Real-time visualization of cross-media transport of microplastics from water to porous media

The environmental persistence and ecological impacts of microplastics have attracted considerable global attention. After entering aquatic environments, microplastics can settle and become trapped by subaqueous sediments, yet the cross-media hydrodynamics of microplastics remain unresolved. This study presents a laboratory-scale experimental investigation of the transport of continuously released microplastics from water to porous media, aiming to examine microplastic plume formation, interfacial impact, radial propagation, and infiltration within porous media as a coupled sequence. The refractive index matching technique combined with high-speed shadowgraph imaging was employed to enable real-time visualization of microplastic cross-media transport. Following impact, microplastics radially propagate along the porous media surface and exhibit two stages with distinct temporal variations of propagation distance, which are dominated, respectively, by impact momentum and by gravity current and vortex entrainment. A dimensionless scaling relationship was established to describe the radial propagation distance of microplastics varying with time across different stages, with the stage transition occurring at a dimensionless time of 0.7 based on a viscous timescale. A transition between sustained microplastic infiltration into the porous media and predominant accumulation on the porous media surface was observed when the microplastic-to-pore size ratio was approximately 1/8. An increased frontal infiltration velocity is obtained for microplastics with a more spherical shape and higher density under a smaller microplastic-to-pore size ratio and a larger mass injection rate. This study elucidates the previously unresolved dominant physical mechanisms and hydrodynamic characteristics governing the cross-media transport of microplastics, thereby advancing the fluid-mechanical understanding and predictive description of microplastic transport.