Settling and drafting of spheres in quiescent fluids in microplastic-relevant regimes

The hydrodynamic settling of microplastic particles is yet to be explored in-depth, particularly in multiple-particle systems where wake interactions may affect settling dynamics. Here, we present an experimental study investigating the settling of spherical particles (∼1.92 mm, 1300 kg m−3) across a transitional particle Reynolds number range (Rep=6–185), relevant to microplastic transport. Using a dual-camera back-lit shadow-imaging system and Particle Tracking Velocimetry, we quantify single-particle terminal velocities and analyze pairwise interactions. Our findings reveal that trailing particles accelerate when entering the wake of a leading particle, with peak enhancements up to 68% above the single-particle terminal velocity. Interaction cases are characterized based on vertical and lateral spacing. In the key observed interaction pattern, drafting begins at a vertical distance of approximately two particle diameters and is accompanied by coupled lateral motion, indicating two-way interaction. These results demonstrate that particle–particle coupling during the drafting stage is stronger and more complex than predicted by classical models, and suggest that even weakly inertial microplastics can experience enhanced settling due to interparticle proximity. This work provides a foundation for improved predictive modeling of microplastic transport in aquatic environments.