On the crystallization of spheres and shells

Physics

Scientific paper

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Water In The Atmosphere, Cloud Physics And Chemistry, Solid-Liquid Transitions

Scientific paper

Isolated drops of many solutions or pure materials readily supercool below their equilibrium crystallization point. In some cases a glass forms; otherwise the crystallization process is usually initiaed at a specific point in the drop leading to a complex structure. Under moderate supercooling, in the first stage of crystallization dendrites grow throughout the drop, leading to latent heat release, and temperature rise. The dendrite growth velocity, tip radius and crystal orientation depend critically on the supercooling.
The second stage of crystallization depends on the geometry of the heat loss from the periphery and results in freezing from one side of the drop for asymmetric heat loss or as a thickening shell for symmetrical heat loss. Most solute is rejected at this stage, to nucleate in a geometry determined by the original dendrite distribution and the growth interface, and the internal pressure as it responds to volume changes and cracking of the shell. New crystal orientation appear for nucleation at large supercooling and also in large drops, following reorientation of dendrite arms separated during Ostwald Ripening.
Crystal growth is usually uninfluenced by the interface in a gas or liquid environment but changes habit to thin needle like crystals, and increases growth velocity for a solid interface. Drops of size of the tip radius will no longer be spherical on crystallization. For a solvent which evaporates, crystals grow faceted (as hydrates); surface tension forces may move solvent over the surface to give well formed faceted crystals.
It is suggested that faceted defect free crystals can be grown from drops under low g using controlled nucleation in a controlled vapor and temperature environment in the absence of convective motion.

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