The sickening grinding noise that a car engine makes if it runs out of oil is unforgettable. Without suitable lubrication, the microscopic 'nanoscale' machines and gears of the future will fail in the same way. US chemists have now worked out how to reduce friction at the molecular level. Their findings should help keep tomorrow's micro-machines running smoothly and may even speed up computer processing.

Hyun Kim and Jack Houston of Sandia National Laboratories, Albuquerque, New Mexico, have separated out the roles played by physical and chemical resistance, which, when combined, cause friction. They studied the friction between gold film and a one-molecule-thick lubricating layer.

They show how a lubricant's molecular structure can make it 'sticky'. This could help to design low-friction layers for specific nanotechnology applications -- just as motorists can select the most suitable engine oil for their cars.

Some microscopic metal surfaces slide past each other more easily if an extra molecule is added to the lubricating polymer between them, for example. The exact number of molecules in the chain seems to be the determining factor, not its overall length. Lubricants with an odd number of molecules have stronger internal bonding, but less adhesion to the surface.

The shape of a polymer backbone changes with the number of molecules it contains. This shape-shift increases or decreases attractive forces between neighbouring chemical groups. If more of these forces are redirected within the polymer, less are available to bind the surface, which reduces its 'stickiness' and hence the overall friction.

"It's a definite step forward," says Alan Burns, who also studies molecular friction at Sandia. "They show that overall friction can be reduced through subtle changes in film structure." This will be useful, he says, in creating new low-friction materials.

By devising inert, polymer structures that do not chemically bind the surface at all, Kim and Houston established how much friction is caused by mechanical forces alone -- that is, how difficult it is physically to move one molecule past another. By building up the surface chemical adhesion, they then show how the overall friction increases. They will report their results in the Journal of the American Chemical Society¹.

"The way they have done their experiment is novel and I think this will help scientists to further understand friction and wear phenomena at the molecular level," agrees Sujeet Sinha, who studies nano-lubrication at the National Institute of Standards and Technology, Gaithersburg, Maryland.

"But I do not think it brings any new model or explanation," Sinha goes on. "The discovery made in this work is a reaffirmation of what some other people have reported previously."

The research could help bring faster, more reliable computers. One way to speed up processing is to place the reading head closer to the hard disk drive, which stores the information. To stop the reading head sticking to the disk and crashing the computer, the head is covered and lubricated by a thin organic layer -- like those studied by Kim and Houston.

Decreasing the friction of this layer will enable computer engineers to bring the reading head even closer to the disk without it sticking. 

• References

  1. Kim, H.I. & Houston, J.E. Separating mechanical and chemical contributions to molecular-level friction. Journal of the American Chemical Society (published on-line) 17 November 2000. | PubMed |