Liquid Nitrogen and Helium

This evening we made liquid nitrogen ice cream.  We used a very simple recipe--one quart of heavy cream, one pint of milk, 3 tablespoons of vanilla, and 1/2 cup sugar.  Then we added about half a gallon of liquid N2 to bring the mix down to 77 Kelvin (-321 degrees F).  The ice cream was quite smooth and delicious.

Ned and Liesel enjoying liquid nitrogen ice cream.

I was curious about some properties of extremely cold matter that I remember learning in high school and decided to read up on it.  Other than being extremely cold, nothing particularly exciting happens around the temperature at which nitrogen is liquid.  Liquid helium, however, is considerably colder (boiling point of 4.2 K), and at these temperatures we start to see very interesting quantum mechanical phenomena.  For example, if you cool liquid helium a couple degrees below its boiling point, it becomes a superfluid, with bizarre properties.  Atoms of most elements settle into a solid at cold enough temperatures due to intermolecular interactions.  Helium, however, is so light and has such weak intermolecular interactions that even at absolute zero it remains a liquid.  Since helium stays liquid near absolute zero, it can transform into a Bose-Einstein condensate, in which all the atoms of the liquid move in unison with each other.  Superfluid helium can leak through its container, finding openings between the container molecules that ordinary fluids would not be able to penetrate.  Superfluids also have zero viscosity, which means they can climb up the walls of their container, seemingly defying gravity, and spill over the edge, eventually emptying the container.


More practically, liquid helium is used for cooling superconducting magnets used in NMR and MRI machines.  Superconducting magnets are made from superconducting wire, which when cooled below a critical temperature, exhibit zero electrical resistance.  Electrical current will flow forever in a loop of superconducting material.


Really cool video on superconductors.

As you might expect, it is pretty simple to make liquid nitrogen, since it makes up 80% of air.  All you have to do is compress air, allow it to radiate off its heat, and then let it expand, which will cause the gas to get extremely cold.  This process is repeated several times until liquid nitrogen is achieved.