ICAPS: Charge effects in dust agglomeration experiments – Results from the TEXUS-56 sounding rocket flight

The growth of dust grains to dust aggregates is a very important process in the chain of events from dust to planetesimals in protoplanetary discs (PPDs). Not only the size, shape, and porosity play an important role in the collisional growth process, but also the collision speed, the type of gas coupling and the charge of the dust particles [1, 2].

The ICAPS (Interactions in Cosmic and Atmospheric Particle Systems) campaign provides an experimental approach to protoplanetary dust growth and all the above parameters under realistic PPD conditions. The first ICAPS experiment flew onboard the TEXUS-56 sounding rocket and consisted of a vacuum chamber with a cloud of micrometer-sized SiO₂ spheres embedded in a rarefied gas inside. The dust particles could be manipulated using temperature and external electric fields. During flight, the particles were observed using two overview cameras and a high-speed camera attached to a long-distance microscope. In total, three electrical scans were conducted to measure the charge distribution of the dust particles. Two of these scans (E1, E3) were applied immediately after the two dust injections, while a longer one (E2) was performed after the Brownian growth phase. Each of these scans consisted of two equal-length phases of different field polarisation. The analysis of the image recordings provided precise particle tracks and velocities as well as the mass and size of the dust aggregates [3]. From the change in velocity, when the external electric field was present, it was also possible to derive the particle charge.

Fig.1:   Charge per monomer plotted against the cumulative normalized frequency of tracked particles during the electrical scans immediately after the first dust injection (E1), after the Brownian motion phase (E2), and after the second injection (E3), respectively. The duration between E1 and E2 was 164s. In this period, the mean charge per monomer grain decreased to less than 40% of the initial value.

In Fig. 1, the electric charge per monomer grain is plotted as a cumulative normalized frequency distribution of all particles tracked during each scan. Over the duration of 164 s between E1 and E2, a reduction of the mean charge per monomer grain to less than 40% of the initial value was observed. This finding is an indication that there was a relatively large number of distributed charges immediately after the injection, which allowed rapid agglomeration due to the charge-enhanced collision cross-section. At a later stage of the experiment run, the agglomeration was likely mainly driven by Brownian motion and dipole-dipole interactions. It is planned that the evolution of grain charging during agglomeration will be explored in more detail as part of the Laplace campaign, which will use a similar setup for a variety of experiments on the ISS. 

 

References

[1] Blum, J., “Dust agglomeration”, Advances in Physics, vol. 55, pp. 881–947, 2006. doi:10.1080/00018730601095039.

[2] Güttler, C., Blum, J., Zsom, A., Ormel, C. W., and Dullemond, C. P., “The outcome of protoplanetary dust growth: pebbles, boulders, or planetesimals?. I. Mapping the zoo of laboratory collision experiments”, Astronomy and Astrophysics, vol. 513, 2010. doi:10.1051/0004-6361/200912852.

[3] Schubert, B., “ICAPS Sounding Rocket - Particle Growth”, 2020. doi:10.5194/epsc2020-567.