Distinct settling dynamics of irregular tire particles in quiescent fluid

We present a laboratory investigation into the collective settling dynamics of irregular tire-derived particles compared to spherical polyethylene particles of similar bulk density and size. Using particle tracking velocimetry (PTV) and multi-camera 3D reconstruction, we tracked particle trajectories in quiescent ethanol following controlled release from the surface via surface tension-driven entry. Two particle sizes (diameters of 275 µm and 390 µm) and three particle counts (ranging from 100 to 600) were examined for each material. For spherical particles, concentrated releases produced a rapidly settling central core and a dispersive outer ring, with the bulk volume growing roughly linearly in time and scaling with particle number and size. In contrast, tire particles exhibited different behavior; their descent was dominated by vertical dispersion, resulting in the formation of an elongated, less coherent structure and scaling trends. The comparison between particle types highlights the critical role of particle geometry in modulating hydrodynamic interactions and collective motion, revealing unique challenges in characterizing irregular, environmentally relevant particles, such as those from tire wear.