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 } } .