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I went to the doctor yesterday and he attacked me with liquid nitrogen.  To be more specific, I had a wart ‘frozen’ off.  Now, I had some similar treatment years ago, in which the doctor used a container of the stuff surrounded by polystyrene foam, and open to the air. Rather like what we use in the lab sometimes. He dipped in what looked like cotton wool buds, and applied those to the wart. Ouch.

This time I when the doctor disappeared out of the room to get the nitrogen, I was expecting him to come back carrying something similar. But instead, he entered holding a very snazzy shiny metallic thermos flask with a spray nozzle on a stalk. It was a case of point the nozzle at the target and spray. Ouch again.  It looked awfully space-age, much more elegant than a container in polystyrene. I want one.

Just how cold is the spray, though? The contents will be under  pressure, and, releasing this pressure is going to cause them to cool very rapidly.  Just what temperature it gets down to will depend a bit on the rate of flow of fluid and the temperature and pressure in the flask, and how the nitrogen mixes with the air on its journey to the skin, but I suspect it is really quite low indeed. Computational fluid dynamics software can probably do the calculations for me.  I wouldn’t have wanted to touch the end of the nozzle immediately afterwards, however.

I note that there are some interesting applications of the sprayer described on the internet, many of them involving unwelcome insects.