n 18.05.24 – Unlocking the Secrets of Supercritical Fluid Dynamics: a New Neutron Scattering Study Reveals Intriguing Insights.
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In a world where matter challenges conventional classification, a groundbreaking study sheds new light on the enigmatic supercritical state of fluids. A team of international researchers led by Dr. Livia Bove from the Laboratory of Quantum Magnetism at EPFL unveils unprecedented insights into the behavior of matter pushed beyond its critical point. Their article, entitled “Crossover from ‘Gas-like’ to ‘Liquid-like’ Molecular Diffusion in a Simple Supercritical Fluid,” is a breakthrough in our understanding of these hybrid states of matter. The work will be published this week in Nature Communications.

The properties of supercritical fluids are unique: they possess the ability to effuse like a gas while they dissolve materials like a highly corrosive liquid. This inherent duality has made them invaluable in a myriad of industrial applications, from pharmaceutical processing to decaffeinating coffee beans. Yet, their significance extends far beyond applications. Understanding the physical behavior of supercritical fluids is the key to unraveling the mysteries of giant planets like Jupiter, Uranus and Saturn, where such states of matter are present.

At the heart of this study lies a fundamental question: can we pinpoint a region of the pressure-temperature (P-T) plane and a physical observable that unequivocally distinguishes between gas-like and liquid-like behavior in supercritical fluids? Previous theoretical frameworks have proposed various transition boundaries, including the Widom line, the Frenkel line, and the percolation line, but experimental validation has remained elusive until now.

This study explores the dynamics of supercritical methane, unraveling key insights into its molecular behavior under varying pressures. At the heart of the investigation lay the determination of the self-diffusion coefficient, a crucial parameter reflecting the mobility of molecules within the fluid. Through extremely challenging high-pressure quasi-elastic neutron scattering (QENS) experiments on supercritical methane, conducted at the Institut Laue Langevin in Grenoble, Dr. Ranieri and collaborators observed a crossover from gas-like to liquid-like behavior in the dynamic structure factor of the fluid, a physical observable describing the spectrum of the space and time correlations between the fluid molecules. The transition from a gas-like Gaussian shape of the dynamic structure factor to a liquid-like Lorentzian shape is completed at the Widom line, validating its pivotal role in the supercritical fluid characterization. This gradual transition, devoid of first-order phase characteristics, hints at a heterogeneous mixture of gas-like and liquid-like molecules within a narrow pressure range. However, simplistic models fail to capture the intricate interplay of molecular exchanges and interface contributions, warranting further investigation.

Prof. Bove remarks, “Our findings not only advance our understanding of supercritical fluid dynamics but also hold significant implications for planetary science, where similar states of matter may govern the behavior of gaseous giants and exoplanets. In gas giants, the existence of a non-homogeneous super critical state would indeed influence whether a boundary between the planets’ interior and their atmosphere can be defined. This would impact planetary properties such as the thermal conductivity and other heat-related physical phenomena like storm activity, as well as mass diffusion related properties like the ionic conductivities and the consequent generation of anomalous magnetic fields. By unraveling the complexities of these hybrid states, we move one step closer to understanding the farther planets of our Solar System and the fundamental properties of condensed matter.”

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