Quote: Furry Logic [Chapter 2]

Furry Logic: The Physics of Animal Life - Matin Durrani, Liz Kalaugher

"Before we deal with how geckos cope with wet surfaces, let’s find out how they hang on to dry ones. Yup, it’s all about physics. As experiments by Kellar Autumn of Lewis and Clark College, US, showed in 2002, the gecko uses van der Waals forces, the tiny attractions between molecules. Named after Dutch physicist Johannes Diderik van der Waals (1837–1923), these forces are like a mini-gravity. The science gets scary but basically the forces arise because each molecule contains electrons whizzing round in random orbits. These charges set up electric fields that can temporarily attract another molecule close by.

 

Since they’re due to weakly charged electrons, these forces are weak too. Typically, they might provide an adhesion energy – a measure of how much the molecules want to stay together – of 50–60 millijoules (mJ) per square metre. So how do van der Waals forces counteract the body weight of a 100g (3.5oz) gecko? There’s another snag: van der Waals forces only work over distances of less than 10 nanometres. That’s smaller than the size of a virus, so the gecko must plonk its feet right up close to the surface it’s trying to cling to; the molecules in its skin and the ceiling must be near enough to attract. To achieve this, species like the tokay gecko have fleshy folds known as ‘lamellae’ covering the soles of their feet. The result is like a rubber tyre tread, with folds spanning the width of each of the toes. In spite of its fleshy feet, the tokay gecko is beautiful, with large eyes with a vertical slit pupil, and a pale blue or grey body speckled with a mix of yellow, orange or bright red spots, as if decorated by a pointillist on acid."