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The physicist and the gecko

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This close up of a gecko walking on a glass wall shows their millions of satae. These satae are able to create friction even on smooth surfaces and must detach very carefully. (Photo: Bjørn Christian Tørrissen, http://bjornfree.com/galleries.html)

Johannes Diderik van der Waals was a Dutch physicist, who made his reputation studying the phase change between liquids and gas, winning the Nobel Prize in 1910. He was responsible for the van der Waals state equation that describes that phase change. In his own words, he "conceived the idea that there is no essential difference between the gaseous and the liquid state of matter;" that liquids "can only be regarded as compressed gasses at low temperature."

But liquids fail to obey Boyle's law. So in order to make the numbers work out, van der Waals was forced to conclude that far from being one-dimensional points, as gas molecules were sometimes regarded at the time, molecules must have some irreducible volume. Furthermore, molecules must have some attraction for each other that was different that of chemical bonds.

This attraction he referred to as "pseudo-association" to reflect the temporary nature of the resultant aggregations. To postulate such an attraction was a leap of faith in the 19th century, as the very existence of molecules was still in doubt. Van der Waals' pseudo-associations are now considered to be the result of the "van der Waals force," a somewhat ambiguous term for all of the complicated attractive and repulsive forces between molecules that are not the result of covalent or ionic bonds. 

Geckos are a family of tropical lizards that, despite having no knowledge of physics, display amazing feats of agility that are based on van der Waals forces. Geckos can walk on glass and hang from ceilings.

Their tricks are based on their amazing toes, which are jointed backwards, and each is covered with half a million or so bristles called setae, with a density of about 3 million per square inch. Each setae, in turn, branches into hundreds of split ends called spatulae. Each individual spatula is only 200 nanometers long, invisible except with the aid of an electron microscope. But the surface area covered by all of the spatulae added to together is enormous.

The spatulae take advantage of all the nooks and crannies available in the fractal dimensions of surfaces, even on something as smooth as glass. The lizard adheres via the combined van der Waals forces between the spatulae and the surface. In principle, if an average sized man had all of the spatulae of a gecko on his fingertips, he too could hang from the ceiling (they should hold up to about 130 kg (220 pounds)). 

If you catch a gecko by surprise and attempt to yank him off a surface, you are apt to leave one of his feet attached.  All that adhesion has to be detached carefully; by lifting the setae at a 30 degree angle the animal breaks the adhesive forces one by one. 

A further mystery is why the gecko's setae do not stay dirty.  Everyone who has worked with duct tape knows that adhesives are useless once they get dusty.  But the gecko's setae are self-cleaning.  They leave dirt behind in their tracks along with a kind of dry phospholipid slime. 

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This close up of a gecko walking on a glass wall shows their millions of satae. These satae are able to create friction even on smooth surfaces and must detach very carefully. (Photo: Bjørn Christian Tørrissen, http://bjornfree.com/galleries.html)
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