A Quantitative Relationship between Settling and Wettability for Weathered Microplastics in Aquatic Systems

Settling plays a crucial role in determining the residence time, distribution, transport, and ultimate fate of microplastics (MPs) in aquatic environments. The settling dynamics of particles are influenced by their macroscale shape, mesoscale roundness, and microscale surface properties, along with ambient fluid flow conditions. Variations in the wettability of submillimeter MPs affect molecular interactions at the particle-water interface, altering the microscopic flow field and subsequently modifying drag forces during settling. This study examines the impact of wettability on the settling behavior of aged acrylonitrile butadiene styrene MPs by measuring their settling velocities and contact angles. It was shown that increased wettability promotes the settlement of the MPs. A drag model incorporating the Eötvös number─a dimensionless ratio of buoyancy energy to contact angle-derived surface energy─is developed to quantify the relationship between interfacial chemistry and hydrodynamic resistance. Unlike the conventional models, our model considers wettability as a key factor controlling the settling of MP particles. The model was validated using independently measured data and four sets of published data for the MPs. Results demonstrate that the model significantly improves the accuracy of the settling predictions for weathering spherical MPs. Additionally, by integrating the shape factor, it effectively accounts for the settling behavior of irregularly shaped MPs using published data sets. This improvement enhances predictability for MP transport pathways, helping assess MP accumulation zones and potential ecological risks in marine and freshwater systems.