  Dust gaps and rings appear ubiquitous in bright protoplanetary disks.
Disk-planet interaction with dust-trapping at the edges of planet-induced gaps
is one plausible explanation. However, the sharpness of some observed dust
rings indicate that sub-mm-sized dust grains have settled to a thin layer in
some systems. We test whether or not such dust around gas gaps opened by
planets can remain settled by performing three-dimensional, dust-plus-gas
simulations of protoplanetary disks with an embedded planet. We find planets
massive enough to open gas gaps stir small, sub-mm-sized dust grains to high
disk elevations at the gap edges, where the dust scale-height can reach ~70% of
the gas scale-height. We attribute this dust 'puff-up' to the planet-induced
meridional gas flows previously identified by Fung & Chiang and others. We thus
emphasize the importance of explicit 3D simulations to obtain the vertical
distribution of sub-mm-sized grains around gas gaps opened by massive planets.
We caution that the gas-gap-opening planet interpretation of well-defined dust
rings is only self-consistent with large grains exceeding mm in size.
