Is water ice an efficient facilitator for dust coagulation?
  Beyond the snow line of protoplanetary discs and inside the dense core of
molecular clouds, the temperature of gas is low enough for water vapour to
condense into amorphous ices on the surface of preexisting refractory dust
particles. Recent numerical simulations and laboratory experiments suggest that
condensation of the vapour promotes dust coagulation in such a cold region.
However, in the numerical simulations, cohesion of refractory materials is
often underestimated, while in the laboratory experiments, water vapour
collides with surfaces at more frequent intervals compared to the real
conditions. Therefore, to re-examine the role of water ice in dust coagulation,
we carry out systematic investigation of available data on coagulation of water
ice particles by making full use of appropriate theories in contact mechanics
and tribology. We find that the majority of experimental data are reasonably
well explained by lubrication theories, owing to the presence of a quasi-liquid
layer (QLL). Only exceptions are the results of dynamic collisions between
particles at low temperatures, which are, instead, consistent with the JKR
theory, because QLLs are too thin to dissipate their kinetic energies. By
considering the vacuum conditions in protoplanetary discs and molecular clouds,
the formation of amorphous water ice on the surface of refractory particles
does not necessarily aid their collisional growth as currently expected. While
crystallisation of water ice around but outside the snow line eases coagulation
of ice-coated particles, sublimation of water ice inside the snow line is
deemed to facilitate coagulation of bare refractory particles.
