Measurement and modeling of hydraulic characteristics of unsaturated porous media with mixed wettability

The spatial distribution of the solid grainʹs wettability strongly influences the phase-interconnection, phase-flow and solute transport in unsaturated porous media. Non-uniform wetting conditions can arise in soils and aquifers from natural or anthropogenic impacts. In this study, a water–air system in porous media consisting of several types of granular materials is considered. A conceptual model for pressure-wise estimation of water saturation and relative unsaturated hydraulic conductivity of such materials is proposed. The model uses the boundary water retention curves and corresponding wetting angles of the pure components of the mixture as the input parameters. The approach is based on simplified statistical and geometrical assumptions for describing the mixed porous material and its capillary behavior. In the experimental part of this study, three materials with different wetting angles (quartz sand, silanized quartz and teflon grains) were used both as pure and as homogeneously mixed packings to conduct experiments with a water–air system in short columns under well-controlled laboratory conditions. The hydraulic characteristics were determined for packings with various mixing ratios. The non-uniform matrix wettability was found to strongly influence the water retention behavior of the packings, whereas its impact on the water-saturation-dependent hydraulic conductivity was weak. The measured data are compared to model predictions. The results show that the model is capable of providing good qualitative estimates of both the boundary water retention curves and the curves of the relative hydraulic conductivity for all investigated materials.