Residual NAPL Intermittent Expulsion With Increasing Freeze–Thaw Cycles in Saturated Porous Media: Findings From a 2.5‐D Microfluidic Platform

Abstract In this study, we developed a 2.5‐D microfluidic experimental platform that enables nondestructive visualization of pore‐throat microstructures and the migration processes of residual non‐aqueous phase liquids (NAPL) within a three‐phase system (water–ice–NAPL) under freeze–thaw conditions. Simulated experiments were then used to investigate the freeze–thaw migration of typical NAPL components. We analyzed the forces within the system during freezing and proposed an expulsion factor (Nf) to evaluate the migration potential of residual NAPL. The results indicated that the residual NAPL exhibited three primary mechanisms during freezing: expulsion, snap‐off, and compression. Expulsion significantly drove the residual NAPL movement during the early stages with fewer freeze–thaw cycles. As the number of cycles increased, snap‐off and compression became dominant. This substantially restricted the expulsion of the residual NAPL. These findings have significant theoretical and practical value to understand and predict contaminant movement in frozen soil regions.