Lizard, lizard on the wall, why is it you do not fall?
Scientists have been asking that question of the limber little gecko for at least 75 years. These scaly climbers can scuttle nimbly across a polished glass ceiling. In a pinch, they can hang by one toe.
Over the decades, experiments and observations have ruled out suction, electrostatic forces, and glue as possible explanations. Now, a West Coast team of scientists and engineers has discovered that a surprisingly large sticking force arises when tiny hairs, or setae, on gecko feet rub up against surfaces.
From each seta sprout even tinier stalks, called spatulae. When a gecko gloms a foot onto a surface, the billion-or-so spatulae that carpet its sole snuggle so close to the surface that intermolecular forces such as van der Waals bonds may come into play, the researchers say.
"Our calculations show that van der Waals forces could explain the adhesion, though we can't rule out water adsorption or some other type of water interaction," says Robert J. Full of the University of California, Berkeley.
He, Kellar Autumn of Lewis and Clark College in Portland, Ore., and their colleagues report the first direct measurements of the force on a single seta in the June 8 Nature. Wowed by the strength of that attraction, the researchers have also set out to develop an adhesive modeled on gecko anatomy.
The scientists got a tip about how setae adhere from the peculiar walking style of geckos.
The lizards set their toes down with motions like tongues uncurling and then lift them again as if peeling up adhesive tape.
The downward step apparently both presses the setae against the surface and tugs them parallel to it. The new tests, done on individual setae, showed that the tugging action increases each seta's grip 10-fold compared with just pressing. Setae hold fast until the angle between them and a surface reaches about 30 degrees, the research also shows. When geckos unpeel their toes, they presumably lever setae to that critical angle, the scientists say.