I've been using LN2 for snap freezing a lot, and the effect is near instant. One second my samples are room temperature, the next second they're frozen solid.
I've read dry ice + 100% ethanol bath can substitute for snap freezing, which would be a lot more convenient for me. However, I tried it and the freezing process took >15 seconds for a modest 200 uL sample (of h2o) in an eppendorf. I'm confused -- is this normal for "snap freezing" with dry ice + ethanol? That's really slow. Am I doing something wrong? It's 200 proof ethanol.
Pure water can be in supercooling form. Super clean water or water droplets of cloud-relevant diameters (∼10–20 µm) freeze homogeneously at temperatures
I think that Alex is sugesting that the dryice/ethanol bath may allow growth of ice crystals (which risks damaging the structural inregrity). The second factor is the rate of heat exchang and in the case lof liquid N2 that is suppressed by the formation of a gas film.
So if dry ice ethanol is more convenient give it a go!
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I have previously used a liquid nitrogen / dry ice slurry. It works very well for rapid cooling and maintaining low temperatures and allows you to use less liquid nitrogen. I agree that ethanol and dry ice should work but as the other answers say you never know until you try it on a real sample.
We use dry ice and pure ethanol SDA (200 Proof) for snap freezing the tissues in OCT in disposable base molds for cryosectioning. Each sample takes 3-5 mins to freeze. I agree that process is slower and time consuming than snapfreezing in Liquid Nitrogen.
Of course, if liquid nitrogen is used, you instantly obtain a temperature of around -195 C, while dry ice surface temperature is -78 C. However if the dry ice is chisel to break off smaller pieces and your sample is into 2-methyl butane (Isopentane) the freezing process can be as fast as 10 or 30s depending on their sizes.
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