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 $gamma_{ice} = 0.190 , mathrm{J , m^{-2}}$.
Download