Particle transport along the non-wavy free surface above turbulent water

Because a large fraction of all plastics are less dense than water, the fate of microplastics depends on the dynamics of free-surface turbulence. Past studies in this area have mostly focused on the influence exerted by the surface on the flow underneath, while the characterization of the transport along the surface itself remains incomplete. I will summarize our recent experiments on tracer particles floating in turbulent water, using laboratory-scale and field-scale facilities: grid turbulence in a large open-channel flow, homogeneous turbulence in a zero-mean flow tank, and an outdoor stream. We focus on regimes in which surface waves are too small to affect the transport. Single-point and two-point statistics in both Eulerian and Lagrangian frames are explored. Although the tracers move in two dimensions, their motion is consistent with the hallmarks of Kolmogorov's theory for three-dimensional turbulence, as revealed by the velocity fluctuations, structure functions, energy cascade, and Lagrangian dispersion. Uniquely to the compressibility of the free surface, the tracers cluster strongly and do so over spatial and temporal scales comparable to the integral scales of the turbulence. Finally, the effect of particle size, shape and concentration will be discussed.