Collective dynamics in supercritical fluids: Can one discriminate between the “liquid-like” and “gas-like” types of dynamics?

The idea of the sensitivity of dynamic properties of fluids on the liquid-like or gas-like state of the system proposed in [1] was a breakthrough in understanding the dynamic dissimilarities of supercritical fluids. For fluids a purely dynamic quantity that was suggested to discriminate between the liquid-like and solid-like states is the so-called positive dispersion of collective excitations and inealstic X-ray scattering (IXS) experiments proved this assumption [2]. The experiments revealed that the positive sound dispersion vanished at the thermodynamic point that could be treated as a continuation of the Widom line (maximum of the specific heat Cp(p) dependence) into the supercritical region. Later on a theory of positive dispersion in supercritical fluids [3] was constructed based on an analytical approach of Generalized Collective Modes (GCM) [4,5]. This theory revealed the leading role of non-hydrodynamic structural relaxation in positive sound dispersion.
In this talk recent results from simulations and theory of collective dynamics in supercritical Argon [6] and soft-sphere fluids will be discussed and a comparison will be made with a recently proposed “rigid”-”non-rigid” fluid dynamic crossover [7].

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