As engineers, chemists and physicists race to design the nanomachines predicted to revolutionize everything from manufacturing to health care, other scientists are worrying about the friction—not to mention adhesion, indentation, dissipation, corrosion, and even downright self-destruction.

Dr. Jacqueline Krim, an NC State professor of physics in the College of Physical and Mathematical Sciences, is one of a new breed of scientists called nanotribologists who know that friction at the nanoscale is no small matter. Recent progress in nanotribology has demonstrated that the laws of macroscale friction simply don’t apply to atomic scale devices, and that the problems friction can generate are overwhelming in machine components with such astoundingly small dimensions.

“The technology has crashed head-on with fundamental physics and chemistry,” says Krim. “At the atomic scale, friction has very little to do with surface roughness. Some dry surfaces slide against each other easier than wet ones. And contrary to what we know about industrial scale machines, gravity (related to volume or weight) is a negligible contributor to friction when opposing objects are only a few atoms or molecules thick.”

Nanotribology [from the Greek tribo, to rub] is such a new science that Krim herself coined the term in 1986 while working at Northeastern University. A number of textbooks mention her pioneering work. She works today in the Nanoscale Tribology Laboratory on NC State’s Centennial Campus with six graduate research assistants and two post-doctoral associates. With annual funding from the National Science Foundation, the Department of Energy, and the U.S. Air Force in the half-million-dollar range, she runs one of the world’s top research groups studying the fundamental origins of friction.

Krim’s goal is to develop a lubricant that can be used at extreme
temperatures without vaporizing or freezing, reducing both heat generation and wear. Either heat or wear could inflict mortal wounds on nanomachines, where melting or shearing off a surface layer of atoms could render a device useless.

To meet their research goal, Krim’s group must first understand why a lubricant lubricates a particular surface. Their most recent discovery, now awaiting publication, occurred on a project sponsored by the Air Force. Researchers at Wright Patterson Air Force Base were searching for jet engine materials and lubricants that could withstand higher temperature stress, reducing the need for heavy, energy-guzzling engine cooling
In response, Krim set out to compare how fast the lubricant particles begin to bind or stick to the materials they are supposed to lubricate. She was astonished by the discovery that a difference in how the lubricant’s molecules flexed and slipped after they stuck was what made the difference in effectiveness. “Nothing is totally flat at the molecular scale, and microscopic irregularities of surfaces touch and push into one another as materials slide. If the lubricant begins to stick to one surface, but the molecular bonds have just the most miniscule amount of flex, sometimes that’s all that’s needed to slide the contact points of the two surfaces past each other.” (See illustration below.) Using a quartz microbalance, she and her students observed that in this case, only a picosecond of slip—one millionth of a millionth of a second—was the critical factor.

Armed with this unforeseen result, Krim is advising Air Force researchers about possible parameters for design of new lubricants and materials that allow the flexible bonding. “It can save the Air Force both time and money to have this experiment done at a university,” she says. “An even greater advantage to them may be that now any of my graduate students could step into a job in the Wright Patterson research lab fully trained to work on this problem.”

Krim says her discovery will be just as important for nanomachines as for jet engine parts. “Scientists working on nanoscale devices must understand friction at the atomic scale or their devices won’t survive the heat they generate.” For now, she’s one of perhaps a hundred nanotribologists in the world. But she predicts that as the need to conserve both energy and raw materials becomes more urgent, nanotechnologists’ rush to understand basic frictional processes can be expected only to accelerate.

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