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.