Abstract
Sodium chloride (NaCl) grown in terrestrial conditions form hopper cubes under diffusion controlled mass transport (Péclet number: ≪ 1), high supersaturations (S > 1.45), and fast growth rates (10–110 µm/s) over periods only maintainable for seconds to minutes yielding hopper cubes typically <250 µm. Here we report on NaCl hopper cubes grown in microgravity on the International Space Station (ISS) by evaporation of brine. They grew under diffusion limited mass transport (Péclet number: ~4 × 10−4 − 4) at low supersaturation (S < 1.002) and slow growth rates (0.34–1 µm/min) over periods of days to weeks. Due to the lack of sedimentation, symmetrical hopper cubes, 2–8 mm were produced. The most striking differences between microgravity and terrestrial gravity hopper growth conditions are low supersaturation and slow growth rates over long periods of time. Large, 1–20 cm naturally occurring symmetrical NaCl hopper cubes are found suspended in brine soaked mud, hypothesized to be produced in a slow growth, diffusion dominated environment. We speculate these geologic conditions allow for hopper growth similar to that of microgravity.
Highlights
The habit of a crystal is given by its internal structure and the external growth conditions
For the 2–3 cm diameter crystallizers, time lapse imagery showed that relative motion between bulk suspended crystal faces and the immediate surrounding brine was of the order of 100 μm/min
For the 0.8 cm diameter crystallizers, the bulk brine motion was effectively suppressed giving a relative motion between the hopper cube faces and the surrounding brine too small to be determined from time lapse imagery
Summary
The habit of a crystal is given by its internal structure and the external growth conditions. The later are mainly determined by impurities, temperature, supersaturation, mass transfer, heat transfer and sedimentation. Small differences in concentration and temperature are generated and under the influence of gravity these result in buoyancy driven convection that in turn affects the mass and heat transfer. Symmetrical growing crystals quickly become influenced by sedimentation, typically sinking to the crystallizer bottom. Due to asymmetrical growth conditions, crystallographic identical faces grow with different rates resulting in different crystal habits (e.g. crystals change from cubic to tabular habitus). To maintain symmetrical growth conditions on earth, crystals have to remain suspended in the solution by stirring or freely hanging in the solution or being submersed in a viscous gelatinous medium. Density driven convection and sedimentation are essentially eliminated
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
