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Mapping microplastic movement: A phase diagram to predict microplastics modes of transport
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Using a 3D particle-tracking flume experiment, researchers mapped how 24 different microplastic particles varying in size, shape, and density move through flowing water, finding that shape is a stronger predictor of transport behavior than size or density alone. Fibers traveled closer to the water surface and moved slower than spheres, while the ratio of fluid force to particle settling speed predicted which transport mode — rolling, bouncing, or suspension — each particle would experience. This phase diagram is a practical tool for predicting where different microplastics will accumulate in rivers and streams.
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.
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Researchers tracked 24 different types of nonbuoyant microplastic particles in turbulent open channel flow to understand how they are transported in aquatic environments. They found that microplastics move similarly to natural sediments through rolling, saltation, and suspension, but particle shape strongly influences transport behavior, with fibers staying closer to the water surface than spheres. The study introduces a new phase diagram for predicting microplastic transport modes based on flow conditions and particle properties.
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Researchers investigated the transport dynamics of 24 negatively buoyant microplastic particles across a spectrum of sizes, shapes, and densities using a 3D particle tracking system in turbulent open channel flow, generating 720 trajectories. They found that particle shape was the dominant determinant of transport behavior, with fibers tending to remain near the water surface at lower forward velocities while spheres stayed closer to the bed with higher forward velocities.
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Using particle tracking in a field-scale meandering stream, researchers found that the shape and size of floating particles — including microplastics — significantly affect how they move with water currents. Irregularly shaped particles behave differently than spheres, which matters for predicting where plastic pollution accumulates in waterways.
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Researchers conducted flume experiments to quantify how microplastic particles of different shapes and densities are transported and retained in artificial river channels under controlled flow conditions. The results show that particle shape and flow dynamics strongly interact to determine where microplastics settle or remain suspended in river systems.
Shields Diagram and the Incipient Motion of Microplastic Particles
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Researchers conducted flume experiments to determine the conditions under which different shapes and sizes of microplastic particles begin to move along a river or ocean bottom, testing spheres, cylinders, disks, cubes, fibers, and irregular particles. They developed a new framework that accounts for differences in friction, surface roughness, and sheltering effects to predict when microplastics start to be transported. For the first time, the study reconciles microplastic movement behavior with the classical Shields diagram used in sediment transport science.
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