Oscillating Instabilities in Multicomponent Diffusion

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Abstract
An apparently new oscillatory phenomenon in gaseous multicomponent diffusion is studied experimentally and theoretically. The interdiffusion of two gases can ``drag along'' a third gas; if the third gas is heavy enough and abundant enough, the upper layers of the mixture can become more dense than the lower layers. The resulting density inversion may cause convection, and irregular oscillations result. A diffusion theory is developed which predicts fairly well the concentrations of the third gas for which the system just becomes unstable. The criterion for the onset of convection if a density inversion exists in a diffusing gas mixture is discussed by analogy with Rayleigh's treatment of convection in a heat‐conducting gas having a temperature inversion. Some multicomponent systems may have several density reversals and will be unstable for any orientation in a gravitational field. It is suggested that diffusion instabilities are quite common and may play a role in some natural and engineering processes.