  We investigate the behaviour of dust in protoplanetary disks under the action
of gas drag using our 3D, two-fluid (gas+dust) SPH code. We present the
evolution of the dust spatial distribution in global simulations of planetless
disks as well as of disks containing an already formed planet. The resulting
dust structures vary strongly with particle size and planetary gaps are much
sharper than in the gas phase, making them easier to detect with ALMA than
anticipated. We also find that there is a range of masses where a planet can
open a gap in the dust layer whereas it doesn't in the gas disk. Our dust
distributions are fed to the radiative transfer code MCFOST to compute
synthetic images, in order to derive constraints on the settling and growth of
dust grains in observed disks.
