Dry Feet Are More Than Just A Big Comfort Factor

Dry feet are more than just a big comfort factor if you want to run up walls as some creatures have a penchant for doing (not including Spiderman of course).

Smooth Feet
Geckos have a strategy of using soft, smooth feet. Their pads deform to give intimate contact over the whole area of their feet. When two materials are within tens of nanometers of each other, London- van der Waals forces become quite significant so they want to stick together and friction is high.
For a Gecko this is great, it can stick to a wall without sliding down and can peel off a foot to place it higher up. Very smooth surfaces like glass are best for vertical running. Stark et al from U Akron wanted to know how much a Gekko gecko could carry up a sheet of glass and what would happen in the rain (1, 2). Their pet gecko was fitted out with a body harness that was then hitched up to an electric motor and the force to drag it off the glass was measured. A gecko can hold on with loads up to 20 times its own weight.
What happens in a rainforest? The glass was misted and the gecko came perilously close to slipping. Put a liquid between to surfaces and the London-van der Walls forces are ameliorated, but also liquids shear easily – hence the slip. But geckos have very hydrophobic feet so when they place them, the water is repelled and their pads can get some close contact in patches so it’s not total disaster.
It may be close, but not total until you stand your gecko in water for an hour and a half to give it thoroughly wetted feet. At that point it can barely hold on, certainly it couldn’t carry a friend.

Hairy Feet
In a related piece the BBC draw attention to Hosoda et al who have an interest in green dock beetles (3). They observe that their beetles have hairy feet, which are oily and hydrophobic. When they walk on a wet surface, the water is displaced to trap an air bubble and the surface tension holds their feet to the leaf, wall or whatever they decide to climb.

Just don’t start spraying soap solution about or the geckos and beetles will be taking out court orders.

  1. http://jeb.biologists.org/content/215/17/i.1.full
  2. http://jeb.biologists.org/content/215/17/3080.abstract
  3. http://www.bbc.co.uk/nature/19149870

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