US scientists have found the first experimental evidence that geckos use electrostatic forces – not capillary action – to walk safely across even very smooth ceilings. To compare the electrostatic and capillary theories, Kellar Autumn of Lewis and Clark College in Oregon and colleagues measured the shear forces exerted by geckos’ toes on a variety of surfaces. The team’s discovery that geckos employ Van der Waals forces in this way could now be exploited to develop a new generation of ‘dry adhesives’ (K Autumn et al 2002 Proc. Natl Acad. Sci. 10.1073 pnas.192252799).
Geckos are well known for their ability to walk on smooth walls and ceilings, and biologists have long proposed that this ‘stickiness’ arises from capillary forces between these surfaces and the hairs on their toes. But this theory could not explain how geckos walk easily across both hydrophilic – or water-attracting – and hydrophobic – water-repelling – surfaces.
So Autumn’s team set out to establish whether an alternative theory – based on Van der Waals forces – could account for this ability. Van der Waals forces are weak electrostatic attractions between adjacent atoms or molecules that arise from fluctuations in the positions of their electrons. If these forces act over a relatively large area, they can build up a significant attractive force.
The researchers placed the toes of live Tokay geckos onto various hydrophilic and hydrophobic semiconductor wafers, with both high and low dielectric constants. As they gently pulled the geckos down the wafers – which were vertical – the researchers measured the shear forces exerted on the wafers by the geckos’ toes.
The geckos’ toes stuck equally well to both the hydrophobic and hydrophilic surfaces, but the researchers found that they slipped on surfaces with very low dielectric constants. If the geckos had used capillary action to stick to surfaces, they would have slipped on the hydrophobic wafers. But they slipped on surfaces with low dielectric constants, which showed that electrostatic forces were at work.
The team believed that the geometry of the hairs on the geckos’ toes made the contact area between the toes and the surfaces large enough for the Van der Waals attraction to become significant. To back up their findings, they made accurate models of geckos’ toes from two different materials, and showed that both sets of toes had similar adhesive qualities. This confirmed that the geometry of the hairs on the toes is more important than any chemical reactions that might take place between them and the surface.
According to the researchers, a new generation of dry adhesives – which would in a wide range of conditions – based on Van der Waals forces could now be developed.
