  Particles floating on the surface of a turbulent incompressible fluid
accumulate along string-like structures, while leaving large regions of the
flow domain empty. This is reflected experimentally by a very peaked
probability distribution function of $c_r$, the coarse-grained particle
concentration at scale $r$, around $c_r = 0$, with a power-law decay over two
decades of $c_r$, $\Pi (c_r) \propto c_r^{-\beta_r}$. The positive exponent
$\beta_r$ decreases with scale in the inertial range, and stays approximately
constant in the dissipative range, thus indicating a qualitative difference
between the dissipative and the inertial ranges of scales, also visible in the
first moment of $c_r$.
