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Settling of finite-size slightly negative buoyant particles under plunging breaking waves
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When scientists studied how tiny plastic particles sink in breaking ocean waves, they found the churning water slows down sinking dramatically, particles settled 28-42% slower than they would in calm water. This matters because it means microplastics may stay suspended near the surface longer than expected, potentially increasing the chances they're ingested by marine life or washed onto beaches, which affects how these particles eventually make their way into our food chain.
Transport of microplastics is strongly influenced by transient surface-breaking waves. In this study, we experimentally investigate the settling dynamics of millimeter-scale slightly negative buoyant spherical particles entrained by a plunging breaker. Particle Image Velocimetry (PIV) is used to characterize the spatiotemporal evolution of the breaker-generated turbulence, while Particle Tracking Velocimetry (PTV) resolves the particle trajectories and settling. The measurements reveal a 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 turbulence and wave-induced sloshing. Under these conditions, the mean settling remains strongly suppressed, with settling velocities only approximately 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 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 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.
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