Issue 111 / May - June 2016
Gecko The Physicist
They can resist gravity and easily hang upside down on flat ceilings. They are geckos, extraordinary lizards that have been perfectly created for their environments. But what allows them to resist gravity? Is it an adhesive secretion? Sucking discs? Some force, one stronger than gravity?
If a geckoâ€™s feet used a secretion to stick to surfaces, they wouldnâ€™t be able to move. And geckos can stick to surfaces, even if there is no air, meaning they donâ€™t use sucking discs. Some scientists suggested an electrostatic charge might help geckos resist gravity, but theyâ€™re too heavy for such a charge. Research has shown that Â geckos can still cling to surfaces, even if the air is charged with electron ions. If geckos were resisting gravity thanks to electrostatic charges, then the air filled with ions would prevent such resistance.
It turns out geckos are able to adhere to surfaces thanks to the numerous microscopic hairs, or setae, on the bottoms of their feet â€“ as many as 2 million hairs on just one foot. Each of these 2 million hairs further branches out to 1000 more miniscule hairs, totaling 2 billion hairs per foot. The thickness of each hair is one five thousandth of a millimeter. To make a comparison, there are around one hundred thousand strands of hair on a human head. If we had the same number of hairs found on the foot of a gecko, then our hair would cover a surface as large as a football field.
The hair on a geckoâ€™s feet are directed towards its heels. As a gecko steps forward, it presses on the ground and pulls back a little, enabling a maximum level of contact and generating a weak attractive force at the molecular level, called van der Waalâ€™s force, which connects the foot and the ground.
Van der Waalâ€™s force is the bonds between molecules that arise from the forces between positively and negatively charged sectors. An atom has a positively charged nucleus surrounded by a cloud of negatively charged electrons. If the positive charge of the nucleus is equal to the negative charge of electrons, the atom does not carry a charge. Electrons move randomly around the nucleus, sometimes swarming on one side of the atom for a short period of time. When this happens, one side of the atom will have a negative charge, while the other side is positive. Such polarized atoms will momentarily enter into electrostatic interactions with other atoms. This results in an attractive force, which usually lasts for a short period of time. Even though this interaction comes into existence momentarily and then disappears, it is sufficient to hold atoms together in an environment where there are many atoms.
Van der Waal forces do not only appear inside matter and among atoms; they exist even between larger objects, like our hand and the wall it touches. Yet these forces are very weak. At the atomic level, our hand looks like a range of mountains, and only the atoms on top of the summits are in contact with the wall. What makes the gecko unique is that contact occurs between the wall and each one of the millions of hairs on the geckoâ€™s feet. When the gecko lifts its foot, the attractive force is broken. Itâ€™s that simple â€“ no suction cups necessary!
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