Non-selective evaporation of ethanol-water binary mixture within heated capillary

In this study, the evaporation of ethanol-water binary mixture within heated capillary is experimentally and numerically investigated. The ratio of ethanol and water evaporation rates was found to be equal to that of their initial concentration in the mixture. This observation contradicts the widely accepted belief of selective evaporation, where the ratio of ethanol-to-water evaporation rates is expected to be considerably higher than the concentration ratio owing to the higher volatility of ethanol. We refer to this novel phenomenon as non-selective evaporation. Subsequently, numerical analysis revealed that, within the heated capillary, changes in component concentration caused by ethanol preferential evaporation appear solely in a small area known as the diffusion layer near the meniscus. When the diffusion layer was fully developed, the evaporation process shifted from selective to non-selective. Because the duration of the selective evaporation stage was short, the evaporation process exhibited distinct non-selective characteristics. Furthermore, by considering the coupled effects of convection and diffusion, an analytical model was proposed, and the Peclet number criteria were established to determine whether the evaporation process of binary mixture exhibits selective or non-selective characteristics.