I could stay on the internet all day looking at pictures of geckos. Geckos are the fluffy kittens of the reptile world.
The house I grew up in was inhabited by these native robust velvet geckos (below). The term 'robust' is appropriate. From time to time, two geckos fighting on our ceiling would lose their footholds and fall onto the floorboards with a loud smack. Fortunately, it takes a lot of force to burst a gecko. Usually they would merely sit looking stunned for a few seconds, then run up the nearest wall.
At the time I never thought about what might allow them to walk on our ceiling in the first place. It's easy to see how an animal might stick to a smooth surface like glass using vacuum suction, or to many surfaces using a sticky substance like mucus. But it's more difficult to understand how it would walk on a dry, comparatively rough surface like a ceiling.
The answer is that geckos' feet are able to 'share' electrons with the surface they are walking on. Chemistry is not my strong point, so please bear with me as I try to explain this.
Here are some pictures of the undersides of geckos' feet. Note the distinctive ridged patterns.
Traditionally, having hairy palms is meant to be a sign of madness. In the case of geckos, it's a sign of awesomeness. At a higher magnification, it's possible to see that the ridges are composed of extremely fine hairs. Each hair is split at the end into hundreds or thousands of smaller branches.
Like all matter, the hairs on the underside of a gecko's foot are made up of atoms. Here's a diagram of an atom. It probably looks familiar from high school. As you can see, the atom is made up of three kinds of smaller particles. In the centre are the positively charged protons, and the neutrons, which do not carry a charge. The electrons, which are negatively charged, orbit the other particles, like the planets orbiting the sun. The number of electrons (which, in a typical atom, is the same as the number of protons) determines what kind of element the atom is. For example, a hydrogen atom has one electron, while an atom of gold has seventy-nine. The foot of a gecko is made up of many different kinds of atoms, including carbon, nitrogen, hydrogen and oxygen.
If you think of one of these atoms, it's possible to imagine the electrons moving around in orbit. At certain times in the orbit, there are likely to be more electrons in certain areas of the atom than in others. This gives one end of the atom a slight negative charge, and the other end a slight positive charge. One end of a gecko's foot hair atom might be negatively charged, and be adjacent to the end of a ceiling atom that has a positive charge. This results in an attractive force, like that between magnets.
Obviously, because the electrons are in constant orbit, the attraction between the two atoms will be fleeting. However, as the electrons circle around, the charges on the two electrons swap. The previously negative end of the foot atom now has a positive charge, and vice versa in the ceiling atom. The attraction is therefore maintained.
As far as I can tell (again, this is not really my subject area), these kinds of attractive forces must exist between all kinds of matter. Perhaps, at this very moment, there are attractive forces occurring between you and your chair. I spend most mornings trying to overcome the attractive forces between myself and my bed. But if you can imagine the entire surface area of a gecko's foot hairs spread flat, it's possible to appreciate the large number of potential attractions that are compressed in the relatively small area of the foot. It's thought that the geometry of the foot, rather than any properties of the substances in it, is responsible for its remarkable adhesive power.
Geckos now have their imitators. Some scientists from the University of California have invented what they describe as a 'hard plastic gecko-inspired synthetic adhesive'. They believe this product may be useful for medical products, sporting goods and 'climbing robots where a controllable and reusable adhesive is needed'. Imagine! In the future, we could all be living in houses inhabited by hard plastic, gecko-inspired robotic geckos. I'm looking forward to the time when my evenings are interrupted by two pimped up gecko-bots blasting each other off the ceiling in a tiny ball of flames.