Microelectromechanical Systems and Nanoelectromechanical Systems

A Splash of Alcohol Eases the Friction When Tiny Gears Meet

MEMS are micro machines that typically range in size from as small as a dust particle up to the size of a grain of rice (20 microns to a millimeter). MEMS are embedded in cell phones, automotive air bags, digital cameras, microphones, and ink jet printers, to name only a few of their multibillion dollar applications. More recently, MEMS accelerometers that measure the motion in handheld devices are the enabling technology in the popular Nintendo Wii gaming system.

Seong Kim has studied the interface between solids and gases for his entire career. A still boyish looking associate professor of chemical engineering at Penn State, he earned degrees at Yonsei University in Korea before taking a Ph.D. from Northwestern University, followed by postdoctoral study at the University of California, Berkeley. Recently he has added the relatively new field of nanotribology, the study of friction and wear at the nano length and time scales, to his portfolio.

Kim recalls that he became interested in MEMS (microelectromechanical systems) at a tribology conference in 2002. "Everyone was talking about the same problem: how to keep coatings from wearing off."

We Don’t Know Why It Works

In the presence of alcohol vapor, MEMS devices keep running and running, but why it works so much better than other lubricants is still not clear.

We found that there is some chemical reaction in the contact region that doesn’t normally occur,” Kim acknowledges. "The friction is forming some kind of oil, a long chain polymer."

Using mass spectrometry to look at the molecular weightof the material forming on the surface, they expected to see a peak of 87 or 88, corresponding to the type of alcohol in the vapor. What they found instead was something with a molecular weight that goes up to about 200.

But whether the long chain polymer is responsible for the remarkable lubrication or whether it is Kim’s first idea, the constant replenishment of lubrication as gas molecules.

Microengine gears
A microengine gear meshes with another gear. Credit: Courtesy Sandia National Laboratorie