  Water ice is one of the most abundant materials in dense molecular clouds and
in the outer reaches of protoplanetary disks. In contrast to other materials
(e.g., silicates) water ice is assumed to be stickier due to its higher
specific surface energy, leading to faster or more efficient growth in mutual
collisions. However, experiments investigating the stickiness of water ice have
been scarce, particularly in the astrophysically relevant micrometer-size
region and at low temperatures. In this work, we present an experimental setup
to grow aggregates composed of $\mathrm{\mu}$m-sized water-ice particles, which
we used to measure the sticking and erosion thresholds of the ice particles at
different temperatures between $114 \, \mathrm{K}$ and $260 \, \mathrm{K}$. We
show with our experiments that for low temperatures (below $\sim 210 \,
\mathrm{K}$), $\mathrm{\mu}$m-sized water-ice particles stick below a threshold
velocity of $9.6 \, \mathrm{m \, s^{-1}}$, which is approximately ten times
higher than the sticking threshold of $\mathrm{\mu}$m-sized silica particles.
Furthermore, erosion of the grown ice aggregates is observed for velocities
above $15.3 \, \mathrm{m \, s^{-1}}$. A comparison of the experimentally
derived sticking threshold with model predictions is performed to determine
important material properties of water ice, i.e., the specific surface energy
and the viscous relaxation time. Our experimental results indicate that the
presence of water ice in the outer reaches of protoplanetary disks can enhance
the growth of planetesimals by direct sticking of particles.
