Surprise: Study Shows Carbon Nanotubes Require More Energy To Roll Than To Slide

The study, which appears in Thursday's (Jan. 21) issue of the journal Nature, shows for the first time that atomic-scale carbon nanotubes appear to behave differently from essentially all larger solid materials. UNC-CH physicists and computer experts believe other atomic-scale particles will be found to behave similarly to nanotubes.

"There is a huge industry devoted to reducing the friction between two surfaces, and friction is why we put oil in our cars, for example, so pistons will slide along cylinder walls more easily," said Dr. Richard Superfine, associate professor of physics and astronomy. "In this work, we found something remarkable that relates to friction. These nanotubes will roll, but not as easily as much larger structures. In fact, more energy is required to roll them than to slide them."

Interactions between electrons on the two surfaces likely are responsible, he and his colleagues believe. Nanotubes, a form of soot, are created by arcing electricity between two sticks of carbon. They measure 10 to 30 billionths of a meter (nanometers) in diameter and about 1 to 5 millionths of a meter long. About eight years ago, a Japanese scientist discovered the tiny tubes, which are proving to be stiffer and stronger than any other known substance.

Besides Superfine, authors of the new paper are graduate students Michael R. Falvo and Aron Helser, Dr. Russell M. Taylor II, research assistant professor of computer science; Vernon Chi, director of the microelectronics systems laboratory; Dr. Frederick P. Brooks Jr., Kenan professor of computer science; and Dr. Sean Washburn, professor of physics and astronomy. The National Science Foundation, the National Institutes of Health, the Office of Naval Research and Topomatrix Inc. supported the continuing experiments.

In a report published in Nature in late 1997, Falvo, Superfine and colleagues discovered carbon nanotubes possess such remarkable flexibility, strength and resiliency that industry should be able to incorporate them into high performance sports and aerospace materials. Carbon fibers already are used in graphite composite tennis rackets and other products because of their strength and lightness, Falvo said. The UNC-CH research indicates that carbon nanotubes are significantly stronger than carbon fibers and hundreds of times stronger than steel.

Many other products also could be made stronger and possibly safer, scientists say, but nanotubes, or comparable spherical molecules call bucky balls, might not become the super lubricants some had hoped.

The earlier work involved bending and recording properties of carbon nanotubes with a unique device the UNC-CH researchers invented. Known as the nanoManipulator, the device combines a commercially available atomic force microscope with a force-feedback virtual reality system. The former employs an atomically small probe capable of bending and otherwise manipulating molecule-sized particles. The latter allows scientists to see and feel a representation of the surface a million times bigger than its actual size.

Researchers used the nanoManipulator in the new work to roll and to slide nanotubes on a graphite plate while measuring the amount energy each process required, Superfine said. The carbon molecules constitute an excellent model system for study because their surfaces are almost perfectly smooth. But because of interactions between electrons and the surfaces, the effect is comparable to rolling a ball bearing across Scotch tape, he said. And for the first time, investigators have found that the age-old principle of the wheel may not be the most efficient way of moving one surface past another on a nanometer scale.

"We are now trying to understand how to use these nanotubes in tiny machines," Superfine said. "For example, can we start making nanotubes move around on surfaces by applying electrical pulses to them? We're also considering other types of objects of this size to move around and study, but the challenge is to find other nanometer-sized particles with atomically smooth surfaces."