Most of the time, in order to cool a material to a very low temperature, we need cryogenic liquids, which means liquids that are already very cold: liquid oxygen (90.2 K), liquid nitrogen [S’AMUSER AVEC L’AZOTE](77.4K), liquid hydrogen (20.3 K), liquid helium (4.2 K).
In order to obtain those liquids, we have to liquefy the corresponding gases, thanks to a technique developed by thermodynamicians in the 19th century: the Joule-Thomson effect. It consists in transferring a gas from a small volume to a bigger volume. The gas particles hence move away from one another. However, this is contrary to their natural inclination, because they are attracted to one another by an electrically caused attraction called Van der Waals force (Van der Waals was a Dutch physicist who was Kammerlingh Onnes’ PhD supervisor). Since the particles have to struggle in order to move away from one another, their energy and speed decrease. If the gas particles are slower, the gas is colder: the Joule-Thomson effect enabled to cool the gas. However, if the gas has not been cooled enough while transferring to the bigger volume, it will not liquefy.
The trick in the Joule-Thomson effect consists in subjecting a gas to a series of successive expansions. In order to do that, the gas is compressed in a small volume using a compressor. It is transferred to a bigger volume through a small porous opening. The Joule-Thomson effect is not sudden but takes time, which enables to recover the expanded gas by pumping in the big volume, before compressing it again in the small volume; repeating this cycle enables to cool the gas continuously. This technique only works when the gas is already cold enough at the beginning, which means it has to be put in contact with already cooled liquids. It is thanks to this kind of technique that K. Onnes was able to liquefy helium in 1908.
Finally, when the gas is liquefied, pumping above the surface of the gas with which it is balanced can still cool the liquid. This forces the liquid to find a new balance by evaporating some molecules, which costs it more energy and cools it. Thanks to this technique, helium can be cooled from 4.2 K to about 1 K.
