Plastic debris is a growing environmental concern for the world's oceans; several million metric tons of plastic enter the oceans each year. Much of the plastic debris is present as small, irregular particles five millimeters or less in length, called "microplastics," that pose a threat to marine organisms, the environment, and human health. This research project involves extending state-of-the-art research about particle-laden flows to situations relevant to the microplastics problem. Irregular particles are being studied in wavy flows to understand how to control the transport and sorting of these microparticles in the ocean and in near-shore environments. Both laboratory experiments and modeling are being performed and tools for predicting the motion of microplastics in the ocean and the length of time particles spend near the shore are being developed. The research team is working with earth scientists, biologists, and policy makers to integrate the resulting knowledge into conservation efforts. The investigators also are closely engaged in mentoring graduate students, teaching undergraduates about the transport of marine microplastics, and creating outreach activities for high school and middle school students.
This research project is identifying and classifying the mechanisms that control transport and tumbling of anisotropic particles in wavy environments and quantifying the dispersion rate and sorting of these particles based on size and shape in waves. While particle-laden flows are well studied in certain limits, the complexities of particle-laden flows involving large, anisotropic particles in surface gravity waves is complicated by the wide range of length and time scales involved. In particular, direct numerical simulations become computationally expensive, and as a result, experiments are a crucial tool. The investigators have advanced facilities for generating and quantifying wavy flows and are performing experiments and empirical modeling to characterize particle transport in these flows. The experiments will allow the researchers to address how anisotropic particles, like microplastics with their additional degrees of freedom for rotational motion relative to spherical particles, may lead to complex dynamics when coupled with waves, turbulence, and the stratification found in the ocean. A better understanding of these interactions will provide guidance regarding assumptions going into simplified models. The governing nondimensional parameters (the density ratio, wave Froude number, and Stokes number) are being varied over ranges relevant to those in the ocean. Pointwise flow characteristics and turbulence are being measured using laser Doppler anemometry; waves are being characterized via wave gauges; and the mean transport due to Stokes drift is being measured via particle tracking. Anisotropic particles, including rods, disks, and ellipsoids, are being created via 3D printing, and the dispersion of these particles is being measured by releasing a cloud of particles and then measuring the phase-averaged shape of the cloud over time and space. The results are expected to enhance our understanding of the fundamental physics governing the transport of non-spherical particles in waves and to inform the empirical modeling of microplastics in the ocean.
