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 H₂O-ice) have not been conducted yet. Thus, we used two different experimental methods to produce micrometer-sized H₂O-ice particles, i.e. by spraying H₂O droplets into liquid nitrogen and by spraying H₂O droplets into a cold nitrogen atmosphere. The mean particle radii of the ice particles produced with these experimental methods are (1.49 ± 0.79) μm and (1.45 ± 0.65) μm. Ice aggregates composed of the micrometer-sized ice particles are highly porous (volume filling factor: ? = 0.11 ± 0.01) or rather compact (volume filling factor: ? = 0.72 ± 0.04), depending on the method of production. Furthermore, the critical rolling friction force of F_Roll,ice = (114.8 ± 23.8) × 10⁻¹⁰ N was measured for micrometer-sized ice particles, which exceeds the critical rolling friction force of micrometer-sized SiO₂ particles . This result implies that the adhesive bonding between micrometer-sized ice particles is stronger than the bonding strength between SiO₂ 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 SiO₂ particles, results in γ_ice = 0.190 J m⁻².