MANHASSET, N.Y. Knowing how much force is needed to pull a material off of another material is essential for most manufacturing. Currenlty no test exists for applying this knowledge to nanoscale structures.
Now, researchers at Purdue University have precisely measured the forces required to peel tiny nanotubes off of other materials, creating a possible standard for nanomanufacturing.
Flexible carbon nanotubes stick to surfaces differently than larger structures due to attractive forces between individual atoms called van der Waals forces.
"These forces are very relevant on this size scale because a nanometer is about 10 atoms wide," said Arvind Raman, an associate professor of mechanical engineering at Purdue (West Lafayette, Ind.).
Mechanical engineering doctoral student Mark Strus made the first peeling-force measurements for nanotubes at the Purdue Birck Nanotechnology Center.
The energy it takes to peel a nanotube from a surface was measured in "nanonewtons." According to Strus, this compares to perhaps a billion times less energy than that required to lift a cup of coffee.
That peeling energy is proportional to the nanotube's "interfacial energy," which is one measure of how sticky something is, according to Strus. "Until now, no such test had been completed to successfully measure and quantify these forces on the nanoscale," said Strus.
Properly integrating high-strength nanotubes into polymers for composite materials requires knowledge of how the nanotubes stick to polymers and to each other.
Nanotubes also must be dispersed uniformly in a solution before being mixed with the polymer to make composite materials, but the tiny rods tend to clump together. Learning precisely how the tubes adhere to each other could lead to a method for dispersing them.
The findings also promise to help researchers understand how geckos are able to stick to surfaces, a trait that could translate into practical uses for industrial and military applications.
Tiny branching hairs called setae on the animal's front feet use van der Waals adhesion. "The question is, How does it stick, and, equally important, if the adhesion force is strong enough to hold its weight onto a surface like a wall, then how does it then unstick, or peel, itself to move up a vertical surface?" Strus said.
Nanotubes also have possible medical applications where knowing precisely how nanotubes adhere to cells can create more effective bone grafts and biomolecular templates to replace damaged tissues.
Another potential application is a "nanotweezer" that might use two nanorods to manipulate components for tiny devices and machines.
Raman and Strus plotted how much force it took to peel nanotubes from surfaces, discovering that the tubes lift off in fits and starts, not smoothly. "We saw these jumps in peeling forces, where the nanotubes would lift off suddenly and then snag, lift off suddenly and then snag. This behavior has a very deep physical significance and can only be appreciated by means of mathematical models," Raman said.
The researchers next fine tuned their theoretical model, which describes the physics of why nanotubes peel off unevenly. The nanotubes used in the research had a length of about 6 microns and meansured 40-nm wide.
The researchers used an atomic force microscope to measure the peeling forces. The nanotube was attached to the end of microsope's microcantilever. As the nanotube was pulled away from a surface, the cantilever bent. The bending movement was tracked with a laser, revealing the forces required to peel the nanotube.
NASA provided the carbon nanotubes for the Purdue research. The research was funded by the National Science Foundation and the Korean Center for Nanomanufacturing and Mechatronics.