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Fast sound in liquid and gas mixtures.

Fast sound in liquid and gas mixtures

T atoms or molecules that make up a gas or liquid are in constant motion. Often, the motion of one particle influences its neighbors, producing large-scale collective motions in which particles behave in a coordinated fashion. In 1986, on the basis of a computer model, Jurgen Bosse of the Freie Universitat Berlin and his colleagues predicted that a liquid consisting of two components -- one made up of lightweight particles and the other of heavy particles -- would have a special collective motion that travels through the mixture like a sound wave but at a speed several times faster than that of ordinary sound in the mixture. They dubbed this novel type of internal motion "fast sound."

Now, researchers at the University of Amsterdam in the Netherlands have detected fast sound in a hydrogen-argon gas mixture. Their results, reported in the Dec. 18 PHYSICAL REVIEW LETTERS, agree well with a computer model extending the analysis from two-component liquids to gas mixtures. "The agreement between computer and experimental results is very good," says E.G.D. Cohen, who devised the computer model with Alessandro Campa at the Rockefeller Unversity in New York City.

However, the details of how the constituent particles carry this type of motion are not yet completely understood. The heavy and light components in this mixture apparently behave somewhat independently, and only the light particles carry the fast sound. Indeed, fast sound travels at roughly the same speed at which ordinary sound would travel through a sample consisting only of the light gas. At the same time, because this type of motion dies out more quickly than an ordinary sound wave would, the heavy particles must exert some influence.

"How the separation of these two phenomena takes place, I don't know," Cohen says. "It's far from clear how this whole thing works."

Recently, Maynard J. Clouter and his colleagues at Memorial University in St. John's, Newfoundland, added another twist to the fast-sound puzzle. By observing a number of different two-component gas mixtures, they discovered that heavy particles in a mixture can also have a special collective motion -- "slow sound" -- that is detectable even in the presence of fairly large concentrations of the lighter component. "Now we require some direction from the theorists," Clouter says.
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Publication:Science News
Date:Jan 6, 1990
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