Exploring salt droplet crystallization dynamics in controlled experimental environment: Microgravity and Low-pressure conditions

Rim Hadidi

Florida A&M university

Research into salt creep under controlled experimental conditions has recently gained considerable attention and activity^{1 2 3}. A comprehensive grasp of drying mechanisms, as well as crystallization and creep dynamics, is contingent upon understanding the evaporation kinetics of individual droplets of salt solution. Our investigation delves into novel facets of sample manipulation amidst a non-equilibrium setting, particularly within reduced-pressure and microgravity environments, facilitated by the acoustic levitation method. These conditions can profoundly influence salt crystallization dynamics by accelerating solvent evaporation, increasing supersaturation levels, and shaping the kinetics and morphology of crystal growth. Acoustic levitation, promising method for sample manipulation in salt crystallization studies, entails suspending a liquid droplet just beneath the pressure nodes of an ultrasonic standing wave, offering substantial benefits by eliminating the complex effects of a contacting surface, thereby enabling controlled access to supersaturated solutions under precisely regulated conditions. Throughout levitation, the gradual reduction in droplet volume due to solvent evaporation leads to an increase in solute concentration⁴. Prior studies^{5 6} have already presented intricate models elucidating the droplet evaporation physics. Building upon this foundation, in our study, we have phenomenologically dissected the in-situ crystallization process of a salt droplet and proposed hypotheses regarding the underlying mechanisms governing salt migration towards levitated droplets poles, stemming from the Marangoni effect induced by concentration gradients⁷. Furthermore, our research offers fresh perspectives on alterations in crystal size post-droplet evaporation under reduced-pressure, focusing on sodium chloride and potassium chloride crystals. We have also explored the mechanisms behind the decrease in the observed "ring effect" under reduced pressure as opposed to atmospheric conditions. Our hypotheses revolve around of the impact of substrate surface tension under reduced pressure influencing the size of the central ring, consequently the crystallization mechanism. These findings furnish more nuanced physical analyses and open up avenues for various applications relevant to astrophysics and environmental science. Ultimately, our results will enhance our understanding of salt crystallization and creep under reduced gravity and pressure conditions, with pivotal implication for the design and operation of systems for extraterrestrial exploration, including NASA missions on the Moon and Mars.

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