  Observed protoplanetary disks consist of a large amount of micrometer-sized
particles. Dullemond and Dominik (2005) pointed out for the first time the
difficulty in explaining the strong mid-IR excess of classical T-Tauri stars
without any dust-retention mechanisms. Because high relative velocities in
between micrometer-sized and macroscopic particles exist in protoplanetary
disks, we present experimental results on the erosion of macroscopic
agglomerates consisting of micrometer-sized spherical particles via the impact
of micrometer-sized particles. We find that after an initial phase, in which an
impacting particle erodes up to 10 particles of an agglomerate, the impacting
particles compress the agglomerate's surface, which partly passivates the
agglomerates against erosion. Due to this effect the erosion halts within our
error bars for impact velocities up to ~30 m/s. For larger velocities, the
erosion is reduced by an order of magnitude. This outcome is explained and
confirmed by a numerical model. In a next step we build an analytical disk
model and implement the experimentally found erosive effect. The model shows
that erosion is a strong source of micrometer-sized particles in a
protoplanetary disk. Finally we use the stationary solution of this model to
explain the amount of micrometer-sized particles in observational infrared data
of Furlan et al. (2006).
