16h00 - 16h25 : A laboratory model for plastic fragmentation in the turbulent ocean.

While marine plastic pollution is ubiquitous and represents a global environmental threat, fundamental questions related to the fate of plastic debris in oceans remain poorly understood. In particular, the fragmentation process of marine plastic items, at the origin of the dispersion of microplastic debris across the globe, remains qualitatively described in the literature and a quantitative description is therefore needed. For instance, the fragmentation model currently used considers impact between solids, which is not relevant for a turbulent fluid environment. Here, I will present a physical model for the fragmentation of plastic debris in the turbulent upper layer of the ocean. Using laboratory experiments on the fragmentation of brittle fibres in a turbulent flow complemented by numerical simulations and theoretical analyses, our results demonstrate that the fragmentation process is limited at small scales by a physical cut-off length. This length originates from the fluid-structure interactions between the debris and the surrounding turbulent flow field, and is shown to be independent of the brittleness of the fibres. Such limitation mechanism of the fragmentation process at small scales is able to reproduce the size distribution of floating plastic debris measured in the ocean and therefore paves the way for a thorough understanding of marine plastic fragmentation.