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Updated: Jun 10, 2026

An Analog Macroscopic Technique for Studying Molecular Hydrodynamic Processes in Dense Gases and Liquids
Published on: December 4, 2017
Collective excitations in supercritical fluids: analytical and molecular dynamics study of "positive" and "negative"
Taras Bryk1, Ihor Mryglod, Tullio Scopigno
1Institute for Condensed Matter Physics, National Academy of Sciences of Ukraine, UA-79011 Lviv, Ukraine. bryk@icmp.lviv.ua
Abstract:
The approach of generalized collective modes is applied to the study of dispersion curves of collective excitations along isothermal lines of supercritical pure Lennard-Jones fluid. An effect of structural relaxation and other nonhydrodynamic relaxation processes on the dispersion law is discussed. A simple analytical expression for the dispersion law in the long-wavelength region of acoustic excitations is obtained within a three-variable viscoelastic model of generalized hydrodynamics. It is shown that the deviation from the linear dependence in the long-wavelength region can be either "positive" or "negative" depending on the ratio between the high-frequency (elastic) and isothermal speed of sound. An effect of thermal fluctuations on positive and negative dispersion is estimated from the analytical solution of a five-variable thermoviscoelastic model that generalizes the results of the viscoelastic treatment. Numerical results are reported for a Lennard-Jones supercritical fluid along two isothermal lines T(*)=1.71,4.78 with different densities and discussed along the theoretical expressions derived.
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