Cooling to very low temperatures

Close to absolute zero

In general, we have little interest in the cold. As a test, imagine you have just got up and are making your breakfast coffee with milk. The coffee is too hot and you decide to speed up the cooling process: do you wait 5 minutes before pouring in the cold milk or do you pour in the cold milk and then wait 5 minutes? And most importantly: what are your reasons for choosing one or the other strategy?

Even if your intuition tells you to pour the milk quickly, the correct answer is to wait first. If you do the test every morning, you will end up finding what Isaac Newton discovered and named Newton’s law of cooling in his honour: the rate at which a body cools is proportional to the difference in temperature between it and the surrounding medium. Therefore, if we pour the milk in quickly, we will have lowered its temperature, the difference with the air will be smaller and it will cool more slowly.

Cold, colder

Antarctica is the coldest place on Earth, where temperatures are 20 to 30 degrees lower than at the North Pole. There, the Vostok research base has recorded the lowest temperature on Earth: -89.2°C on 21 July 1983. That may sound like a lot, but it is nothing compared to the temperatures reached on other planets in our Solar System. Europa, Jupiter’s satellite, has a surface temperature of -170ºC. This Jovian satellite looks completely frozen and beneath its surface, with ice between 10 and 100 kilometres thick, lies a vast sea of salt water.

But for cold, cold, our own universe. Its average temperature is about -270ºC. But although it may seem incredible, space is not the coldest place we can find. In fact, if we want to take a tour of the coldest place in Spain, all we have to do is go to one of the laboratories of the Institute of Materials Science of Aragon (ICMA). There you will find a dilution refrigerator that cools to a temperature of 12 thousandths of a degree above absolute zero (-273.16 ºC).

Cold in the kitchen

The search for procedures to preserve food for more or less prolonged periods of time has been a constant in the history of mankind. The natural preservation of food is affected by bacteriological activity, favoured by heat. However, bacteria reproduce very slowly at 0°C. At lower temperatures, they stop their action. At lower temperatures they stop their destructive action. Other micro-organisms stop their activity at -7ºC and enzymes (which affect the taste and texture of food), at -18ºC. The mission of refrigeration techniques is to stop or slow down the bacteriological and enzymatic processes that destroy food.

A freezer refrigerator (four stars) must reach a temperature of -30°C in its cold room. A preserving refrigerator (three stars) will reach -18ºC. It is not suitable for freezing, although it is suitable for preserving frozen food. Freezing the food freezes the water that makes up the cells of the food. If the application of cold is slow (preservative), the ice crystals are large and sharp-edged. They tear the cell walls and on thawing the cellular liquid, which contains much of the nutritional value of the food, is poured out. On the other hand, if it is fast (freezer), the crystals are smaller and rounded, and the food retains its nutritional qualities and flavour.

The absolute scale

If we listen to a low-temperature physicist, we will discover that he usually talks about another scale than the centigrade scale. If we pay attention we will hear him talk about Kelvin. What does he mean by 3 Kelvin or 3 K? The relationship is quite simple: 0º C is 273 K. Or the other way round, 0 K is -273 ºC (more precisely, -273.16 ºC)? What is so special about -273.16º C? Can’t anything be colder than that? The answer is no. Temperature is simply a measure of the agitation of atoms and molecules. The more they move, the more temperature we measure. At absolute zero all motion stops and therefore cannot go any lower.

This result is generalised by the so-called Third Law of Thermodynamics: absolute zero cannot be reached in a finite number of steps. This has led some physicists to express the three laws of thermodynamics in an ironic way:

  • First law: Heat can be converted into work.
  • Second law: But completely only at absolute zero.
  • Third law: And absolute zero is unattainable!

Close to absolute zero

How do we reach lower and lower temperatures? Of course, we are not talking about putting a piece of meat in a freezer, but how do we reach really low temperatures, for example -270°C? For each temperature range there are different techniques. Below -100°C we use cryogenic liquids. Basically two are used, liquid nitrogen down to -200°C and liquid helium, with which we can go down to -269°C (4.2K on the absolute scale). The most obvious way to cool the material is to immerse it directly in the liquid. If we want to go lower, we have to use more complex techniques. With a cooler using a mixture of two helium isotopes (He3 and He4) we can go down to 0.01 K, but to get closer to absolute zero we must use techniques such as electronic demagnetisation, with which we can go down to 0.003 K, and nuclear demagnetisation, down to 50 K.

The surprising properties of cold matter

The study of matter at very low temperatures has yielded surprising results. For example, superconductivity. The Dutch physicist Heike Kamerlingh Onnes succeeded in liquefying helium in 1908. A great achievement considering that helium boils at -269°C (helium has the lowest boiling temperature and at atmospheric pressure never freezes). Onnes used liquid helium to cool other substances, such as mercury, which solidifies at -38.89º C. When he measured its electrical resistance, he found, as expected, that the lower the temperature of the mercury, the lower its resistance. But when he reached -269º C he discovered that its electrical resistance became zero: Kamerlingh Onnes had just discovered superconductivity. Since then, different materials have been discovered which, below a certain critical temperature, offer no resistance to the passage of electricity: superconductors.

Superfluidity

Kamerlingh Onnes missed one of the most surprising properties of liquid helium. In 1938, Russian Peter Kapitsa and Canadians John Allen and Austin Misener found that below -271°C liquid helium became an excellent conductor of heat, 200 times better than copper. Not only that, but it had a viscosity less than one ten-thousandth of that of hydrogen gas: this is the phenomenon of superfluidity.

All liquids have an opposition to flow: it is viscosity, the product of friction between the molecules of the solid and those of the surface on which they slide. Some, like shampoo or honey, are very viscous. Others, such as water, are not so viscous. The same is true of liquid helium. However, below -271°C, its viscosity practically disappears and it becomes superfluid. This means that we can see how the helium literally rises up the walls of the vessel containing it and spills out. This fact has important technological applications, such as the location of micro-holes in ducts and pipes: superfluid helium can be easily squeezed through holes smaller than 2 ten-thousandths of a millimetre.

At the limit

In this amazing world of very low temperature phenomena, a group at the Joint Institute for Laboratory Astrophysics (JILA) in Boulder, Colorado, culminated in 1995 a two-decade effort by scientists around the world to experimentally verify a prediction made almost 80 years ago by Albert Einstein and the Indian Satyendra Nath Bose. At normal temperatures, the atoms of a gas are distributed throughout the volume of the container that holds them. But at extremely low temperatures, of the order of a few billionths of a degree above absolute zero, the atoms behave as if they were a single superatom: this is the Bose-Einstein condensate, a new state of matter. The JILA group succeeded in cooling 2,000 rubidium atoms to below one hundred billionths of a degree absolute for 10 seconds, creating the first ever Bose-Einstein condensate. They received the Nobel Prize for it.

Why is cooling important?

At room temperature, the thermal energy that the atoms of a material have masks a number of interesting phenomena that we could observe if the atoms had less energy. Imagine a room full of people in a hurry, running around, constantly bumping into each other and unable to stop and talk to each other. As the mood cools down, people start to slow down, talking to each other, exchanging things, forming groups, making plans to do things together. Something similar happens to matter. When it cools down, interactions between atoms that were masked by thermal agitation give rise to a very interesting phenomenology that is not observable at room temperature. This is why it is so important to be able to study materials at very low temperatures.

To find out more:

Absolute temperature

Superfluidity

Bose-Einstein condensity

Dilution refrigerator

Instituto de Nanociencia y Materiales de Aragón