Discovery of different regimes in supercritical fluids

Temperature verses pressure plot showing the dynamic transition between rigid liquid and a supercritical gas-like fluid

Our researchers in the Centre for Condensed Matter and Materials Physics in the School have discovered surprising properties of supercritical fluids: substances at a temperature and pressure above the critical point, which is the point at which there are no phase boundaries between the liquid and gas phases.  By studying the amount of heat required to change the temperature of an object, known as its heat capacity, they have discovered that there are two distinct regimes can in the supercritical state: rigid liquids and supercritical gas-like fluids. Rigid liquids are rigid like a solid on short time scales, but flow like a liquid on long time scales; while a supercritical gas-like fluid has the dynamic motions of a gas but is able to dissolve materials, like a liquid.

You might not have heard about supercritical fluids before but they have every day applications, ranging from the extraction of fragrance from flowers to the production of decaffeinated coffee and are used in pharmaceuticals, cosmetics, biotechnology, fossil and bio-fuels and microelectronics. Thus, it is crucial to understand their properties and in particular, the heat capacity.  While physicists have a good theoretical understanding of the heat capacity of solids, liquids and gases, a general theory of the heat capacity of supercritical fluids has always remained elusive.

In the paper published this month in Nature Communications, our researchers describe their investigation of the thermodynamic properties of the supercritical state.  They formulate a theory of system thermodynamics, finding good agreement between the calculated and experimental heat capacity. They go on to derive a power law describing the relationship between heat capacity and viscosity and analyse supercritical scaling exponents in the system above the Frenkel line, which has previously never been done before.