Non-spherical particle orientation dynamics under a wind-driven free surface boundary layer

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 plastic particles are seeded in the flow and are tracked using a large-scale shadow imaging technique. The results provide insight into the physical processes governing particle transport in the ocean surface layer. Particle concentration and orientation are investigated as a function of depth, and are used to explore the effects of particle size and shape on the amount of solar irradiation nonspherical particles would receive in the environment.