Understanding the Salt Crystallizations from Droplets under Various Gravity and Pressure Environments: Display of the Marangoni Effect?

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Understanding the drying mechanisms, crystallization, and creep dynamics in salt solutions relies on a thorough understanding of the evaporation kinetics of individual droplets. These processes are of significant interest due to their implications in fields such as astrochemistry, environmental science, and material science. However, under nonequilibrium conditions like reduced pressure (<1 atm) and microgravity (<1g), they remain poorly understood, creating a pressing need for studies addressing these gaps. Our study investigates the evaporation of droplets from saturated salt solutions under nonequilibrium conditions, with experiments conducted in low-pressure and microgravity environments using a vacuum chamber and an acoustic levitation setup. We focused on the in situ crystallization dynamics of sodium chloride, potassium chloride, and ammonium chloride droplets, hypothesizing that the poleward migration of salt within levitating droplets is driven by the Marangoni effect, which arises from concentration gradients.1 Additionally, we explored the droplet evaporation mechanism under low pressure, examining factors contributing to the reduced "ring effect" in these conditions. Our findings indicate that the surface tension of the substrate under low pressure plays a crucial role in crystallization mechanisms, influencing the size of the central ring. These insights not only enhance our understanding of salt crystallization dynamics but also address the demand for innovative approaches to studying these processes in extraterrestrial and extreme environments. Furthermore, they provide a refined physical analysis and suggest potential applications in astrophysics, space exploration, and sustainable source management.