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 H(2)O-ice) have not been conducted yet. Thus, we used two different experimental methods to produce micrometer-sized H(2)O-ice particles, i.e. by spraying H(2)O droplets into liquid nitrogen and by spraying H(2)O droplets into a cold nitrogen atmosphere. The mean particle radii of the ice particles produced with these experimental methods are (1.49 +/- 0.79) mu m and (1.45 +/- 0.65) mu m. Ice aggregates composed of the micrometer-sized ice particles are highly porous (volume filling factor: phi = 0.11 +/- 0.01) or rather compact (volume filling factor: phi = 0.72 +/- 0.04), depending on the method of production. Furthermore, the critical rolling friction force of F(Roll,ice) = (114.8 +/- 23.8) x 10(-10) N was measured for micrometer-sized ice particles, which exceeds the critical rolling friction force of micrometer-sized SiO(2) particles (F(Roll,SiO2) = (12.1 +/- 3.6) x 10(-10) N). This result implies that the adhesive bonding between micrometer-sized ice particles is stronger than the bonding strength between 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 SiO(2) particles, results in gamma(ice) = 0.190 J m(-2). (C) 2011 Elsevier Inc. All rights reserved.
