  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.
