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Posted: July 24, 2006

Twin friction-tricks grease nano-wheels

(Nanowerk News) Two tricks that could eliminate friction on the nanoscale have been revealed by physicists in Canada, Switzerland and the US.
One of the techniques involves subtly vibrating nano components to move surfaces apart. The other uses electric charges to reduce friction between contacting surfaces.
Efforts to develop nanoscale mechanical machines will prove futile unless the debilitating effects of friction can be overcome on this scale. Lubricants normally used to ease friction inside larger machinery simply do not work at the micro or nano scale.
"Traditional liquid lubricants become too viscous when confined in layers of molecular thickness," explains Anisoara Socoliuc at the University of Basel in Switzerland, who developed one of the friction-defeating tricks with her colleagues.
Surface wear
In a washing machine or car engine gears, cantilevers and shafts experience friction when components come into contact with one other. Their relative motion causes surface wear and also converts kinetic energy into heat. But the problem is even more serious for components measured in nanometres, or billionths of a metre.
"The much greater surface-to-volume ratios characteristic of these devices lead to serious adhesion and wear problems," Socoliuc explains.
Socoliuc and colleagues worked alongside Roland Bennewitz at McGill University in Montreal, Canada, to develop a solution. They reduced friction between the tip of an atomic force microscope and a flat salt crystal surface by vibrating the silicon tip 56,000 times per second. These vibrations reduced friction 100-fold and also eliminated any noticeable wear on the salt surface. Socoliuc says this could prevent nano-devices from becoming locked together through "stiction", or friction between two static surfaces.
Voltage changes
The second friction-busting ploy was developed by Jeong Young Park and colleagues at Lawrence Berkeley National Laboratory in California, and Iowa State University in Ames, both in the US. This involved applying different voltages to semiconducting silicon to see how it affected surface friction.
They found they could vary friction levels by simply altering the voltages. By making nanomachines from semiconducting components and altering applied voltages, they hope to alter the frictional forces experienced at will.
"Micro and nano-devices with sliding interfaces like actuators and microgears cannot yet be commercialised because their surfaces wear out and seize too rapidly," says Robert Carpick, a physicist at the University of Wisconsin in Madison, US. "There is a critical need to control the effect of friction at this scale."
Carpick adds that both the new friction-relieving tricks provide "beautiful and enticing" ways to solve the problem of nanoscale friction.
Journal reference: (Science: vol 313 p186, p207)
Source: Paul Marks, NewScientist.com news service
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