A settling velocity formula for irregular shaped microplastic fragments based on new shape factor: Influence of secondary motions

Microplastic (MP) fragments are prevalent in rivers and lakes and cause considerable pollution in natural water environments. Determining the settling velocity of microplastic fragments is crucial for predicting their migration and fate in aquatic systems. Predicting the settling velocity of MP fragments is challenging because of their complex and variable geometries and the uncertainties associated with secondary motions. To better understand the secondary motions of irregular MP fragments, a numerical model was developed to study the entire settling process, and an experiment was conducted to validate the numerical model. The model results showed the temporal changes in the settling velocity and orientation of MP fragments during the settling process. The MP fragments were classified according to their shape factors into fragments undergoing stable, transitional, and oscillating settling on the basis of velocity fluctuations caused by secondary motions. To describe the shape of irregular MP fragments appropriately, a new irregular shape factor (ISF) was derived by performing a theoretical analysis of forces and demonstrated to be more suitable than the Corey shape factor (CSF) for irregular MP fragments exhibiting considerable secondary motions. The settling velocity data were fitted to obtain an explicit settling velocity formula that includes the ISF for irregular MP fragments. Compared with machine learning methods and existing formulas, the proposed formula provides more accurate predictions of the settling velocity for irregular MP fragments.