  We study the outflow of dust particles on the surface layers of optically
thick disks. At the surface of disks around young stars, small dust particles
(size < 10 micron) experience stellar radiation pressure support and orbit more
slowly than the surrounding gas. The resulting tail-wind imparts energy and
angular momentum to the dust particles, moving them outward. This outflow
occurs in the thin surface layer of the disk that is exposed to starlight, and
the outward mass flux is carried primarily by particles of size ~0.1 micron.
Beneath the irradiated surface layer, dust particles experience a head-wind,
which drives them inward. For the specific case of a minimum-mass-solar-nebula,
less than a thousandth of the dust mass experiences outward flow. If the
stellar luminosity is 15 times brighter than the sun, however, or if the gas
disk mass is as small as ~100 M_earth, then the surface outflow can dominate
the inward flux in certain radial ranges, leading to the formation of rings or
gaps in the dust disks.
