The amazing ability of some geckos to scale smooth walls and cling to ceilings could be primarily a result of contact electrification. That is the claim of researchers at the University of Waterloo in Canada, who have made a new study of the electrostatic interactions between the lizard’s feet and two different surfaces. Their conclusion contrasts with conventional thinking, which attributes the stickiness of gecko feet to Van der Waals forces.
The exceptional climbing ability displayed by many geckos comes from their specially adapted toe pads. Each pad is covered in layers of microscopic, hair-like structures – or setae – that split into smaller, spatula-shaped tips. Being so small, the tips can get close to the surfaces on which the geckos walk, forming an intimate contact. Each seta contributes only a tiny attraction, but together they produce a combined adhesive force of about 10 N for each foot, which allows geckos to hang from a ceiling by a single limb. Letting go is not a problem because the adhesive effect is directional, allowing a gecko to detach by simply re-orientating its foot.
According to conventional theory, the attraction is a result of Van der Waals interactions. These are the weak dipole–dipole forces that act between adjacent atoms and molecules as a result of shifting electron concentrations.
Exchanging electric charges
In their new study, Alexander Penlidis and colleagues looked at how contact electrification could contribute to gecko adhesion. This effect occurs when two materials touch and exchange electric charges. The result is a net negative electrostatic charge on one material and a positive charge on the other, which causes an attractive force between the two.
To test whether these interactions could be contributing to the adhesive abilities of geckos, the researchers measured the electric charges and adhesive forces generated when gecko toe pads were stuck on two insulating polymer surfaces – one of Teflon AF and one of polydimethylsiloxane. In both cases, on contact, the geckos’ toe pads became positively charged and the surfaces negatively charged. Furthermore, the adhesion strength correlated with the magnitude of the electrostatic charge that was generated. Despite having a lesser potential for generating Van der Waals forces, Teflon AF was seen to have a much stronger adhesion than the other substrate. This, say the researchers, suggests that contact electrification plays a major role in gecko adhesion.
Still clinging in ionized air
The finding could overturn 80 years of conventional wisdom that electrostatic interactions are not involved in gecko adhesion. Penlidis and colleagues believe this dismissal can be traced back to an experiment described in 1934 by the German scientist Wolf-Dietrich Dellit. Ionized air – which would neutralize electrostatic interactions – was blown towards a gecko clinging to a metal surface and had no effect on its ability to hang on. Penlidis and colleagues explain that Dellit’s observation is consistent with their conclusion because the contact between seta and substrate is so close that ionized molecules in the air would not be able to get between the two to neutralize the interaction.
While contact electrification could play an important role in a gecko’s ability to scale smooth walls, it is not clear whether the force is any help on rougher surfaces. “The [research] clearly shows how electrostatic forces can play an additional role in enhancing adhesion in geckos, which is an aspect that had not been previously considered,” says Duncan Irschick, a biologist at the University of Massachusetts who is developing a synthetic, reusable adhesive based on gecko feet. Irschick questions, however, “whether such forces are relevant for natural surfaces that geckos have evolved to use, such as leaves, trees, etc”.
In 2002 Kellar Autumn of Lewis & Clark College in Oregon was the first to observe the Van der Waals interaction in gecko feet. Commenting on this latest research, he says that “This is a novel and important discovery, and suggests that electrostatic forces could contribute to adhesion in geckos on some surfaces, such as Teflon.” However, Autumn is not convinced that electrostatic forces are dominant, pointing out that this conclusion is not supported by the results of the study. “Moreover, the use of whole animals rather than isolated setae, and only one axis of force measurement, makes the results difficult to interpret,” he adds.
The research is described in the Journal of the Royal Society Interface.