Clustering of floaters on the free surface of a turbulent flow: an   experimental study
  We present an experimental study of the statistical properties of millimeter-size spheres floating on the surface of a turbulent flow. The flow is generated in a layer of liquid metal by an electromagnetic forcing. By using two magnet arrays, we are able to create one highly fluctuating flow and another, more stationary flow. In both cases, we follow the motion of hundreds of particles floating at the deformed interface of the liquid metal. We evidence the clustering of floaters by a statistical study of the local concentration of particles. Some dynamical properties of clusters are exposed. We perform spatial correlations between particle concentration and hydrodynamical quantities linked with inertial effects; with vortical motion, and with horizontal divergence (corresponding to compressibility in the surface). From comparing these correlations, we propose the so-called surface compressibility as the main clustering mechanism in our system. Hence, although floaters are not passive scalar and move on a deformed surface, the scenario is similar to the one reported for passive scalar on an almost flat free surface of a turbulent flow. 
Pablo Guti\'errez and S\'ebastien Auma\^itre
physics.flu-dyn