Micrometer-sized ice particles for planetary-science experiments - I. Preparation, critical rolling friction force, and specific surface energy

Coagulation models assume a higher sticking threshold for micrometer-sized ice particles than for micrometer-sized silicate particles. However, in contrast to silicates, laboratory investigations of the collision properties of micrometer-sized ice particles (in particular, of the most abundant water ice) have not been conducted yet. Thus, we used two different experimental methods to produce micrometer-sized water ice particles, i. e. by spraying water droplets into liquid nitrogen and by spraying water droplets into a cold nitrogen atmosphere. The mean particle radii of the ice particles produced with these experimental methods are $(1.49 \pm 0.79) \, \mathrm{\mu m}$ and $(1.45 \pm 0.65) \, \mathrm{\mu m}$. Ice aggregates composed of the micrometer-sized ice particles are highly porous (volume filling factor: $\phi = 0.11 \pm 0.01$) or rather compact (volume filling factor: $\phi = 0.72 \pm 0.04$), depending on the method of production. Furthermore, the critical rolling friction force of $F_{Roll,ice}=(114.8 \pm 23.8) \times 10^{-10}\, \mathrm{N}$ was measured for micrometer-sized ice particles, which exceeds the critical rolling friction force of micrometer-sized $\mathrm{SiO_2}$ particles ($F_{Roll,SiO_2}=(12.1 \pm 3.6) \times 10^{-10}\, \mathrm{N}$). This result implies that the adhesive bonding between micrometer-sized ice particles is stronger than the bonding strength between $\mathrm{SiO_2}$ particles. An estimation of the specific surface energy of micrometer-sized ice particles, derived from the measured critical rolling friction forces and the surface energy of micrometer-sized $\mathrm{SiO_2}$ particles, results in $\gamma_{ice} = 0.190 \, \mathrm{J \, m^{-2}}$.