Naked Science Forum

General Science => General Science => Topic started by: quells on 11/05/2011 12:43:04

Title: Why is there a "halo" of unfrosted moisture around a salt crystal during frost?
Post by: quells on 11/05/2011 12:43:04

This photo was taken during the winter after I had salted my driveway that was subsequently rained on. Can anyone explain why there is a ring of non-frozen moisture around the larger crystals that were not washed away?

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Title: Why is there a "halo" of unfrosted moisture around a salt crystal during frost?
Post by: RD on 11/05/2011 14:47:27

An area of salt water has leached out from the salt crystal via rain water, (concentration gradient (http://en.wikipedia.org/wiki/Concentration_gradient)).
The salt water has a lower freezing temperature than rain water and hasn't frozen.

The difficult bit to explain is why the bulls-eye pattern ? …

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Perhaps some sort of reaction–diffusion pattern ?,  e.g. ...

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http://en.wikipedia.org/wiki/Reaction%E2%80%93diffusion_system#Two-component_reaction.E2.80.93diffusion_equations


Variations in temperature may be a factor ...

Quote

Well-controllable experiments in chemical reaction-diffusion systems have up to now been realized in three ways. First, gel reactors or filled capillary tubes may be used. Second, temperature pulses on catalytic surfaces have been investigated.

http://en.wikipedia.org/wiki/Reaction%E2%80%93diffusion_system#Experiments

Title: Why is there a "halo" of unfrosted moisture around a salt crystal during frost?
Post by: RD on 11/05/2011 20:15:16

Quote

Mathematics and the modeling of reaction-diffusion processes

Liesegang patterning is a special type of chemical pattern formation in which the spatial order is formed by density fluctuations of a weakly soluble salt ...

... accidentally dropped a small crystal of silver-nitrate onto a specimen, and was astonished that the precipitate formed concentric rings about it.

(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.insilico.hu%2Fliesegang%2Fhistory%2Fpatt.jpg&hash=69f579caba837bd7685086a67039f72b)

http://www.insilico.hu/liesegang/history/history.html

Title: Why is there a "halo" of unfrosted moisture around a salt crystal during frost?
Post by: quells on 12/05/2011 16:06:32

Thanks RD but diffusion doesn't cut it for me. The ring is about 4 inches in diameter and 2 inches away from the crystal. And why is the moisture frozen close to the crystal? Or put another way, why is the salt concentration more in the ring than close to the crystal?

Title: Why is there a "halo" of unfrosted moisture around a salt crystal during frost?
Post by: RD on 12/05/2011 23:40:20

You could try posting the photos on the New Scientist "last word" blog : they specalise in these type of phenomena,
 e.g. ... http://www.last-word.com/content_handling/show_tree/tree_id/3958.html