Settling and transport of microplastics: shape, size, and mass distribution effects

Microplastics are widely recognized as a contaminant of emerging concern by both the scientific community and the general public. They appear throughout the earth’s ecosystems, from the top of Mount Everest to the bottom of the Mariana Trench—and everywhere in between. However, the fluid dynamics of microplastic transport remains an active area of study. Microplastics occur in a wide variety of shapes, including not only spheres (and other shapes typically canonical to studies of particle-laden flow) but also fragments, films, foams, fibers, and more. They also span a range of sizes, from millimeters to nanometers. Commonly used plastics include both positively and negatively buoyant materials. These physical features—shape, size, and composition—are integral to our understanding of how microplastics are transported. A further complication is that these properties can be dynamic: plastic can be degraded or colonized by microorganisms, changing both the overall density as well as the mass distribution (and in some cases, size and shape as well). We will examine how shape and mass distribution affect settling velocity and trajectory for two cases: an idealized case, in which low-aspect-ratio cylinders are fabricated with specified mass distributions, and a realistic case, in which microplastics are exposed to bacterial growth. In the idealized case, we compare uniform-density cylinders with bipartite cylinders of the same overall size, shape, and mass density. In the realistic case, we culture a bacterial biofilm on either one side or all sides of positively buoyant polyethylene nurdles. We compare the still-water settling and rising velocities for each case, and find significant differences. We will discuss these findings in the context of overall microplastics transport, and what our results might imply for the future of microplastics research.