PRELIMINARY INVESTIGATION OF THE FALL MECHANICS OF MICROPLASTICS PARTICLES IN AIR

Costello and Ebert (2016a, b) documented the aeolian transport of microplastic particles to coastal dunes from Great Lakes beaches and noted that microplastic particles were 1.5 to 2.0 phi grades coarser than the modal size of the sand collected with the microplastics. To better constrain the mechanics of aeolian transport of microplastics, structure from motion analysis to characterize particle shapes and laser assisted photogrammetry in settling experiments were applied to microplastic particles collected from the dunes on the shores of lakes Erie and Ontario. Particles ranged in size from 1.2mm to 6.8mm with shapes characterized as spheroids and disks (nurdles). Theoretical terminal velocities were also calculated using classical Newtonian mechanics.

Force system analysis for different particle shapes (Zingg 1935) showed that the drag force on the particles at terminal velocity is equivalent to the gravitational force acting on the same particle. A closed system drop test without evacuation enabled calculation of the drop force using known normalized drag coefficients and measured average mass and surface area of microplastic particles. This method facilitated calculation of terminal velocities for each of Zingg’s (1935) shape categories. Applying the empirical equation of Gibbs, Matthews and Link (1971) to the terminal velocities for spheres and disks enabled graphing of the settling velocities for these shape classes as a function of their radii for the most common densities of microplastics. The maximum velocity of the test particles (spheres and disks) was 1.70 m/s. Kinematic analysis of the results of our settling experiments determined that terminal velocity was not achieved in our apparatus and so results were significantly lower than the theoretical terminal velocity derived from the Newtonian calculations.

This study, although preliminary, defines a flexible methodology for future study and comprises a first attempt at understanding the settling and terminal velocities in air for particles that are less dense than quartz.