By Jose Angel Martin Gago y Mar Gulis (CSIC)*
At some point in life, who else has enjoyed drinking a hot coffee in a very cold, remote environment or in which, for example, there are very few chances of finding a cafeteria. The most common way to achieve this is by using a simple and cheap thermos. But, have you ever wondered about the mechanism that makes this ‘miracle’ possible? It has to do with emptiness. Here we explain it to you.
A thermos consists of two vessels: an interior one, in contact with the liquid that we want to maintain at a given temperature; and another exterior, in contact with the environment and that generally supports the thermos. The interior is held by the neck with the exterior through a minimal portion of material and leaving a small space, empty of air, between the two vessels. In this way, the thermos isolates the interior space, where our coffee is kept at 40 °C, from the exterior, which can be at 4 °C.
If the container that contains the coffee were in direct contact with the environment, in a few minutes the coffee would acquire the temperature of the environment and we would drink it cold. Instead, if we empty the space between the two vessels of air, we manage to insulate them thermally. This is explained by the kinetic theory of gases: heat transfer is basically due to the exchange of energy between the hottest and coldest molecules when they collide with each other. With this intermediate vacuum chamber it is achieved that the thermal conductivity between both containers is practically null. That is to say, without air molecules that transfer heat, the inner vessel will remain insulated and, therefore, its temperature will not vary.
Interestingly, this development is not as recent as one might assume. The first to do so was the Scottish physicist James Dewar in 1892. Hence these containers that provide thermal insulation are known as Dewar or Dewar glasses.
A very illustrative fact of the effectiveness of this process is that, if the vacuum were in the ultra-high vacuum range (with pressures similar to those that can exist in interplanetary space) and the contact between both containers was non-existent or minimal, it could keep coffee hot more than ten years. However, in the case of a thermos designed for liquids or food, the intermediate vacuum corresponds to what we call under vacuum (the pressure is slightly less than atmospheric), which causes the air molecules to come into contact with both surfaces, and our coffee ends up cooling.
Cryogenics: from the thermos of coffee to the transport of liquid nitrogen
However, for many technological applications liquid nitrogen or helium is used, elements that must be kept at very low temperatures and are transported in metallic containers of hundreds of liters. The thermal difference between the interior and exterior walls in these cases is very large (more than 200 °C). If we were to use a mechanism like that of a normal thermos, liquid nitrogen or helium would easily sublime and turn from a liquid to a gas. To avoid this, it is necessary to have a high vacuum between both surfaces (pressures less than a million times atmospheric pressure, or less than 10-6 millibars of pressure). When this is achieved, the tanks or Dewar vessels that carry these substances can hold and store liquid nitrogen for several weeks at -196°C.
The use of cryogenic temperatures is much more extensive than we might imagine. In biology, biochemistry or medicine, cryogenics is very important for the preservation of cells and cultures, such as sperm and eggs; medicines, such as some vaccines; or to treat some foods. Also in diagnostic tests, such as nuclear magnetic resonance. From a technology standpoint, many research devices, such as radiation detectors or superconducting magnets, require liquid nitrogen or helium to function. Therefore, indirectly, the vacuum helps to preserve and transport these cryogenic substances and makes these technologies possible in our day to day.
*Jose Angel Martin Gago He is a CSIC researcher at the Madrid Materials Science Institute (ICMM-CSIC) and author of the popular book ¿Qué sabemos de? The vacuum (CSIC-Waterfall).