Matter at extreme conditions of very high temperature and pressure turns out to be remarkably simple and universal

Scientists at Queen Mary University of London have made two discoveries about the behaviour of 'supercritical matter' - matter at the critical point where the differences between liquids and gases seemingly disappear.  

Previously, while the behaviour of matter at reasonably low temperature and pressure was well understood, the picture of matter at high temperature and pressure was blurred. Above the critical point, differences between liquids and gases seemingly disappear, and the supercritical matter was thought to become hot, dense and homogeneous.  

The researchers believed there was new physics yet to be uncovered about this matter at the supercritical state.  

By applying two parameters - the heat capacity, and the length over which waves can propagate in the system, they made two key discoveries. First, they found that there is a fixed inversion point between the two where the matter changes its physical properties – from liquid-like to gas-like. They also found that this inversion point is remarkably close in all systems studied, telling us that the supercritical matter is intriguingly simple and amenable to new understanding.  

As well as fundamental understanding of the states of matter and the phase transition diagram, understanding supercritical matter has many practical applications; hydrogen and helium are supercritical in gas giant planets such as Jupiter and Saturn, and therefore govern their physical properties.

In green environmental applications, supercritical fluids have also proved to be very efficient at destroying hazardous wastes, but engineers increasingly want guidance from theory in order to improve efficiency of supercritical processes. 

Study co-author Kostya Trachenko, Professor of Physics at Queen Mary University of London, said: 'The asserted universality of the supercritical matter opens a way to a new physically transparent picture of matter at extreme conditions. This is an exciting prospect from the point of view of fundamental physics as well as understanding and predicting supercritical properties in green environmental applications, astronomy and other areas.  

This journey is ongoing and is likely to see exciting developments in the future. For example, it invites the question of whether the fixed inversion point is related to conventional higher-order phase transitions? Can it be described by using the existing ideas involved in the phase transition theory, or is something new and quite different needed? As we push the boundaries of what is known, we can identify these new exciting questions and start looking for answers.' 

Research paper: 'Double universality of the transition in the supercritical state'.