Table 2 NaCl hopper morphology in normal gravity and in microgravity
From: Comparison of sodium chloride hopper cubes grown under microgravity and terrestrial conditions
Terrestrial (normal gravity) | ISS (microgravity) |
|---|---|
Hopper cubes | Hopper cubes |
Hopper cubes form in symmetrical growing conditions. They grow randomly orientated in stirred solutions in the bulk of the brine or displacively in soft mud sediments. | Hopper cubes form in symmetrical growing conditions. They grow randomly orientated in the bulk of the brine. |
Tabular hopper crystals | Tabular hopper crystals |
Grow as thin square plates attached to the air/brine surface. Upon reaching a size of about 1 mm, they detach from the surface and sink to the brine bottom where they continue to grow as a lumped mass, or they nucleate and grow directly lying on the bottom. | Grow as thin square plates attached to the air/brine interface and remain attached by surface tension forces until they agglomerate into an interlocking crust. |
Hopper pyramids (fleur de sel) | Hopper pyramids (fleur de sel) |
These are hollow four sided regularly stepped pyramids with an open square base attached to the brine surface with the apexes oriented downward. They grow macro stepwise at elevated temperatures of 25–80 °C in the localized supersaturated surface film. When one face remains in contact with the surface, growth is only at the highest-lying edges.33 At ~3–4 mm in size, surface tension forces can no longer keep them attached to the brine surface and they sink to the bottom stopping their characteristic growth. At the surface of a shallow brine pool cm sized floating hopper pyramids have been observed.34,35 | We did not observe hopper pyramids in microgravity. We speculate hopper pyramid morphology requires gravitational force to keep the apex directed into the bulk brine thus allowing the growth of a regular sequence of new macro steps. Our brine temperature was 19 °C where hopper pyramid morphology requires a minimum of 25 °C. Fleur de sel is naturally produced in solar salt ponds at temperatures of ca 30 °C and 40% rel. humidity.36 This temperature is needed to create by evaporation of brine the highly supersaturated surface layer within the stepwise growth occurs. |
