[M-IS19] Ocean Plastics, an earth science perspective

[MIS19-04] Modelling the fragmentation of marine plastics in response to fracture energy: A statistical mechanics approach

*Kunihiro Aoki¹, Ryo Furue¹, Ettore Barbieri¹ (1.Japan Agency for Marine Earth Science and Technology)

Keywords:Microplastics, Size distribution, Fracture mechanics, Statistical mechanics

A physical model for the fragmentation of marine plastics is necessary to predict the size and abundance of the microplastics produced by the fragmentation. The present study proposes the model for the size distribution of the microplastics in response to the fracture energy potentially associated with the oceanic phenomena such as waves or winds, borrowing the framework of the statistical mechanics. The model is premised on the following two principles: 1) a larger fracture energy is required to produce a smaller fragments; 2) the occurrence probability of the fracture energy follows the Boltzmann distribution. The former comes from the proportionality between the surface energy and the area of the broken surface, and the latter from the law of partitioning a limited energy in the statistical mechanics. The modeled size distribution has the feature of the power law and the lognormal distribution, which well explain the observed size distribution across the size range in the micro- and meso-plasitcs (from 10μm to 10cm order). Although the underestimation of the observed abundance of tiny microplastics is suggested by previous studies in terms of the sinking of the microplastics or sampling error, the present model may not conflict this fact. In such a case, the model may predict quantitatively that the original plastics are exerted by a larger fracture energy. Following those, the present model may pave the way for predicting the size and abundance of the microplastics by quantifying the environmental energy on beaches which is the origin of the microplastics. At the meeting, we will also discuss a potential extension of the model in the case when the strength of plastic pieces depends on the size, and will compare with the kinetics of the fragmentation in the perspective of a stochastic process.