October 10, 2008: The race for the best “gecko foot” dry adhesive got a new competitor this week with a stronger and more practical material conjured by a team of researchers from four US institutions.
Scientists have long been interested in the ability of gecko lizards to scurry up walls and cling to ceilings by their toes. The creatures owe this amazing ability to microscopic branched elastic hairs in their toes that take advantage of atomic-scale attractive forces to grip surfaces and support surprisingly heavy loads. Several research groups have attempted to mimic those hairs with structures made of polymers or carbon nanotubes.
In a paper published in the October 10 issue of Science, researchers from the U. of Dayton, the Georgia Institute of Technology, the Air Force Research Laboratory, and the U. of Akron describe an improved carbon nanotube-based material that creates directionally-varied (anisotropic) adhesive force. With a gripping ability nearly three times the previous record — and ten times better than a real gecko at resisting perpendicular shear forces — the new carbon nanotube array could give artificial gecko feet the ability to tightly grip vertical surfaces while being easily lifted off when desired.
Beyond the ability to walk on walls, the material could have many technological applications, including connecting electronic devices and substituting for conventional adhesives in the dry vacuum of space. The research has been sponsored by the National Science Foundation and the US Air Force Research Laboratory at Wright-Patterson Air Force Base near Dayton, Ohio.
“The resistance to shear force keeps the nanotube adhesive attached very strongly to the vertical surface, but you can still remove it from the surface by pulling away from the surface in a normal direction,” explained Liming Dai, the Wright Brothers Institute Endowed Chair in the School of Engineering at the University of Dayton. “This directional difference in the adhesion force is a significant improvement that could help make this material useful as a transient adhesive.”
Scanning electron microscope images of the vertically aligned multi-walled carbon nanotubes grown for this research. (Credit: Image courtesy of Liangti Qu)
The key to the new material is the use of rationally designed multi-walled carbon nanotubes formed into arrays with “curly entangled tops,” said Zhong Lin Wang, a Regents’ Professor in the Georgia Tech School of Materials Science and Engineering. The tops, which Wang compared to spaghetti or a jungle of vines (see image above), mimic the hierarchical structure of real gecko feet, which include branching hairs of different diameters.
When pressed onto a vertical surface, the tangled portion of the nanotubes becomes aligned in contact with the surface. That dramatically increases the amount of contact between the nanotubes and the surface, maximizing the van der Waals forces that occur at the atomic scale. When lifted off the surface in a direction parallel to the main body of the nanotubes, only the tips remain in contact, minimizing the attraction forces, Wang explained.
“The contact surface area matters a lot,” he noted. “When you have line contact along, you have van der Waals forces acting along the entire length of the nanotubes, but when you have a point contact, the van der Waals forces act only at the tip of the nanotubes. That allows us to truly mimic what the gecko does naturally.”