Bottle of supercooled water + agitation = some ice and water

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Spinnor
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In an experiment plastic bottles of supercooled water were agitated causing visible amounts of ice to form (the water/ice mixture was filtered into a measuring cup, not a large fraction of ice formed). Why did the agitation ("not a lot needed") set off ice formation?

Is it the large scale water stillness that prevents ice formation?

Is there some kind of "barrier" here that must be overcome?

After the ice forms can we say anything about the temperature of the water?

If we knew the temperature of the water, say 1 degree C below freezing, should we be able to calculate the amount of ice formed after we agitate the water?

Does the amount of agitation needed to cause ice formation depend on the temperature of the water below freezing?

Thanks for any thoughts!
 

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Is there some kind of "barrier" here that must be overcome?
The "barrier" is the formation of an initial ice crystal. Two or three water molecules sticking together don't form a proper ice crystal, and their bonds won't be very strong compared to a larger crystal. The water is so cold that an existing crystal will grow, but it is not so cold that a crystal will easily form if there is none present.

Moving the water around is likely to produce some places with different pressure, rearranging some particles floating in the water or whatever, and it increases the chance that there are good conditions for the formation of an initial crystal somewhere.
If we knew the temperature of the water, say 1 degree C below freezing, should we be able to calculate the amount of ice formed after we agitate the water?

Does the amount of agitation needed to cause ice formation depend on the temperature of the water below freezing?
Sure. You can calculate how much heat is released when x gram of ice form, and see how much heat is needed to heat the water to 0 degree C.
 

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