The counter-current diffusion-driven mixing process of two miscible fluids is studied in the absence of gravity, assuming that the mixture is non-regular, that is its volume is smaller than the sum of the initial volumes of the two components. Two competing effects are present in the mixing region: on one hand, the mass flow rate of each species increases, due to the larger density of the fluid; on the other hand, though, the volumetric flux is retarded by the inward convection due to volume

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... appearance, which opposes the outward velocity field due to diffusion. This intuition is confirmed by the analytical result of a 1D non-ideal mixing process, showing that, in the presence of the convection induced by a volume decrease: (a) the process is self-similar; (b) the mass flux of each species at the interface increases by approximately 0.8 , where is the maximum relative volume decrease; and (c) the volume flux of each species decreases by approximately a 0.2 amount. This result is further confirmed by a perturbation analysis for small . C 2013 AIP Publishing LLC.