Unmixing gases with sound

Thermoacoustic processes use an oscillating pressure field to create a thermal gradient, or vice versa. (See the article in Physics Today, Physics Today 0031-9228 48 7 1995 22  https://doi.org/10.1063/1.881466 July 1995, page 22 .) In a new twist, thermoacoustics is being used to separate the constituents of a binary gas mixture by mass. At Los Alamos National Laboratory, Greg Swift and his colleagues coupled speakers to a long pipe connecting two reservoirs and filled the system with a mixture of two gases. During the pressure maximum of an acoustic cycle, the heavier gas diffused to the viscous boundary layer near the pipe’s wall, while the lighter gas was carried along the pipe. In the second half-cycle, during the pressure minimum, the heavier gas was drawn out of the boundary layer and transported in the opposite direction along the pipe. The first separation experiments were done in Swift’s lab by...