Buoyant, non-spherical particles in turbulent wind-driven waves

Buoyant particles, such as microplastics, debris, and ice crystals, are ubiquitous at the surface of the ocean. However, it is not known how particle size, shape, and inertia interact with wind-driven turbulence and waves at the free surface to affect their transport and dispersion in the water column. Experiments are performed to measure particle depth and orientation in a laboratory wind-wave tank over a range of wind speeds relevant to the ocean surface. Buoyant rod-, disk-, and sphere-shaped HDPE particles are seeded in the flow, and particles are tracked using a large-scale shadow imaging technique. The results provide insights into the physical processes governing particle transport in the ocean surface layer. Particle diffusivities are calculated from Lagrangian statistics and compared to observed concentration profiles. Orientation is investigated as a function of depth, and we find that particles have variable orientations near the surface of the water but adopt a preferential orientation deeper in the water column. This bimodal behavior suggests competing forcing on particle orientation due to buoyancy, turbulence, and waves.