Vertical dispersion of buoyant particles in a free surface flow

Microplastics are buoyant particles that accumulate at the ocean surface. In this talk we consider the vertical transport and mixing of buoyant particles below the free surface

when forced by a wind-driven boundary layer flow. While this system may mirror that of sediment transport in an open channel flow, differences can arise due to the particles’

relative density, low concentration, and the wavy free surface. Using laboratory experiments in a wind-wave tank, we observe particle vertical transport and dispersion using both PIV and particle shadow tracking techniques. We find that the particles with higher rise velocities generally have lower diffusivities, consistent with the crossing trajectories effect. We also find that the effective rise velocity of the particles can be affected by both the turbulence and the waves. Together, these findings show that the effects of particle inertia and particle buoyancy are nontrivial when modelling the vertical distribution of microplastics in a wavy, turbulent flow characteristic of the ocean surface boundary layer.