A pore-scale perspective of pollutant wettability on the soil infiltration process

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Contaminants with strong wetting behavior in soil systems tend to have greater affinity for soil surfaces, which promotes pore occupation and helps overcome capillary resistance, thereby increasing their potential for migration in the vadose zone. However, how wettability regulates capillary entry and invasion thresholds in heterogeneous pore-throat geometries remains unclear. In this study, microfluidic chips were used to simulate the infiltration of water, perfluorooctanoic acid (PFOA) solution, and ethylbenzene in vadose-zone pore networks. Under horizontal injection, ethylbenzene showed breakthrough saturations 6.22-17.45% higher than water and 3.10-8.85% higher than PFOA. During vertical invasion, downward displacement assisted by gravity and capillary forces consistently produced higher saturation than upward displacement. Mechanistically, the stronger wetting behavior of ethylbenzene, mainly related to its lower surface tension and smaller contact angle on the pore surface, reduced the capillary entry barrier. Based on the pore-throat network constructed in this study, a quantitative method was developed to determine liquid-entry thresholds across different pore structures. The results show that this threshold expands the throat-size range over which capillary forces shift from resistance to driving, thereby enhancing pore filling. A pore-scale framework was further established to link critical capillary pressure with heterogeneous pore-throat structures and system-dependent wettability, providing a mechanistic indicator for estimating contaminant breakthrough tendencies in nonuniform pore networks. These findings improve the mechanistic understanding of soil contaminants with transport potential and can inform future risk assessment and remediation model development. Synopsis A capillary entry threshold for contaminants in porous media was established based on wettability-dependent capillary forces to characterize their influence on spreading and migration behavior.

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