Drainage and imbibition along main and scanning curves: A pore-scale morphology approach

This study quantifies the three-dimensional morphologies of pore air and pore water, and examines drainage and imbibition behaviours along main and scanning curves in water retention tests. The aim was to clarify the pore-scale drainage and imbibition mechanisms and their relationships to specimen-scale water distribution patterns. The water retention tests involved two drying and wetting cycles, during which X-ray micro-computed tomography (CT) imaging was performed at each equilibrium point. From segmented CT images, pore air and pore water clusters, as well as drainage and imbibition clusters, were extracted and analysed. The cluster volume distributions, number of clusters, and continuity of the pore air and pore water phases were evaluated, revealing distinct transitions of water distribution patterns that depend on the degree of saturation and the drying–wetting history. The shape complexity and spatial positions of the drainage and imbibition clusters were evaluated, and the drainage and imbibition mechanisms along the main and scanning curves were identified. Five distinct saturation regimes were defined based on phase continuity, which were linked to the complexity of the drainage and imbibition cluster shapes. These findings offer insights into the pore-scale mechanisms that underlie macroscopic water retention behaviour and provide implications for improving theoretical water retention curve models.