Printer Friendly

New physics for liquids in tight spots.

Liquids confined in very narrow spaces don't always behave as expected. But the way they do act suggests that some basic physical principles may be involved, theorists now conclude on the basis of computer simulations.

Increasingly, researchers depend on simulations to predict and visualize the molecular mechanisms of friction (SN: 5/30/92, p.360). During the past two years, Peter A. Thompson, a physicist at Exxon Research and Engineering Co. in Annandale, N.J., and his colleagues have used simulations to study lubricants made of sphrerical molecules.

Now, Thompson, Exxon colleague Gary S. Grest and Mark O. Robins, a physicist at Johns Hopkins University in Baltimore, have examined lubricants made with flexible, chain-like molecules.

On the computer, the researchers compress a lubricant between two surfaces and watch on the terminal screen how the molecules -- represented by beads or chains of beads -- behave. Confined spherical molecules crystalize much more quickly when squeezed than do molecules that are not hemmed in, Thompson says. When rubbed hard enough between one moving and one stationay surface, these molecules briefly escape their orderly array, then recystallize, creating a friction called "stick-slip" motion.

Long molecules also work this way, "but here the solid state is a glassy , entangled system," Grest says. The scientists simulated lubricants with six to 20 building blocks, called monomers. All of the long molecules remained fluid much longer than the spherical ones, they report in the June 8 Physical REVIEW LETTERS.

Also, unexpectedly, these fluids showed a similar response to rubbing -- shearing -- by a surface moving at increasing speeds: Friction increased exponently as the shear rate increased, and that exponential relationship held constant over a range of molecules and surfaces. "That's very surpricing," Thompson says.

Last years, Steve Granick's group at the University of Illinois at Urbana-Champaign observed a similar relationship between shear rate and friction in ultra-thin films of the flexible molecule dodecane. Some researchers questioned those results, says Grest, " but seeing it in simulations of a totally different system in a totally different situation helps" support the earlier finding. Thompson's group suggests a universal relationship may exist among polymer fluids because, when confined, these fluids convert so easily to glassy states.
COPYRIGHT 1992 Science Service, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:molecular mechanism of friction
Author:Pennisi, Elizabeth
Publication:Science News
Article Type:Brief Article
Date:Jun 20, 1992
Previous Article:Two steps forward in AIDS vaccine search.
Next Article:Poll finds global environmental concern.

Related Articles
Friction features.
Liquids that tiptoe on the edge of solidity.
Quantum swirls in superfluid helium.
Friction flicks; computer animation offers insights into friction's molecular underpinnings.
'Baked Alaska' cooked up in liquid helium.
New rubber friction testing machine.
Skating on thin water.
Snowboard superstar. (Physical Science: Newton's Law of Motion).
Piddly puddle peril: little water pools foil road friction.

Terms of use | Copyright © 2017 Farlex, Inc. | Feedback | For webmasters