22-EPN3-095: Behaviour of Saline Liquid Droplets in Wind Tunnel Conditions Relevant to the Plumes of Enceladus
Visit by Mark Fox-Powell of the Open University (UK), to TA2.4 Planetary Environment Facilities (PEF), AU (Denmark)
Dates of visit: 19-23 February 2024
Report summary:
The main aim of this project was to investigate the freezing of droplets of saline water injected at high velocity into low pressure conditions, in order to better understand the formation history of salt-rich ice grains encountered by Cassini in the plumes of Saturn’s moon Enceladus. Our objectives were to (1) investigate freezing times of sub-millimetric droplets under pressures representative of the subsurface icy vents at Enceladus; (2) investigate the tendency for ice grains to fragment into smaller grains on impact with solid surfaces; and (3) identify the effects of dissolved salts on both factors. Experiments were conducted in the Aarhus large wind tunnel. Liquids were sprayed into a range of pressures spanning 0.1 – 6 mbar. Spray droplets were imaged with a high-speed camera as they impacted an aluminium target plate, and velocity distributions of droplets were recorded using a laser doppler velocimeter. Real-time observations indicated a strong pressure dependence on freezing times, with droplets at the lowest pressures freezing in less than one second. We observed that the presence of salts enabled droplets to remain liquid longer at lower pressures. In ongoing work, we are analysing high speed footage to quantify the proportions of frozen vs. liquid droplet impacts, and fragmenting vs. non-fragmenting frozen grains, and how these proportions vary across all experiments. These data will allow us to predict likely freezing behaviour of similar droplets in the Enceladus plumes, leading to new understanding of the processes controlling ice grain formation at Enceladus.
