Experimental investigation of settling velocity for cuboidal microplastic

Accurate settling velocity estimation is crucial for modeling microplastic distribution in the ocean, and this estimation can be achieved using nondimensional correlations. This study experimentally investigates the settling velocity of irregularly shaped microplastics and proposes new empirical correlations for drag coefficient and dimensionless settling velocity. Microplastic shape and size were standardized using 3D printing: 150 cuboidal microplastics with minimum dimensions of 0.2 mm and aspect ratios ranging from fiber-like (1:1:10) to plate-like (10:10:1) were fabricated. Experiments were conducted in water and a water-glycerol mixture, spanning a Reynolds number range of 0.03 to 200. New empirical correlations for drag coefficient and dimensionless settling velocity were developed using equivalent and longest length scales, incorporating various shape factors. The root mean square error in predicted settling velocity using these correlations averaged 11.7 %. Comparisons with existing drag models demonstrate the improved accuracy of our proposed correlations, providing more reliable methods for predicting microplastic behavior in diverse marine environments, from coastal regions to the open ocean.