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, 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."
In tests done on a variety of surfaces including glass, a polymer sheet, Teflon and even rough sandpaper the researchers measured adhesive forces of up 100 Newtons per square centimeter in the shear direction. In the normal direction, the adhesive forces were 10 Newtons per square centimeter about the same as a real gecko.
The resistance to shear increased with the length of the nanotubes, while the resistance to normal force was independent of tube length.
Though the material might seem most appropriate for use by Spider-Man, the real app
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Georgia Institute of Technology Research News