Measuring Air-Water Interfacial Areas: Contributions of Capillary and Film Domains in Natural Porous Media

Abstract

The air-water interface in variably saturated porous media is recognized to influence interfacial retention of organic and inorganic contaminants, and mediate various mass-transfer processes. The formation and presence of water films commonly solvating the surfaces of soil/sediment grains in unsaturated systems, as well as their impact on flow and retention processes have been of sustained interest. X-ray microtomography was used to measure air-water interfacial area at multiple wetting-phase saturations for natural porous media. First, a study was conducted to evaluate image-processing procedures suitable for characterizing fluids and associated interfaces in natural porous media. A simple method was developed for the analysis of all phases in the system, using global threshold for phase identification and combination of binary files (M1). This method was then compared to a simultaneous multiphase segmentation approach using locally adaptive threshold selection (M2). Both methods were used to process data sets comprised of multiple drainage steps for water-saturated packed columns imaged via synchrotron x-ray microtomography. The results of both methods were evaluated based on comparison of values determined for porosity and specific solid surface area to independently measured porosity and specific solid surface areas. The results show both methods are suitable for determination of total air-water interfacial area, which requires characterization of only the non-wetting phase. Conversely, determination of capillary interfacial area requires characterization of all phases present and thus, is more sensitive to the challenges associated with image processing. The simultaneous multiple-phase segmentation (M2) method provides an integrated and consistent analysis of the phases, and anticipated to improve water-phase detection. Using the advanced segmentation approach, the air-water interfacial area is presented as a result of direct measurement of contact areas between the two fluids. This is in contrast to previously reported data, which were derived indirectly from calculations based on individually measured phase surface areas and conceptualizations of fluid distributions. The effects of these assumptions on the capillary interfacial behavior are evaluated. Results from this study confirmed the initial hypothesis that the behavior of fluid surface areas will affect the theoretical shape of the capillary curve. The results support the understanding of the capillary interfacial area behavior in response to changes in the configuration of fluid surface areas during a drainage cycle. Furthermore, results for the measured air-water interface allows for further identification of fluid domains, such as the relationship between film interfacial area, capillary domains (menisci), and the total-measurable interfacial area. Experiments were also conducted using aqueous-phase interfacial partitioning tracer tests for comparison. Results support the hypothesis that different methods provide characterization of different interfacial domains. Overall, this study provides an imaging-based approach for evaluation of water configuration, and presents a measurement-based framework for further understanding of the role of fluid-fluid interfaces in natural porous media.