# Morphology Characteristics of the Liquid-Vapour Interface in Porous Media
## 5 Conclusions  
This study employs X-ray \(\upmu\)-CT technology to investigate the l-v interface of porous media at different saturation levels. We tested sandy soils at various saturations, using KI solution instead of pure water to enhance phase contrast and obtain high-quality images. Flickering issues in the image stack were preprocessed with deflicker. For artifacts generated by scanning heterogeneous materials, an interactive machine learning algorithm was used for enhanced recognition, allowing accurate differentiation of phase and characterizing the pore features of the porous medium. However, due to limitations in experimental approaches, this method has yet to be extended to the micropores of silty and clayy soils. From this, the three-dimensional pore structure was reconstructed based on microscopic permeability images, and the liquid-vapour interface was identified and extracted for geometric and topological analysis. Results show that as evaporation progresses, the liquid phase gradually disintegrates into separate entities. We quantified saturation, liquid surface area, interface area, curvature, and phase topology during the pore water evaporation process. The interface area distribution is bell shaped at certain saturation levels, indicating that the area available for evaporation initially increases and then decreases with drying, with the maximum interface area occurring at around 30% saturation. The connectivity of the liquid and vapour phase is complementary, suggesting that availability and accessibility are in competition. During evaporation, the vapour phase, serving as the pathway for evaporation, faces less obstruction, whereas the liquid phase, serving as the source of evaporation, is less abundant. The interface changes with evaporation can be categorized into three types based on the interface morphology: annular liquid bridges, porous liquid bridges, and attached droplets, with trends in capillary pressure changes with saturation for each morphology. The grain size distribution and particle shapes (such as roundness and sphericity) of natural granular soils create narrower pores, thus promoting capillary action.  
Research on evaporation mechanisms can be applied in various fields, such as soil moisture conservation in arid regions, seawater desalination, and energy-saving buildings. This study provides detailed interface information on sand to further explore the evaporation mechanism of soil water. It also morphologically classifies previously difficult-to-identify l-v interface, thereby accurately understanding their impact on evaporation. Additionally, it may reveal characteristic relationships between interface curvature radius and intrinsic evaporation rate during soil water evaporation.  
Conceptualization, Y.D.; Methodology, Y.Z. and Y.D.; Software, Y.Z.; Validation, Y.Z.; Formal analysis, Y.Z.; Investigation, Y.Z.; Resources, Y.D.; Data curation, Y.D.; Writing--original draft, Y.Z.; Writing--review & editing, Y.Z. and Y.D.; Visualization, Y.Z.; Supervision, Y.D.; Project administration, Y.D.; Funding acquisition, Y.D. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Science Foundation of China (NSFC Grants 42277178).  
Not applicable.  
The raw data supporting the conclusions of this article will be made available by the authors on request.  
The authors declare no conflicts of interest.