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Diffusion takes a controversial spin.

Diffusion takes a controversial spin

A drop of red dye in the middle of a large body of undisturbed liquid spreads out evenly, expanding into a spherical cloud of color centered on the dye's starting point. What would happen if the same diffusion experiment were done in a body of liquid being rapidly rotated as a whole? Recent theoretical calculations predict that for particles the size of protein molecules whirled in a high-speed centrifuge, such rotation may significantly slow down diffusion of the molescules. Moreover, the expanding cloud's shape would be oval rather than spherical.

This prediction contradicts a long-standing assumption that thermodynamic processes such as diffusion and the flow of heat are largely unaffected by the motion of the medium in which the process takes place. Researchers usually assume that the rotation of the medium is so much slower than any particle motion that it would have a negligible effect on the particles. However, the question of whether an effect could ever be observed has remained controversial.

"This situation is exactly equivalent to our usual neglect of the Earth's rotation when we do experiments in laboratories," says chemical engineer Gregory Ryskin of Northwestern University in Evanston, Ill. "In most practical situations, when you have diffusion, say, of sugar dissolving in water, it's actually an extremely good approximation. However, diffusion can also occur with particles that are so large that the approximation may break down." Ryskin's calculations, reported in the Sept. 26 PHYICAL REVIEW LETTERS, provide one of the clearest examples of a situation in which the assumption may not hold.

In Ryskin's model, a diffusing particle pushes aside liquid, causing the liquid to flow out of the way. The rotation of the liquid as a whole twists this particle-induced liquid flow into cylindrical columns above and below the diffusing particle. Those columns move with the particlea and slow it down. "In this phenomenon, it's not the force on a particle itself that gives rise to the effect," Ryskin says. "It is the Coriolis force on the fluid motion caused by the particle."

Normally, particle diffusion occurs from a region of high concentration of particles toward one of low concentration. In a sense, the particles flow down the concentration gradient. Ryskin's calculations suggest that in a rotating frame of reference, some difusion also occurs at right angles to the gradient, a surprising effect that no one has ever thought to look for.

Ryskin's theoretical predictions have yet to be tested experimentally. The effect would be most apparent in a system in which the diffusing particle's density matches that of the liquid, he says.

"If diffusion is influenced by the rigid-body rotation of the system, the theory of the centrifuge, used in biochemistry and polymer science for the characterization of macromolecules, needs modification," Ryskin says. A decrease in the mobility of large molecules diffusing through a rotating liquid may account, at least in part, for some puzzling anomalies observed in the use of high-speed centrifuges for analyzing DNA molecules.
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Author:Peterson, Ivars
Publication:Science News
Date:Oct 15, 1988
Words:498
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