Microplastics Transport in Turbulent Flow: Investigating the Effects of Physical Characteristics and Flow Dynamics

The surge in global plastic production has led to increasing plastic pollution in aquatic environments, where plastic debris fragments into microplastics (MPs), particles smaller than 5 mm, through weathering processes. MPs are transported by ambient flow across different aquatic compartments, posing ubiquitous risks to the ecosystem health. Effective mitigation of MPs' risks requires a comprehensive understanding of MPs' transport and mobility. Turbulence and the natural settling or rising movements of MPs are fundamental transport mechanisms, yet many aspects of how MPs' diverse physical properties affect these processes remain underexplored. Density, size, and shape are amongst critical physical properties of MPs that shape their transport and affect flow interactions. This PhD dissertation investigates the effects of MPs’ physical properties on their transport and mixing in turbulent flows using numerical and experimental approaches. The findings of this research elucidate how density, size, and shape affect the behaviour of MPs, providing explanations for their selective abundance and distribution in aquatic environments. Results of this PhD dissertation illustrate that lower marginal densities relative to the ambient fluid, smaller sizes, and non-spherical shapes make MPs more susceptible to the transient dynamics of the ambient flow as such MPs deviate significantly from their terminal behaviours. The findings explain the distant transport of smaller non-spherical MPs and the absence of smaller MPs of common polymers in offshore surface layers, as such particles are more likely transported to deeper water columns by in-depth currents. This research also explores the advantages of dynamic Lagrangian modelling over commonly used kinematic approaches, emphasizing the importance of particle acceleration for MPs with higher mixing levels, particularly those with smaller sizes, lower marginal densities, and non-spherical shapes. These findings contribute to understanding MPs' transport and distribution based on their physical properties and flow dynamics and offer a foundation for developing effective strategies to mitigate the ecological impacts of MPs.