Preparation and characterization of micrometer-sized ice particles for planetary-science experiments

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 established two different experimental methods to produce micrometer-sized water ice particles, i. e. by spraying water droplets into liquid nitrogen an by spraying water droplets into a cold nitrogen atmosphere. We found that the size of the ice particles, produced with the different methods, are very similar (arithmetic means of the particle radii for the two different methods: (1.49+-0.79) micrometer and (1.45+-0.65) micrometer, respectively). 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, a rolling friction force of (114.8+-23.8)*10^(-10) N was measured for micrometer-sized ice particles, which exceeds the rolling friction force of micrometer-sized SiO2 particles ((12.1+-3.6)*10^(-10) N). 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 rolling friction forces and the surface energy of micrometer-sized SiO2 particles, results in a specific surface energy of the ice particles ranging between 0.100 J m^(-2) and 0.185 J m^(-2).