Materials processing for future technology 

The Science of Supercritical Fluids

Supercritical Fluid Electrodeposition - PCCP Journal Front Cover

You can learn more about specific aspects of supercritical fluid electrodeposition on our other science pages. 

Let's take water as an example. If you put water in a freezer it turns into ice, a solid. If you then leave it out in a bowl it melts into water that you can drink, a liquid. If you put the water in a kettle and heat it up it turns into steam, a gas. These are the normal forms in which you will find materials during everyday life. There is however a fourth form, one that you can reach by increasing the temperature and also the pressure of the material, a 'supercritical fluid' or SCF.
Pond Skater Fly

Supercritical fluids are interesting because they have some of the properties of liquids and some of the properties of gases. Let's go back to the water example. Just like liquid water you can dissolve things in a SCF, for example sugar or salt crystals. But like steam it can flow through fine holes because it doesn't form drops. This is because SCFs like gases do not have a property called 'surface tension'. This is force that pulls liquids into their droplet shapes and allows pond flies to walk on water.

In this simple animation you can see how a liquid can be heated at high pressure until it becomes a SCF and the exciting behaviour as it is cooled back to the liquid state. 

  1. Initially you can see the meniscus (surface) of the liquid as it sits in the chamber. Above the liquid is some of the material in the gas state.
  2. As the liquid is heated it begins to boil, bubbling,  until suddenly the meniscus disappears and there is only one 'phase' present, i.e. it is a SCF.
  3. The SCF is then cooled and, as it approaches the 'critical point', droplets of liquid and bubbles of gas simultaneously appear giving the appearance of bubbles that go both up and down.
  4. As the cooling continues below the critical point, the liquid settles and the meniscus is clearly visible again.

For a full video explained by one of our Research Fellows, visit the Videos page or YouTube.

Supercritical Fluid Phase Behaviour

For Scientists... The pressure-temperature plot on the left shows the high-T and high-P regime in which a medium is in the supercritical state. The point on the plot at which the liquid, gas and SCF regimes meet is called the critical point, a value unique to a given medium. By controlling the pressure and temperature to bring a medium closer towards the SCF-liquid or SCF-gas phase boundaries, the properties of a SCF can be 'tuned' to be more liquid-like or more gas-like. This is particularly noticeable close to the critical point where a small change in T or P can lead to a significant change in density. This behaviour is valuable in controlling SCFED for applications development. The absence of surface tension in a SCF suppresses the formation of bubbles which can be problematic when using aqueous electrolytes.