Submillimetre-sized dust aggregate collision and growth properties

Abstract

Context. In the very first steps of the formation of a new planetary system, dust agglomerates grow inside the protoplanetary disk that rotates around the newly formed star. In this disk, collisions between the dust particles, induced by interactions with the surrounding gas, lead to sticking. Aggregates start growing until their sizes and relative velocities are high enough for collisions to result in bouncing or fragmentation. With the aim of investigating the transitions between sticking and bouncing regimes for colliding dust aggregates and the formation of clusters from multiple aggregates, the Suborbital Particle and Aggregation Experiment (SPACE) was flown on the REXUS 12 suborbital rocket. Aims. The collisional and sticking properties of sub-mm-sized aggregates composed of protoplanetary dust analogue material are measured, including the statistical threshold velocity between sticking and bouncing, their surface energy and tensile strength within aggregate clusters. Methods. We performed an experiment on the REXUS 12 suborbital rocket. The protoplanetary dust analogue materials were micrometre-sized monodisperse and polydisperse SiO 2 particles prepared into aggregates with sizes around 120 µm and 330 µm, respectively and volume filling factors around 0.37. During the experimental run of 150 s under reduced gravity conditions, the sticking of aggregates and the formation and fragmentation of clusters of up to a few millimetres in size was observed. Results. The sticking probability of the sub-mm-sized dust aggregates could be derived for velocities decreasing from ∼22 to 3 cm s −1 . The transition from bouncing to sticking collisions happened at 12.7 +2.1 −1.4 cm s −1 for the smaller aggregates composed of monodisperse particles and at 11.5 +1.9 −1.3 and 11.7 +1.9 −1.3 cm s −1 for the larger aggregates composed of mono-and polydisperse dust particles, respectively. Using the pull-off force of sub-mm-sized dust aggregates from the clusters, the surface energy of the aggregates composed of monodisperse dust was derived to be 1.6×10 −5 J m −2 , which can be scaled down to 1.7×10 −2 J m −2 for the micrometre-sized monomer particles and is in good agreement with previous measurements for silica particles. The tensile strengths of these aggregates within the clusters were derived to be 1.9 +2.2 −1.2 Pa and 1.6 +0.7 −0.6 Pa for the small and large dust aggregates, respectively. These values are in good agreement with recent tensile strength measurements for ∼mm-sized silica aggregates. Conclusions. Using our data on the sticking-bouncing threshold, estimates of the maximum aggregate size can be given. For a minimum mass solar nebula model, aggregates can reach sizes of ∼1 cm.

Low-velocity collision behaviour of clusters composed of sub-millimetre sized dust aggregates

“…The first consisted of monodisperse dust, composed of monodisperse spherical particles of 0.76 µm radius, manufactured by Micromod (see Figure 3b. in Brisset et al 2016). The second were compacted aggregates of monodisperse dust.…”

“…The other polydisperse dust listed is irregular and manufactured by Sigma-Aldrich ( Table 2). As these experiments were performed with a test version of the experiment hardware (later to be flown on a suborbital rocket; see Brisset et al 2016), not all cells were filled with dust aggregates and not all drops delivered useful experimental data. The agglomerate sizes are indicated with their standard deviation.…”

“…The compaction of the monodisperse aggregates during drop 3 is described in the text. type were aggregates of polydisperse dust, which is composed of irregularly shaped particles with radii between 0.05 µm and 5 µm, manufactured by Sigma-Aldrich (see Figure 3a. in Brisset et al 2016). Finally, the last aggregate type were aggregates of polydisperse dust consisting of polydisperse spheres with radii between 0.8 µm and 1.2 µm, manufactured by Admatechs (type SO-E3, see Figure 2).…”

“…This is of the same order as the experiment cell size (Figure 1), and the mean aggregate collision velocity can therefore be determined as in Brisset et al (2016). It is assumed that free-flying aggregates have a speed of about 1.8v max , where v max is the maximum velocity of the cell walls (see Figure 6 in Brisset et al 2016, where the mean collision velocity between aggregates and clusters during the SPACE suborbital flight was determined in a similar manner). The resulting mean collision velocities were 4.5 cm s −1 for drop 2 and 13.6 cm s −1 for drop 3.…”

“…One of the challenges of observing dust collisions in many particle systems at such gentle velocities is the necessity to conduct these experiments under microgravity conditions. In addition to flying on the REXUS 12 suborbital rocket (∼150 s of consecutive microgravity time) in 2012 (Brisset et al 2016), the SPACE experiment also performed a flight campaign at the Bremen drop tower in 2011. This work presents the experimental results obtained from these ∼9 s microgravity experiments.…”