Vapour transfer into ethanol drops drying in a humid atmosphere*

We report adsorption-absorption and/or condensation of water vapour into ethanol drops during their evaporation in humid air. Combining drop profile measurements with Gas Chromatography allows to directly quantify the amounts of ethanol evaporated and of water intake over time. Increases in the rate of ethanol evaporation and in that of the water intake are observed at higher relative humidity (RH). The increase in ethanol evaporation at higher RH is attributed to the greater diffusion coefficient of ethanol into moist air than dry air. Moreover, the drop surface temperature is found to fall below the dew point due to evaporative cooling in at higher RH, leading to more water condensation onto the drop. The heat released by the water vapour intake gives rise to a higher temperature at the drop surface and hence a greater driving force for evaporation. Based on the experiments, we propose an empirical correlation of the water intake combined with the diffusion model for drop evaporation, which agrees remarkably well with experimental observations.