Improved Settling Velocity for Microplastic Fibers: A New Shape-Dependent Drag Model

Area of Science:

• Environmental Science
• Fluid Dynamics
• Materials Science

Background:

• Microplastics are prevalent pollutants in aquatic ecosystems.
• Accurate prediction of microplastic settling velocity is crucial for understanding their environmental fate, residence time, and concentration distribution.
• Existing drag models struggle to accurately predict microplastic settling velocities, particularly for fibers, due to limitations in shape factor differentiation.

Purpose of the Study:

• To introduce a novel shape factor, the Aschenbrenner shape factor, to differentiate microplastic morphologies (fibers, films, fragments).
• To develop and validate a new drag model incorporating the Aschenbrenner shape factor for improved prediction of microplastic settling velocities.
• To assess the model's performance against experimental data and existing literature.

Main Methods:

• Development of the Aschenbrenner shape factor to distinguish between fiber, film, and fragment morphologies.
• Formulation of a new drag model based on the Aschenbrenner shape factor.
• Experimental validation using a unique dataset and comparison with four published datasets.

Main Results:

• The proposed drag model, utilizing the Aschenbrenner shape factor, significantly enhances the accuracy of predicting terminal settling velocities for microplastic fibers.
• The new model demonstrates applicability to microplastic films and fragments.
• The model's predictive capability for weathered microplastics in field conditions requires further research.

Conclusions:

• The Aschenbrenner shape factor provides a morphological distinction crucial for accurate microplastic settling velocity prediction.
• The new drag model offers a substantial improvement over existing models for microplastic fibers, films, and fragments.
• Future work should focus on refining the model for weathered microplastics and field applications.