Mapping microplastic movement: A phase diagram to predict microplastics modes of transport

Microplastics pose numerous threats to aquatic environments, yet general understanding of the mechanisms governing their transport remains limited. Drawing upon research on natural sediment provides a valuable resource to address this knowledge gap. One key dimensionless number used to describe sediment transport is the transport stage, referring to the ratio between the fluids shear velocity and the particle settling velocity. However, differences in physical properties (e.g., shape, density) raise concerns regarding the applicability of existing sediment transport theories to microplastics. To address this challenge, we employed a physical modelling approach to examine the trajectories of 24 negatively buoyant microplastic particles varying in size, shape and density. Utilizing a 3D particle tracking setup, we captured the movement of the particles in turbulent open channel flow. A total of 720 trajectories where recorded and analysed. The results revealed a strong correlation between the transport stage and the mean forward velocity of the particles, as well as their mean position in the water column. Notably, particle shape emerged as a critical factor influencing particle transport dynamics. Fibres showed a tendency to be transported closer to the water surface while experiencing slower mean forward velocities compared to spheres. The microplastic particles exhibited rolling/sliding, saltation and suspension as transport modes, comparable to natural sediment. The results show a strong correlation between the transport stage and the percentage of time in which microplastics experienced a certain mode of transport. Based on the laboratory results, a new phase diagram for microplastics is introduced, analogous to an existing diagram for sediments.