Clustering and dynamic decoupling of dust grains in turbulent molecular
  clouds
  We present high resolution ($1024^3$) simulations of super-/hyper-sonic
isothermal hydrodynamic turbulence inside an interstellar molecular cloud
(resolving scales of typically 20 -- 100 AU), including a multi-disperse
population of dust grains, i.e., a range of grain sizes is considered. Due to
inertia, large grains (typical radius $a \gtrsim 1.0\,\mu$m) will decouple from
the gas flow, while small grains ($a\lesssim 0.1\,\mu$m) will tend to better
trace the motions of the gas. We note that simulations with purely solenoidal
forcing show somewhat more pronounced decoupling and less clustering compared
to simulations with purely compressive forcing. Overall, small and large grains
tend to cluster, while intermediate-size grains show essentially a random
isotropic distribution. As a consequence of increased clustering, the
grain-grain interaction rate is locally elevated; but since small and large
grains are often not spatially correlated, it is unclear what effect this
clustering would have on the coagulation rate. Due to spatial separation of
dust and gas, a diffuse upper limit to the grain sizes obtained by
condensational growth is also expected, since large (decoupled) grains are not
necessarily located where the growth species in the molecular gas is.
