CAREER: Particle dynamics at the free surface: waves, turbulence, and microplastics

Plastic pollution is a ubiquitous issue impacting the health of marine ecosystems worldwide. Yet, critical knowledge gaps surrounding the fate and transport of plastic once it enters the ocean impede remediation and prevention efforts. Predicting transport is difficult for any particle in the ocean, but microplastics present a particular challenge because their size and density fall outside the regimes of traditionally studied environmental particles such as low-density bubbles and high-density sediment. Thus, the goal of this award is to (1) observe plastic particles in realistic ocean flows recreated in a controlled laboratory setting, and (2) use the observations to build a modelling framework that describes the vertical mixing and transport of plastic at the ocean surface. By examining fundamental interactions between particles and ocean flows, this award will enable more accurate modelling of buoyant particles at the ocean surface. The investigator will partner with local beach-cleanup organizations to share this work through outreach events. This award will additionally support educational activities for both undergraduate and graduate students. The ocean surface boundary layer is a multiphase flow forced by the overlying wind, which generates waves and turbulence. These processes affect the transport and mixing of materials ranging from nutrients and pollutants to bubbles and organisms. Microplastics are a new environmental particle that is increasing in concentration at the ocean surface; yet, predicting the behavior of microplastics is non-trivial because it depends on poorly understood interactions among waves, turbulence, and particle inertia in the ocean surface boundary layer (where most microplastics reside). To help close this foundational knowledge gap in particle-laden flows and improve microplastics transport modeling, this award seeks to describe how the rise velocity of buoyant particles is modulated by the combined effects of waves and turbulence, the processes by which particles are entrained beneath the surface, and the controls on particle diffusivity in waves and turbulence. The research will be conducted in a laboratory facility that can generate both wind and waves. Advanced optical techniques will be used to track particles both at the surface and beneath it under a range of surface conditions, including breaking waves. These observations will be used to develop a wave phase-resolved model for the transport and mixing of buoyant particles in a free-surface boundary layer, a model which will be broadly applicable to microplastics, and other particles found at the ocean surface. 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.