I'm interested to know how long it takes to change states of matter and why it takes the time that it does. For example, what is the very quickest that we could freeze a packet of chicken breasts? In the food industry, flash freezing is often used to prevent damage from occurring by freezing meat very quickly. But why does it take the time that it does? Would it be possible to freeze my packet of chicken in say, a nano second instead of a few minutes?
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$\begingroup$ It's rather for Physis SE $\endgroup$– MithoronAug 18, 2015 at 12:24
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$\begingroup$ I think it's fine for Chem SE. States of matter and interrelations thereof are perfectly cromulent area of chemistry. $\endgroup$– buckminstAug 18, 2015 at 13:21
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$\begingroup$ Heat transfer takes time. The greater the temperature difference the faster heat transfer but it still takes time. Even at absolute zero I doubt you could freeze a packet of chicken in a nano second. Also it is expensive to create very low temperatures. $\endgroup$– paparazzoAug 18, 2015 at 14:45
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$\begingroup$ I understand that but why does it take the time it does? What dictates the time it takes for the change of state to occur? Is there some absolute constant at work here which cannot be got around or will human ingenuity continue to find new ways of freezing faster? $\endgroup$– SeaCatAug 18, 2015 at 15:05
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1$\begingroup$ @SeaCat The nature of heat, temperature, and entropy is bit broad. I am not seeing the piratical value of the question. $\endgroup$– paparazzoAug 18, 2015 at 18:35
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1 Answer
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Part of the answer is that the rate of freezing depends on the heat of fusion of the material being frozen (relatively large for water, the chief ingredient of uncooked chicken), but smaller for other substances).
Another piece of the answer is that an object can lose heat by convection, conduction, and/or radiation. You would increase the rate of heat loss by:
(1) increasing the surface area of the chicken as much as possible (flattening it),
(2) placing it against a surface held as cold as possible and made of a material such as metal with high heat capacity;
(3) hitting it with a continuous blast of extremely cold air or gas at the highest possible wind speed (remember wind chill factor?) and maximizing radiant heat loss by previously having painted the chicken with flat, black paint and conducting the experiment in a mirrored, insulated chamber. Some unusual materials can be induced to cool by irradiation with a laser (this is how we have obtained the lowest recorded temperatures ever) but this will probably not work with chicken.
I do not think there is an absolute limit on cooling rate, just diminishing returns for increasing energy expenditure. But I may be wrong. I hope you pursue this until you get an answer that satisfies you. That is how new discoveries are made.
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$\begingroup$ Just spaced out the paragraphs a bit - makes this good answer clearer to read. $\endgroup$– user15489Aug 19, 2015 at 21:40