OCE-PRF: Vertical transport of buoyant, non-spherical particles in the wind-mixed ocean surface boundary layer

This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2).

Particles at the ocean surface are plentiful: microorganisms, ice crystals, debris, microplastics. Although many are buoyant, these particles do not passively float on the surface of the water; they are continually mixed within the upper regions of the ocean by the turbulence that results from wind blowing over the water surface. Unlike small, spherical particles, these non-spherical and sometimes large particles do not perfectly follow the fluid flow due to their size and shape, and their interaction with turbulence at the ocean surface is not well understood.

The PI will conduct an experimental and analytical investigation to study how buoyant, non-spherical particles move, rotate, and disperse during wind-mixing at the surface of the ocean. Knowledge of how these particles behave will have far-reaching applications in ocean science, from ocean optics to plankton biology to sea ice dynamics. In particular, understanding the transport of non-spherical microplastic particles is critical for quantifying both the scale of microplastic pollution and its risks to human health and the environment.

The results of this project will inform future policy, health guidelines, consumer behavior, and microplastic removal efforts. In addition, the PI will work to broaden participation in science within the local community through mentoring at the university level and involvement in extracurricular activities at the high school level.

The PI will measure vertical transport and dispersion of non-spherical particles in a laboratory wind-mixed surface boundary layer using particle shadow tracking techniques. Experiments will be performed for a range of particle geometries and wind speeds relevant to the ocean surface layer. These measurements will then be used to develop an analytical model of particle spatial distribution over the water column.

The project addresses a major gap in the current understanding of how particle shape and inertia affect particle transport in the wind-driven ocean surface layer. The Stokes number, a key parameter which quantifies how responsive a particle is to turbulent fluid motions, of particles in the ocean spans from zero to O(1), but most studies to date have focused only on particles with a Stokes number of zero, which perfectly trace the fluid flow.

In addition to the Stokes number, other parameters that define the fluid-particle dynamics include the floatability parameter, the aspect ratio between the dimensions of non-spherical particles, and the particle Reynolds number. Non-spherical particle transport in the surface boundary layer spans a wide parameter space; however, systematic experimental studies throughout this parameter space have not yet been done.

A complicating factor is that particle transport by turbulence is an inherently Lagrangian process, i.e., the relevant frame of reference is one that follows each particle along its trajectory. Lagrangian measurements which track particles in space and time are difficult to obtain in situ, and studies often describe particle transport and vertical concentration fields in an Eulerian frame of reference, i.e., within a stationary coordinate system.

This project will systematically explore the parameter space of non-spherical, non-tracer particles in wind-driven turbulence and will obtain both Eulerian and Lagrangian statistics of particle concentration, depth, and orientation. The results will provide insights into the physical processes governing a wide range of ocean transport phenomena, including that of microplastic pollution.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.