, University of Southampton

Supercritical Fluids

Science Made Simple


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.


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.

Pond Skater Fly


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.


Supercritical Fluid Phase Behaviour


Click Here

To Watch The Full Video Of
A Liquid Being Heated To The
Supercritical Fluid State






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.


As the cooling continues below the critical point, the liquid settles and the meniscus is clearly visible again.


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.


As the cooling continues below the critical point, the liquid settles and the meniscus is clearly visible again.




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.