Comparison of sodium chloride hopper cubes grown under microgravity and terrestrial conditions

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.