  Abundances and partitioning of ices and gases produced by gas-grain chemistry
are governed by adsorption and desorption on grains. Understanding
astrophysical observations rely on laboratory measurements of adsorption and
desorption rates on dust grains analogs. On flat surfaces, gas adsorption
probabilities (or sticking coefficients) have been found close to unity for
most gases. Here we report a strong decrease of the sticking coefficients of
H2O and CO2 on substrates more akin to cosmic dust, such as submicrometer-sized
particles of carbon and olivine, bare or covered with ice. This effect results
from the local curvature of the grains, and then extends to larger grains made
of aggregated small particles, such as fluffy or porous dust in more evolved
media (e.g. circumstellar disks). The main astrophysical implication is that
accretion rates of gases are reduced accordingly, slowing the growth of cosmic
ices. Furthermore, volatile species that are not adsorbed on a grain at their
freeze-out temperature will pertain in the gas phase, which will impact gas-ice
partitions. We also found that thermal desorption of H2O is not modified by
grains size, and thus the snowlines temperature should be independent on the
dust size distribution.
