Characterization of Liquid-Vapor Interfaces in Pores During Evaporation

The evolution of the liquid-vapor interface plays a key role in multiphase flow, heat and mass transfer, and fluid phase change in porous media. In the soil water evaporation process, the vaporization occurs only on the liquid-vapor interfaces rather than the apparent soil surface. Yet, the interfaces evolve with high distortion and great complexity along the drying path. Hence the microscale characteristics of interfaces especially geometrical and topological features in soil water evaporation are barely investigated. In this work, we scanned glass bead samples using X-ray micro tomography and scrutinized the development of liquid-vapor interfaces with different degrees of saturation. The liquid-vapor interfaces are identified by morphological operations and extracted using the watershed segmentation technique. The liquid phase disperses into individual ganglia and distributes wildly in the pores as the saturation decreases, leading to low specific interface areas at saturated and dry state but a maximum value at a threshold saturation around 30%. The topological analysis reveals that the liquid and vapor phases present complementary connectivity behaviors quantified by normalized Euler characteristic numbers. The local mean curvature distribution of each typical individual interface cluster quantitatively describes the intricate progression of interface geometry and morphology along with drying. The overall mean curvature evolution of the sample separates the negative curvature component and confirms the capillary pressure increase during the pore water evaporation. The interfacial area and curvature analysis provide a cornerstone to determine the authentic interfacial evaporation rate for the soil under drying. Plain Language Summary Soil water evaporation is a common phenomenon. Yet, how exactly water evaporates from soil is still inscrutable and unsettled. Evaporation occurs at the air-water interfaces but usually the total soil surface was used to evaluate the evaporation rate. This work scanned the glass bead samples using X-ray and reconstructed each phase of the sample at various degrees of saturation. Air-water interfaces were identified and extracted to examine the interfacial area and curvature developments during the pore water evaporation. The investigation reveals how the patterns of interfacial area mean curvature evolve during evaporation and provides a quantitative description of the connectivity for liquid and vapor phase. The results can help further determine the true evaporation rate based on interfacial area and its correlation with interfacial curvature.