Prediction of air-water interfacial area in wet unsaturated porous media

A numerical model is developed to predict the air-water interfacial area of water-wetted unsaturated porous media. A model space is first designed consisting of elements capable of containing a single material. Water, air, and quartz are considered in this work. The elements are square in two dimensions and cubic in three dimensions. The contents of elements containing quartz are considered immobile, while the contents of elements containing fluid (air and water) are treated as mobile. The fluid elements are then arranged in a manner to minimize the total interfacial energy of the system by the simulated annealing optimization technique. Air-water interfacial area predictions are compared to experimental data for two systems of glass beads. Predictions are excellent at high water contents, but fail at low water contents. The failure of this and other thermodynamic models at low water contents is explained by low hydraulic conductivity, which prevents low water content systems from reaching thermodynamic equilibrium.