Sinking behavior of biofilm-covered microplastics with irregular shape: computer aided simulation and experimental validation*

We use computational fluid dynamics to investigate the sinking behavior of biofilm-covered microplastics (BMPs) with realistic shape in a water column. The structure of individual BMPs is either acquired from imaging experiments [1], or generated through computer simulation [2]. The Navier-Stokes-Brinkman formulation is used for the flow pattern around the BMP and within the poroelastic biofilm. The gravity-driven motion of the BMP is simulated with the immersed boundary method. Simulations are parameterized by the Reynolds and Darcy numbers that capture the combined effects of particle size, excess density, seawater viscosity and biofilm permeability. At steady-state sinking, high-resolution finite element analysis provides the distribution of elastic stresses within the biofilm and the locations of maximum likelihood for detachment. The computed terminal velocity is compared with measurements from particle tracking velocimetry experiments [1], across a wide range of aggregate geometry and biological fraction.

[1] Nguyen TH, Tang FHM, Maggi F (2020). Sinking of microbial-associated microplastics in natural waters. PLoS ONE 15(2): e0228209.

[2] Kapellos, G.E., Alexiou, T.S. and Pavlou, S (2015). Fluid-biofilm interactions in porous media, Elsevier, pp. 207-238.