Free-settling dynamics of irregular microplastic particles in water

Irregular particles play a crucial role in numerous industrial and natural processes, such as manufacturing and the oceanic transport of microplastics. In many cases the settling dynamics is of fundamental importance. Previous studies focus on spherical or regular-shaped particles (such as ellipsoids or discs). In this work irregular particles are generated from an industrial granulation process of five different polymers. The variation in mechanical properties among the polymers result in a wide range irregular shapes. The study investigates settling in the intermediate settling regime at particle Reynolds number 100-1000. The characteristic length of the particles is 2-10 mm and the densities between 1030-1400 kg/m³. A stereo vision technique is developed that allows for simultaneous tracking of both the 3 translational and the 3 rotational degrees of freedom of the particle. Additionally, a particle resolved CFD model is validated with the experiment and used to extend the range of densities and Reynolds numbers while keeping the shape constant. The subsequent analysis focuses on the coupling between the rotational and translational dynamics. It is observed that the variation in shape resulting from the granulation process, of the same polymer, yields particles settling in all possible regimes. Furthermore, the study investigates the impact of settling regime on velocity, instantaneous cross-sectional area, and drag coefficient.