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

Aug 30, 2026

Microplastics threaten aquatic ecosystems, yet the mechanisms controlling their transport through water columns remain insufficiently understood. Drawing on sediment-transport research, this study applies the transport stage concept—the ratio of fluid shear velocity to particle settling velocity—to microplastic particles with varying size, shape, and density. Using 3D particle tracking in turbulent open-channel flow, 720 trajectories were recorded to quantify correlations between transport stage and particle velocity, water-column position, and transport-mode timing.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages24

Reading Time1 h 0 min

AccessFree to download

Abstract

Introduction

Methods

Experimental setup

Trajectory analysis

Results

Transport stage correlations

Transport modes and particle shape

Phase diagram

Conclusion

This manuscript is a preprint and has been submitted for publication. Subsequent versions may have slightly different content. The DOI of the peer reviewed publication will be provided if accepted. Please contact the authors if you have any questions or comments on this manuscript. 1 Title: Mapping microplastic movement: A phase diagram to predict microplastics modes of transport 2 Authors: Hadeel Al-Zawaidah1*, Merel Kooi2, Ton Hoitink1, Bart Vermeulen1 & Kryss 3 Waldschläger1 4 1 : Wageningen University and Research, Hydrology and Environmental Hydraulics Group, 5 Wageningen, The Netherlands 6 2 : Wageningen University and Research, Aquatic Ecology and Water Quality Group, 6700 AA 7 Wageningen, The Netherlands 8 *: Corresponding author, [Email:](Email: hadeel.alzawaidah@wur.nl)[ ](Email: hadeel.alzawaidah@wur.nl)[hadeel.alzawaidah@wur.nl](Email: hadeel.alzawaidah@wur.nl) 9 Key words: microplastics transport, the transport stage, phase diagram, bedload transport, suspension 10 Highlights 11 • The transport stage U*/Ws can predict microplastics trajectory characteristics 12 • Microplastics modes of transport can be predicted by the transport stage 13 • Microplastics density and nominal diameter governed their particle trajectories 14 • Particles susceptibility to suspension increase as they deviated from perfect spheres 15 • A phase diagram was developed to predict microplastics modes of transport 16 Abstract 17 Microplastics pose numerous threats to aquatic environments, yet general understanding of the 18 mechanisms governing their transport remains limited. Drawing upon research on natural sediment 19 provides a valuable resource to address this knowledge gap. One key dimensionless number used to 20 describe sediment transport is the transport stage, referring to the ratio between the fluids shear velocity 21 and the particle settling velocity. However, differences in physical properties (e.g., shape, density) raise 22 concerns regarding the applicability of existing sediment transport theories to microplastics. To address 23 this challenge, we employed a physical modelling approach to examine the trajectories of 24 negatively 24 buoyant microplastic particles varying in size, shape and density. Utilizing a 3D particle tracking setup, 25 we captured the movement of the particles in turbulent open channel flow. A total of 720 trajectories 26 where recorded and analysed. The results revealed a strong correlation between the transport stage and 27 the mean forward velocity of the particles, as well as their mean position in the water column. Notably, 28 particle shape emerged as a critical factor influencing particle transport dynamics. Fibres showed a 29 tendency to be transported closer to the water surface while experiencing slower mean forward velocities 30 compared to spheres. The microplastic particles exhibited rolling/sliding, saltation and suspension as 31 transport modes, comparable to natural sediment. The results show a strong correlation between the 32 transport stage and the percentage of time in which microplastics experienced a certain mode of 33 transport. Based on the laboratory results, a new phase diagram for microplastics is introduced, 34 analogous to an existing diagram for sediments. 35 1. Introduction 36 Microplastics are abundant in aquatic environments, posing a pervasive environmental concern (Browne 37 et al., 2011; Koelmans et al., 2019; Rhodes, 2018; Sundt et al., 2015) . In response, various initiatives, 38 spanning legislative, scientific, and social realms, target reduced microplastic emissions and the removal 39 of environmental microplastics (Barcelo and Pico, 2020; Nikiema et al., 2020) . The success and 40 advancement of these initiatives are impeded by our limited ability to assess and monitor their 41 performance and outcomes (Whitehead et al., 2021) . A comprehensive understanding of microplastic 42 transport pathways, su

FAQs

What does the transport stage represent in this study?

The transport stage is defined as the ratio between fluid shear velocity and particle settling velocity. It is used to connect flow conditions with microplastic transport behavior.

How were microplastic trajectories measured and analyzed?

The study used a 3D particle tracking setup in turbulent open-channel flow. A total of 720 particle trajectories were recorded and analyzed.

What transport modes were observed for microplastics?

Microplastics exhibited rolling/sliding, saltation, and suspension. These modes were comparable to those of natural sediment.

Aug 30, 2026

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