Aggregation of microplastic and biogenic particles in a homogeneous and isotropic turbulence

We investigate the aggregation of microplastic and biogenic particles in a homogeneous and isotropic turbulence using direct numerical simulations and Lagrangian tracking of point particles. The range of particle properties (size and density) and mixture characteristics (turbulence intensity and particle number densities) analyzed correspond to scenarios relevant to the transport of microplastics in marine systems. We examine the spatio-temporal distribution of the disperse and continuous phases, the mechanisms and rates of particle collisions, and the composition of the resulting aggregates. The main findings are that (i) microplastics can be found in a large fraction of aggregates in scenarios with different average diameter of the mixture and number density ratios between microplastic and biogenic particles, (ii) microplastic-containing aggregates will sink to the deeper ocean layers particularly in situations where the biogenic particles are larger than or are of similar size compared to microplastics. By examining the collision mechanisms, a model for the collision rate that reproduces the computational results is proposed.