If you've ever spent any time watching a gecko（壁虎）, you may have wondered about their uncanny（神秘的） ability to adhere to any surface -- including upside down on ceilings. It turns out the little lizards can turn the "stickiness" of toe hairs on the bottom of their feet on and off, which enables them to run at great speeds or even cling to ceilings without expending much energy. In the Journal of Applied Physics, from AIP Publishing, Oregon State University (OSU) researchers describe their work exploring the subtleties of geckos' adhesion system mechanism.

"Since the time of the ancient Greeks, people have wondered how geckos are able to stick to walls -- even Archimedes is known to have pondered this problem," said Alex Greaney, co-author and an assistant professor of engineering at OSU. "It was only very recently, in 2000, that Kellar Autumn and colleagues proved unequivocally（明确地） that geckos stick using van der Waals forces."

Van der Waals forces are weak atomistic level forces, "but geckos are able to take advantage of them because of a remarkable system of branched hairs called 'seta' on their toes," Greaney explained. "These seta and their hierarchy can deform to make intimate contact with even very rough surfaces -- resulting in millions of contact points that each are able to carry a small load."

Geckos -- as well as spiders and insects -- have independently evolved the same adhesion system mechanism and have been using it for millions of years.

"Understanding the subtleties of the process for switching stickiness on and off is groundbreaking," said Greaney. "By using mathematical modeling, we've found a simple, but ingenious, mechanism allows the gecko to switch back and forth between being sticky or not. Geckos' feet are by default nonsticky, and this stickiness is activated through application of a small shear force. Gecko adhesion can be thought of as the opposite of friction."