The effect of the disjoining pressure on the surface evaporation of a porous medium

The behavior of surface evaporation of a porous medium is investigated experimentally. A theoretical analysis is used to describe the experimental results in this work. Quartz sand (10-20 mesh) is packed as the porous medium. Pure water with electric resistance greater than 10 M Omega and aqueous solution with various concentration of sodium sulfate are used as working liquid in the experiments. Electric light radiation is used as the heat source. Free convection is used in the experiments. The results of the experiment show that the concentration of the salt has great influence on the rate of surface evaporation. A model of microlayer evaporation is developed to predict the drop of evaporation rate. The agreement of experimental results and theoretical results show that the evaporation of the microlayer, driven by the gradient of the disjoining pressure, as well as the interface of the liquid surface curvature due to meniscus effects is important.