Magazine article Science News

Diffusion Takes a Controversial Spin

Magazine article Science News

Diffusion Takes a Controversial Spin

Article excerpt

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. …

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