Collective settling and wake Interactions in mixed-density particle arrays

We experimentally investigate the collective settling of inertial particles in a quiescent fluid, focusing on how density heterogeneity and spatial arrangement influence settling dynamics. Arrays of 100 spherical particles (diameter 4 mm) were arranged in a 10x10 grid with initial spacing 2d, spanning five configurations: two homogeneous arrays with density ratios 1.14 and 1.28, and three heterogeneous arrays combining both densities in distinct spatial patterns. Each configuration was repeated ten times. Particle trajectories were tracked using 3D particle tracking velocimetry, and the induced flow field was measured with planar and stereo PIV. The results show that settling behavior is strongly governed by particle distribution. Homogeneous arrays consistently formed parachute-like structures, with central particles lagging. In heterogeneous arrays, the descent pattern was either amplified or inverted, depending on the spatial layout. Flow measurements reveal that wake-induced shear and entrainment significantly alter the trajectories of particles, particularly those of lighter particles. Pair-dispersion analysis indicates robust vertical superdiffusion with R_z^2 ~ t^(3/2) and early-time lateral ballistic scaling R_L^2 ~ t^2, followed by a transition to configuration-dependent diffusive behavior. The mesoscale hydrodynamic coupling mediated by wake interactions leads to emergent settling behaviors not captured by single-particle or dilute approximations.