Hold Key To Future Technologies

So-called "peel tests" are used extensively in manufacturing. Knowing how much force is needed to pull a material off of another material is essential for manufacturing, but no tests exist for nanoscale structures, said Arvind Raman, an associate professor of mechanical engineering at Purdue.

Researchers are trying to learn about the physics behind the "stiction," or how the tiny structures stick to other materials, to manufacture everything from nanoelectronics to composite materials, "nanotweezers" to medical devices using nanotubes, nanowires and biopolymers such as DNA and proteins, he said.

Flexible carbon nanotubes stick to surfaces differently than larger structures because of attractive forces between individual atoms called van der Waals forces. "Operating in a nanoscale environment is sort of like having flypaper everywhere because of the attraction of van der Waals forces," Raman said. "These forces are very relevant on this size scale because a nanometer is about 10 atoms wide."

The energy it takes to peel a nanotube from a surface was measured in "nanonewtons," 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, Strus said. "This whole idea of measuring the stickiness of something is a standard material test in industry," he said. "There are certain tests that you need to have for measuring strength, toughness and adhesion."

Nanotubes offer promise to produce a new class of composite materials that are stronger than conventional composites for use in aircraft and vehicles. "This is a big area of research primarily because the strength of nanotubes can be much greater than that of carbon nanofibers," Raman said.

However, properly integrating high-strength nanotubes into polymers for composite materials requires a knowledge of how the nanotubes stick to polymers and to each other.

"One of the big areas in composites, in general, and nanocomposites, in particular, is how to coat a fiber with a material that makes it stick better to the matrix," Raman said. "So it's really important to know how to judge which coatings work best for specific types of fibers. For larger fibers, industry knows which coatings work best, but such knowledge is scarce for nanoscale fibers. It's all about how to make nanotubes 'sticky' to the surrounding matrix."

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.

COMPAMED.de; Source: Purdue University