Collisions of small ice particles under microgravity conditions

Abstract

Context. Planetisimals are thought to be formed from the solid material of a protoplanetary disk by a process of dust aggregation. It is not known how growth proceeds to kilometre sizes, but it has been proposed that water ice beyond the snowline might affect this process. Aims. To better understand collisional processes in protoplanetary disks leading to planet formation, the individual low velocity collisions of small ice particles were investigated. Methods. The particles were collided under microgravity conditions on a parabolic flight campaign using a purpose-built, cryogenically cooled experimental setup. The setup was capable of colliding pairs of small ice particles (between 4.7 and 10.8 mm in diameter) together at relative collision velocities of between 0.27 and 0.51 m s −1 at temperatures between 131 and 160 K. Two types of ice particle were used: ice spheres and irregularly shaped ice fragments. Results. Bouncing was observed in the majority of cases with a few cases of fragmentation. A full range of normalised impact parameters (b/R = 0.0-1.0) was realised with this apparatus. Coefficients of restitution were evenly spread between 0.08 and 0.65 with an average value of 0.36, leading to a minimum of 58% of translational energy being lost in the collision. The range of coefficients of restitution is attributed to the surface roughness of the particles used in the study. Analysis of particle rotation shows that up to 17% of the energy of the particles before the collision was converted into rotational energy. Temperature did not affect the coefficients of restitution over the range studied.

Comet formation in collapsing pebble clouds

“…Weidling & Blum (2015) performed microgravity experiments with mmsized dust aggregates and found that a coefficient of restitution uniformly distributed in a range between 0.29 and 0.81 with a mean value of 0.55 explains their experiments best. On the other hand, microgravity experiments with mm-to cm-sized ice pebbles have shown that the values for the coefficient of restitution are uniformly distributed between 0.06 and 0.84 with a mean value of 0.45 (Heißelmann et al 2010) or between 0.08 and 0.65 with a mean value of 0.36 (Hill et al 2015). Thus, our choice of coefficient of restitution is in agreement with both types of materials.…”

“…Δv i stick = 10 × Δv d stick , and similarly for bouncing and fragmentation. This factor of ten is motivated by the fact that the specific surface energy and the sticking threshold velocity of water ice is also ten times higher (Gundlach et al 2011;Hill et al 2015). The measured compression curve of ice shown in Fig.…”

“…Starting with μm-sized silica monomers, the growing aggregates encounter a bouncing barrier, which leads to the required compression . However, based on the fact that ice is stickier, harder to fragment, and harder to compress (Gundlach et al 2011;Hill et al 2015), Okuzumi et al (2012) showed that 0.1 μm-sized ice monomers grow to very porous aggregates, which do not experience significant collisional compression. Thus, we leave the issue of whether or not compact icy aggregates can form for future studies.…”

“…Collision models based on laboratory experiments Windmark et al 2012a) so far only deal with silica aggregates, because experiments with ice particles are difficult to conduct. However, early results suggest that the threshold velocities for sticking, bouncing, and fragmentation of ice particles are by a factor of 10 higher than for silica (Gundlach et al 2011;Hill et al 2015). The compression curve of ice aggregates, on the other hand, cannot be scaled as easily as the threshold velocities (see Fig.…”

Micrometer-sized Water Ice Particles for Planetary Science Experiments: Influence of Surface Structure on Collisional Properties

“…But, we can only quantify collisional outcomes empirically, to learn how icy dust sticks under proto-planetary conditions (e.g. Gundlach & Blum 2015;Hill et al 2015;Bridges et al 1996;Higa et al 1996).…”