Design of a cryo-vacuum drop tower for simulating water ice aggregates collisions in planetesimal formation

Water ice aggregates play a crucial role in the formation of solar system planets. The growth of water ice are considered a driving force for planetesimal formation, particularly in the outer regions where temperatures are low enough for water to freeze. An investigation to the collisional behavior of aggregates made of μm-sized water ice particles under microgravity will contribute to understanding the growth of planetesimals and formation of ice giants.

We constructed a cryo-vacuum drop tower to simulate the growth and collisional evolution of ice aggregates under microgravity. This drop tower enables a systematic process for preparing ice aggregates through sedimentary growth and supports collision experiments at velocities ranging from 0.2 m/s to 0.5 m/s. The experimental scheme in this work aims to determine the physical properties of ice aggregates, such as velocity thresholds, sticking probabilities, and collision parameters.

We designed a multifunctional vacuum drop tower with a height of 3.6 m and an inner diameter of 1 m. The experimental devices connected to the top cover of the drop tower can be flexibly replaced or customized according to specific requirement of each experiment. For the ice aggregates collision experiments, we developed a cylindrical sample chamber where the temperature can be maintained below 130K by using a refrigerator. The chamber, attached to the top cover, includes two holders functioning as the release mechanism, each with a crystallization ladle at its end. Inside the chamber, 𝜇m-sized water droplets are introduced, and random ballistic deposition (RBD) aggregates are formed on the crystallization ladles. The sedimentary growth process is monitored through an observation window located at the top of the sample chamber. After preparing the aggregates, the internal pressure of the drop tower is reduced to below 1 Pa. The release mechanism then sequentially releases two samples, and the samples collide at a settled relative velocity during free fall. Two high-speed cameras mounted on the outer rail of the drop tower will be released simultaneously and record the entire collision process.

Compared to previous experimental studies, our work offers several advantages. The drop tower integrates sample preparation and collision experiment, enabling in-suit preparation and release of aggregates. Additionally, a refrigerator is used for colling instead of liquid nitrogen, providing safer and more reliable refrigeration. High-speed cameras are mounted on rails, ensuring a more stable observation platform. We plan to conduct collision experiments to investigate the physical properties and growth mechanisms of water ice aggregates.

We will present the design and techniques of the cryo-vacuum drop tower and demonstrate its capabilities.