Spreading and entrainment characteristics of continuously released microplastic suspensions in water

Continuous releases of microplastics represent an important pathway for microplastic pollution in aquatic environments, but they have been far less understood than single microplastic dynamics. This study presents the first experiments on the release of microplastic suspensions into water, focusing primarily on their vertical settling, transverse spreading, and entrainment behaviors. High-speed shadowgraph imaging and Particle Image Velocimetry reveal that such releases lead to the formation of microplastic plumes, and that microplastic transport is strongly affected by initial momentum, buoyancy fluxes, and wake effects. Significant vortices are observed both within the plume interior and around its periphery, resulting in non-uniform distributions and enhanced transverse dispersion of microplastics. The settling velocity of microplastic plumes follows a power-law decay with depth (with an exponent of -2/3) and reaches a weakly varying regime at 3.3-4.3 times the terminal settling velocity of singly released microplastics. Predictions are made regarding the spreading rate and entrainment velocity of microplastic plumes: the former increases with mass injection rate and ratio of injection velocity to terminal velocity but decreases with source mass flux; the latter increases with initial buoyancy flux, but decreases with initial Stokes number. This study elucidates previously unresolved plume dynamics induced by continuous releases of microplastics, demonstrating their ability to enhance vertical settling, transverse spreading, and entrainment, thereby intensifying microplastic dispersion and impact in aquatic environments. The findings provide a scientific basis for effective sampling, prediction, and management of microplastic pollution.