Settling of finite-size slightly negatively particles under plunging breaking waves

Vertical transport of microplastics is strongly influenced by highly transient, energetic surface breaking waves. In this study, we experimentally investigate the settling dynamics of millimeter-scale slightly negatively buoyant spherical particles entrained by a laboratory plunging breaker. Particle image velocimetry is used to characterize the spatiotemporal evolution of the breaker-generated turbulence, while particle tracking velocimetry resolves the particle trajectories and settling response. The measurements reveal a strongly non-stationary post-breaking environment: the turbulent dissipation rate decreases from 2.60 to 6.91 × 10−4 m2/s3, while the particle diameter-to-Kolmogorov length-scale ratio remains between 3.28 and 54.6 and the settling number spans 0.18–1.45. This parameter range indicates a transient finite-size regime in which gravitational settling competes with rapidly decaying turbulent fluctuations and coherent wave-induced sloshing. Under these conditions, the mean settling response remains strongly suppressed, with measured settling velocities only ∼28%–42% of the corresponding quiescent values. The suppression is particle size-dependent: smaller particles exhibit broader and more non-Gaussian velocity distributions, stronger early-time retardation, and a more persistent response to the combined action of turbulent fluctuations and coherent sloshing. Concentration measurements further show that the cloud evolves from an initially well-mixed intrusion to a progressively weakened, broadened, and downward-shifted structure. Comparison with a recent finite-size settling model developed for homogeneous isotropic turbulence reproduces the qualitative recovery trend as turbulence decays, but generally overpredicts the settling response. These results show that turbulence-based parameterization alone is insufficient for breaking-wave flows and that coherent sloshing introduces an additional suppression of net downward transport. The measurements provide a physically grounded basis for incorporating breaking-induced settling retardation into predictive models of microplastic transport.