Non-destructive quantification of microplastics mobility in soil during saturations-desaturations cycles

Microplastics are persistent contaminants in soils and aquifers, accumulating and potentially remobilizing within groundwater and surface flows. Predicting their fate requires pore-scale insights in realistic porous media, especially under saturation-desaturation conditions. Despite the growing body of research on MP transport in porous media, the methodological divide between qualitative visualization and quantitative measurement limit predictive understanding under realistic subsurface conditions. Microfluidic systems enable direct pore-scale observation of MP mobility but rarely provide domain-wide quantification, while column-scale experiments offer robust mass balance yet obscure the underlying pore-scale mechanisms. This disconnect hinders the development of mechanistic understanding linking pore geometry, multiphase flow, and MPs fate. To address this limitation, this study introduces an integrated, non-destructive microfluidic imaging framework that couples high-resolution time-lapse visualization with semi-automated image analysis to simultaneously resolve pore-scale processes and quantify domain-scale retention across saturation-desaturation cycle. Four statistically distinct micromodels representing heterogeneous porous domains were used to capture realistic flow and retention behaviors. The segmentation pipeline classified individual phases (air, water, solid, and MPs), allowing spatially and temporally resolved quantification of retention and release throughout the entire flow field. The results demonstrate that MPs mobility depend not on porosity alone but on the combined effects of throat-size distribution, connectivity, and capillary dynamics, which control the persistence or remobilization of particles under transient saturation. Beyond its immediate findings, this work establishes a reproducible and scalable experimental-computational framework that bridges the gap between visualization and quantification, offering a mechanistic basis for predicting MPs fate in soils and aquifers under dynamic environmental conditions.