Imagine a dry compound that can conductively bind components to a printed-circuit board without the high heat associated with various soldering processes. Or, how about an adhesive that never dries out in a vacuum—a common problem in aerospace applications? For the adventurous, imagine wearing a suit that would allow you to easily scale walls and hang from ceilings like a superhero.
These fantasies may not be too far from reality as scientists look to the gecko for answers. For some time now, researchers have observed the ability of geckos to cling to surfaces, be they rocks, trees, walls, and ceilings, vertically and horizontally, and even upside down, with their toes. This ability is attributable to microscopic hairs with an elastic quality in the gecko’s toes that exert attractive forces on an atomic level (Fig. 1).
GECKOS AND NANOTUBES
In their attempts to replicate these adhesive hairs, scientists have been experimenting with certain polymers and carbon nanotubes. In October, researchers at the University of Dayton, Georgia Institute of Technology, Air Force Research Laboratory, and University of Akron unveiled the first carbon nanotube material that not only apes the gecko’s anisotropic adhesive properties, but does so with an adhesive ability about three times better than previous models and with 10 times greater resistance to perpendicular shear forces than the gecko.
Using the unique carbon nanotube array, these researchers expect to develop artificial gecko feet that can grip vertical surfaces while being easy to remove. According to Georgia Tech Regents Professor Zhong Lin Wang, the unique material relies on rationally designed, multiwalled carbon nanotubes that form arrays with entangled tops (Fig. 2). These tops mimic the architecture of gecko feet and hairs of varying diameters.
Pressing these nanotubes upon a surface, their tangled tops align with the surface, and contact between them significantly increases. This increase in contact of the tops with each other causes an increase in van der Waals forces occurring on the atomic level, which promotes adhesion.
“The contact surface area matters a lot. 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,” Wang says. “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.”
Liangti Qu, a research assistant at the University of Dayton, fabricated the vertically aligned, multiwalled nanotube arrays via a low-pressure, chemical-vapor deposition process on a silicon wafer. The first segments grew in random directions, creating the coiled and entangled tops.
During lab tests, performed with a number of surface types including glass, Teflon, polymer sheets, and sandpaper, the researchers measured adhesive forces in the shear direction of about 100 Newtons per square centimeter. Measurement in the normal direction reveals an adhesive force approximately the same as a gecko at 10 Newtons per square centimeter (Fig. 3). In essence, resistance to shear force increases with the length of the nanotubes while resistance to normal force appears to be independent of nanotube length.
APPLICATIONS AND FUTURE RESEARCH
Since carbon nanotubes conduct both heat and electrical current, a dry adhesive formed with the gecko-like nanotubes could provide an efficient method for connecting electrical components. “Thermal management is a real problem, and if you could use a nanotube dry adhesive, you could simply apply the devices and allow van der Waals forces to hold them together. That would eliminate the heat required for soldering,” Wang says.
With eyes on the skies, the researchers also foresee adhesives that will hold up for long periods in outer space. “In space, there is a vacuum and traditional adhesives dry out. But nanotube dry adhesives would not be bothered by space environments,” says University of Dayton researcher Liming Dai.
A bit more work is necessary before we see these dry adhesives on the market. The researchers need to analyze various surface interactions to boost adhesive force as well as determine longterm reliability.
“As surfaces may not be uniform, the adhesive force produced by a larger patch may not increase linearly with the size. There is much we need to learn about the contact between nanotubes and different surfaces,” Dai says.