The physics of sticky gecko feet


Geckos are known for their remarkably adhesive feet. In a study published Tuesday in the Journal of Applied Physics, researchers describe just how remarkable those feet are.

It turns out that gecko feet are able to stick and unstick themselves instantly — so when a gecko goes from hanging off of a ceiling to scurrying down a wall, it does so without losing speed or energy. This mechanism makes gecko feet, which were already being used as inspiration for wall-climbing robots and space-age adhesive materials, even more interesting to engineers.

In 2000, researchers determined that the reptiles rely on van der Waals forces — the very small forces between molecules — to accomplish the feat. Their toes are covered with hairs called setae, arranged in gill-like rows. Each tiny hair produces only a small van der Waals force when interacting with a surface, but together the multitude create reliable suction.

There’s more to it than that, according to lead study author and Oregon State University assistant professor of mechanical engineering P. Alex Greaney. “These are really fascinating nanoscale systems and forces at work,” Greaney said in a press release. “It’s based not just on the nature of the seta, but the canted angles and flexibility they have, and ability to work under a wide range of loading conditions.”

The setae work at the nanoscale to turn stickiness on and off instantly, Greaney said, and expend hardly any energy in doing so. It only takes a small angled forced to push setae into their “sticky” position, and without that force they unstick seamlessly. That’s why, according to Greaney, setae can support 50 times a gecko’s body weight as it hangs from the ceiling but don’t stop the gecko from running at speeds of 20 body-lengths per second.

Complex networks of setae, which can create millions of contact points between a gecko’s feet and the rough surface it’s climbing on, have already inspired dry adhesives for robots and other technology. But Greaney thinks this mathematical model will improve on those efforts.

“While we don’t envision ‘Mission Impossible’ sticky gloves, which are inspired by or based on the concept of gecko adhesion,” Greaney said in a release, “we envision that robots will use gecko adhesion in extreme environments in the future.”