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Tight spaces make lubricants misbehave.

As engineers design ever smaller mechanical and electronic components, they need to reconsider the types of lubricants they use to keep those devices working smoothly. New research has confirmed that lubricating molecules confined to narrow spaces do not behave the same way they do in bulk form. A polymer lubricant, when squeezed between two pieces of mica separated by just a few nanometers, will act more like rubber than oil, says Steve Granick, a materials scientist at the University of Illinois at Urbana-Champaign.

Granick compared the energies needed to slide these plates when separated by the polymer, and when farther apart and separated by oils or rubbers. The polymer films proved less fluid and more elastic than scientists had thought, he says.

This rubbery nature means that as computer hard drives or engines start up, they may be subject to damage because the lubricant in narrow spaces isn't doing its job, he adds.

In addition, "dry" lubricants, called boundary layer systems, do not always act as expected, reports Jacob N. Israelachvili of the University of California, Santa Barbara. These lubricants consist of hydrocarbon chains that anchor into the surface being lubricated. They are most useful in devices employed in clean and high-vacuum environments.

Using a surface forces apparatus, he and his colleagues determined that these chains sometimes develop a different sort of slip-stick, the motion in which surfaces slide in jerks (SN: 5/30/92, p. 360). The molecules become tangled and untangled and switch from exerting normal friction to exerting very low friction, so they slide by each other more easily. "It's like combing your hair. It starts out messy, then [the comb] suddenly goes smoothly," says Israelachvili.

At high velocities, "one might be able to have low friction, an order of magnitude lower than one normally has." he adds.
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Title Annotation:polymer films prove less fluid and more elastic when squeezed into narrow boundaries
Author:Pennisi, Elizabeth
Publication:Science News
Article Type:Brief Article
Date:Dec 19, 1992
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