One of my duties is keeping stocks of cell cultures. I have a -80°C freezer and a dewar for that purpose. Ice crystals appear on drawers when I open the freezer. Crystals grow because I open the freezer periodically so, I need to clean it from time to time. Interestingly, there is no ice in the dewar although I take cell containers out and they are covered with ice when I put them back. The next day I do the revision and the containers are clean from ice. The temperature of liquid nitrogen is -196°C and logically ice should be trapped in dewar as it happens in the freezer but the reality is the opposite. I would like to get an explanation of how ice escapes from the dewar. I have one hypothesis: gas escaping the dewar constantly so even under very low temperatures water can evaporate and simply squeezing out of the dewar by flowing gas. But why so fast?
I can suggest three main factors: 1) the high specific surface of frost, 2) the lowest partial pressure of water in dry nitrogen, 3) the relatively high value of the vapor pressure of water even at low temperatures due to the phase diagram.
Dear Dzmitry Pasrednik , I think you are right, the N2 gas flow is continously removing water molecules from the ice crystals, a kind of sublimation but without the need of low pressures. Here is the N2 flow what is removing the water molecules from the surroundings of the ice crystals, something that is not happening with the freezer, where ice crystals are just exposed to the air conditions, which has already some % of humidity.
In the following research paper by Liu and Zhang, they used a version of the Hertz-Knudsen equation to study a similar problem, how air flow from air conditioners affects to ice evaporation in ice rinks. There they remove water molecules from ice with dry air, but I think the problem is the same, as pointed by E. Vasil'ev the low partial pressure of water in dry nitrogen should be the main reason for this observation.
Article Effect of air parameters on ice sublimation in ice rink
Thanks for making us think... out of equilibrium systems.
Hope it helps.
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