Particles floating on turbulent water

Every year, millions of tons of plastics enter the ocean. Devising effective strategies to mitigate such pollution requires the quantitative understanding of how floating particles travel and spread in turbulent waters. Past studies have mostly focused on the influence exerted by the surface on the flow underneath, while the characterization of the transport along the surface remains incomplete. I will summarize our recent experiments using laboratory and field-scale facilities. Microscopic particles faithfully follow the free surface flow, revealing a rich structure that shares similarities with both three-dimensional and two-dimensional turbulence, as well as unique features. In particular, due to the compressibility of the surface velocity field, the particles cluster over a wide range of spatial and temporal scales. For millimetre-sized particles, capillarity-driven attraction breaks the equilibrium between cluster formation and breakup, leading to aggregates that grow steadily in size. Even larger particles filter the small-scale velocity fluctuations, which results in a more time-correlated motion and, in turn, faster dispersion.