Thin synthetic fibers sinking in still and convectively mixing water: laboratory experiments and projection to oceanic environment

Abstract

Synthetic fibers with diameters of several tens of micrometers are the most abundant type of microplastics in the marine environment, yet the most unknown regarding dynamics in the water column. Experiments proposed here are a proof-of-concept of qualitative and quantitative characteristics of fibers' motion in still water and in the presence of thermal convection. For 12 sets of fine fibers (nylon (1.12 g/cm³) and polyester (1.35 g/cm³), 1.9-14.8 mm long, diameters 13 and 20 μm), 84 measurements of sinking velocity in still water were acquired. In still conditions, fibers settled smoothly and slowly, preserving their initial (accidental) orientation. Sinking rates of fibers with lengths <5 mm varied between 0.5 and 3.7 mm/s (the bulk mean of 1.6 mm/s). Fibers with similar properties showed 4-fold different sinking velocity, which is supposed to be the effect of their different orientation while settling: vertically oriented fibers (19% in the experiments) settled faster than those with inclined orientation (48%), and horizontally oriented fibers (33%) settled with the smallest velocities. Convective mixing of water, heated from below, principally changed the manner of sinking of fibers: their motions became unsteady and 3-dimensional. In 78 measurements for 4-mm long nylon fibers (using the "light knife" technique), only about 56% of fibers showed downward velocity component (mean 1.33 ± 0.78 mm/s), which was twice as small as in still water, however the ratio of max/min values increased up to 14. Fibers could move in different directions and follow circular motions of a convective cell. Our findings suggest two possible mechanisms retaining fibers in the water column: entrainment of some particles in horizontal and vertical motions and slowed sinking due to unsteady flow around the fiber. The retention of fibers leads to decrease in integral downward particle flux (up to 4 times in our experiments).

Keywords: Convection; Fibers; Microplastics; Sinking velocity; Turbulence.