Change in vertically-multi-walled carbon nanotubes during adhesion measurements.

Change in vertically-aligned multi-walled carbon nanotubes during adhesion measurements.

What can a team of highly trained researchers from four different U.S. universities learn from the feet of a gecko lizard? According to the researchers, how to improve carbon nanotube-based material so that, for the first time, it demonstrates “directionally varied (anisotropic) adhesive force” and gripping power nearly three times the level of existing nanotube dry adhesives.

The team – comprised of researchers from the University of Dayton, the Georgia Institute of Technology, the Air Force Research Lab and the University of Akron – describes its achievement in a paper published in the Oct. 10 edition of Science magazine.

As Science reports, the team believes their advance could lead to solder-free connections between electronic devices, longer-lasting adhesives for use in outer space and a broad range of other important applications.

In the article, team members explain that a gecko’s ability to scale vertical walls is due to atomic-scale van der Waals interactions that occur naturally in the microscopic hairs on the lizard’s toes.

These hairs – actually minute setae – give the gecko resistance to perpendicular shear force, enabling it to grab vertical surfaces with surprising strength. The setae also allow the gecko to easily release its strong hold.

By manipulating carbon nanotubes to simulate and intensify the anisotropic adhesive forces at work in gecko hairs, the team has created a carbon nanotube dry adhesive that is “ten times better than a real gecko at resisting perpendicular shear forces.”

According to team member Zhong Lin Wang, a Regents Professor in Georgia Tech’s School of Material Science and Engineering, the newly developed adhesive’s performance depends on the use of “rationally designed multi-walled carbon nanotubes formed into arrays with curly entangled tops.”

Wang likens the tangled tops to a “jungle of vines” that replicates the structure of a gecko’s foot, down to its “branching hairs of different diameters.”

These tangled tops become aligned with a surface when pressed against it, significantly increasing the contact area between the tops and the surface, Wang says.

Wang says, “When lifted off the surface in a direction parallel to the main body of nanotubes, only the [nanotube] tips remain in contact [with the surface], minimizing the forces of attraction.” He claims this “allows us to truly mimic what the gecko does naturally.”

Wang explains that, “When you have line contact along [a surface], 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.”

As the Science article reports, the researchers have tested their new adhesive’s grip on a number of surfaces, including glass, polymer, Teflon and rough sand paper. Wang says they found it measured up to 100 newtons per square centimeter in the shear direction and only 10 newtons per square centimeter in the normal direction. The team’s conclusion, he says, is that resistance to shear increases with nanotube length, while resistance to normal force is independent of tube length.

Funded by NSF, the project is led by the University of Dayton’s Liming Dai, the Wright Brothers Institute Endowed Chair in the UD’s School of Engineering. The research team also includes the UD’s Liangti Qu, Morley Stone from the Air Force Research Lab and Zhenhai Xia from the University of Akron.