for walking. And that's just what they'll do. The stickiness ends when the hairs are moved beyond a certain angle, so the gecko can just peel its feet off easily.

The fatal weakness? It doesn't work underwater or on teflon. But that's not so bad, even Superman couldn't take Kryptonite. Or magic, apparently.


I really like this video of the pebble toad climbing up sheer rock and escaping predation. He looks like such an intrepid veteran at the end!

Also, just in case you didn't get the reference and for fun as well:

That's Bing Crosby in the middle surrounded by a lunatic and a man covered in some kind of varnish. I have no idea what to think of the singing of those ladies except that times change.

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