Geckos, renowned for the ability to stick to walls or cling upside down to overhead surfaces, have another trick up their sleeves -- well, on their feet -- the ability to turn that stickiness on and off at will, researchers have discovered.
Scientists have long been attempting to duplicate the feet and the feat of geckos, and have successfully created technologies including robots that can climb walls and incredibly sticky space-age adhesives.
However, those are all-sticky-all-the-time duplications, which the geckos can go one better by being sticky-footed or not at will.
Geckos are able to cling to vertical or overhead surfaces through their broad toes, which are equipped with hundreds of microscopic hairs known as setae, each of which branches into further hundreds of tinier bristles dubbed spatulae.
Those tufts of microscopic hairs can conform so closely to the surface contours of a surface that an effect known as the van der Waals force can operate.
That's a physical bond that occurs when tiny forces between electrons in closely adjacent molecules -- for example in the gecko's toe bristles and a wall -- begin to interact and create a type of electromagnetic attraction.
What's been discovered in the latest study, reported in the Journal of Applied Physics, is that the geckos can turn that nanoscale attraction on and off at will, allowing them to unstick and then re-stick their feet as they scurry at high speed across a wall or ceiling.
A gecko's toe bristles are incredibly flexible, the researchers found, and a combination of flexibility, angle, and extensibility of the bristles yields incredibly strong and robust adhesion, but one the geckos can unstick without having toe expend energy.
"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," says study co-author Alex Greaney, an engineering professor at Oregon State University. "Geckos' feet are by default nonsticky, and this stickiness is activated through application of a small shear force."
The angled force needed to push a gecko's setae into a "sticky" position is very small, and when the force is removed they quickly unstick seamlessly, Greaney says.
That's why although the setae are able to support 50 times the creature's weight as it clings to a ceiling, they don't immobilize it, allowing the gecko to move along a surface at a rate of 20 body-lengths per second.
This understanding will help improve man-made technologies that mimic the geckos' ability, Greaney says.
"While we don't envision Mission Impossible sticky gloves, which are inspired by or based on the concept of gecko adhesion, we envision that robots will use gecko adhesion in extreme environments in the future," he says.
