Contacts of Water Ice in Protoplanetary Disks-Laboratory Experiments

Water ice is abundant in protoplanetary disks. Its sticking properties are therefore important during phases of collisional growth. In this work, we study the sticking and rolling of 1.1 mm ice grains at different temperatures. We find a strong increase in sticking between 175 and 200 K, which levels off at higher temperatures. In terms of surface energy this is an increase with a factor of 63.4, e.g., from gamma = 0.0029 to gamma = 0.19 J m(-2), respectively. We also measured critical forces for inelastic rolling. The critical rolling distance is constant with a value of 0.19 mm. In view of planetesimal formation at low temperatures in protoplanetary disks, the surface energy is not larger than for silicate dust, and ice aggregation will share the same shortcomings. In general, water ice has no advantage over silicates for sticking, and collisional growth might not favor ice over silicates.