Collective excitations in supercritical fluids: analytical and molecular dynamics study of "positive" and "negative" dispersion.
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.
Bryk
Taras
T
Institute for Condensed Matter Physics, National Academy of Sciences of Ukraine, UA-79011 Lviv, Ukraine. bryk@icmp.lviv.ua
Mryglod
Ihor
I
Scopigno
Tullio
T
Ruocco
Giancarlo
G
Gorelli
Federico
F
Santoro
Mario
M