Is the dust-to-gas ratio constant in molecular clouds?
  We perform numerical simulations of dusty, supersonic turbulence in molecular
clouds. We model 0.1, 1 and 10 {\mu}m sized dust grains at an initial
dust-to-gas mass ratio of 1:100, solving the equations of combined gas and dust
dynamics where the dust is coupled to the gas through a drag term. We show
that, for 0.1 and 1 {\mu}m grains, the dust-to-gas ratio deviates by typically
10-20% from the mean, since the stopping time of the dust due to gas drag is
short compared to the dynamical time. Contrary to previous findings, we find no
evidence for orders of magnitude fluctuation in the dust-to-gas ratio for 0.1
{\mu}m grains. Larger, 10 {\mu}m dust grains may have dust-to-gas ratios
increased by up to an order of magnitude locally. Both small (0.1 {\mu}m) and
large ($\gtrsim$ 1 {\mu}m) grains trace the large-scale morphology of the gas,
however we find evidence for 'size-sorting' of grains, where turbulence
preferentially concentrates larger grains into dense regions. Size-sorting may
help to explain observations of 'coreshine' from dark clouds, and why
extinction laws differ along lines of sight through molecular clouds in the
Milky Way compared to the diffuse interstellar medium.
