Dynamics and Thermodynamics beyond the critical point

Gorelli, Federico A.; Bryk, Taras; Krisch, M.; Ruocco, G.; Santoro, Mario; Scopigno, T.

doi:10.1038/srep01203

Cited by 83 publications

(98 citation statements)

References 28 publications

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“…We therefore show that the supercritical state of water can be divided into a gas-like region and a liquid-like region by the WL anticipating the phase separation and the coexistence that is found below the critical point. These results are in line with recent experimental findings on supercritical noble gases 26,27 , and together with those indicate a new perspective in the characterization of the fluid-matter thermodynamics of the supercritical state of water and possibly of other liquids with all the important consequences that this can have for many areas of application. The supercritical state cannot, in fact, be considered as an indistinguishable fluid phase any longer.…”

Section: Discussionsupporting

confidence: 77%

“…The WL of supercritical noble gases, Lennard-Jones and van der Waals fluids was recently studied by experiments and simulations [23][24][25][26][27] . In particular, a recent experimental work has paved the way to a more profound knowledge of hot dense fluids by detecting a crossover in sound velocity of supercritical fluids of noble gases connected to the WL 26 .…”

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confidence: 99%

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Widom line and dynamical crossovers as routes to understand supercritical water

2014

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Supercritical water is fundamental in many fields of applications and a precise characterization of the supercritical state is of uttermost importance for this liquid. In a fluid, when moving from the critical point into the single-phase region, the thermodynamic response functions show maxima reminiscent of the critical divergence. Here we study the thermodynamic properties of water in the supercritical region by analysing both available experimental data and our computer simulation results. We find that the lines connecting the maxima of the response functions converge on approaching the critical point in a single line, the Widom line. We further show that the Widom line coincides with a crossover from a liquid-like to a gas-like behaviour clearly visible in the transport properties. These thermodynamic and dynamic features show that the supercritical state in water is far more complex than what was so far believed, indicating a new perspective in the characterization of the thermodynamics of this state.

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Section: Discussionsupporting

confidence: 77%

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confidence: 99%

Widom line and dynamical crossovers as routes to understand supercritical water

2014

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“…In this context it is worth to mention recent studies [25][26][27][28] that have discussed the existence of a transition between a low-density gaslike and a high-density liquidlike dynamical regime of a supercritical fluid. This crossover was identified by Brazhkin and coworkers [29] with the crossing of the so-called Frenkel line, which has been eventually connected directly to the onset of a nonmonotonic time dependence of the VAF [30].…”

Section: Introductionmentioning

confidence: 99%

Density of states and dynamical crossover in a dense fluid revealed by exponential mode analysis of the velocity autocorrelation function

et al. 2017

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Extending a preceding study of the velocity autocorrelation function (VAF) in a simulated Lennard-Jones fluid [Phys. Rev. E 92, 042166 (2015)] to cover higher-density and lower-temperature states, we show that the recently demonstrated multi-exponential expansion method allows for a full account and understanding of the basic dynamical processes encompassed by a fundamental quantity as the VAF. In particular, besides obtaining evidence of a persisting long-time tail, we assign specific and unambiguous physical meanings to groups of exponential modes related to the longitudinal and transverse collective dynamics, respectively. We have made this possible by consistently introducing the interpretation of the VAF frequency spectrum as a global density of states in fluids, generalizing a solid-state concept, and by giving to specific spectral components, obtained through the VAF exponential expansion, the corresponding meaning of partial densities of states relative to specific dynamical processes. The clear identification of a high-frequency oscillation of the VAF with the near-top excitation frequency in the dispersion curve of acoustic waves is a neat example of the power of the method. As for the transverse mode contribution, its analysis turns out to be particularly important, because the multi-exponential expansion reveals a transition marking the onset of propagating excitations when the density is increased beyond a threshold value. While this finding agrees with the recent literature debating the issue of dynamical crossover boundaries, such as the one identified with the Frenkel line, we can add detailed information on the modes involved in this specific process in the domains of both time and frequency. This will help obtain a still missing full account of transverse dynamics, in both its nonpropagating and propagating aspects which are linked through dynamical transitions depending on both the thermodynamic states and the excitation wavevectors.

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“…Intensive theoretical studies based on extended kinetic theory [14][15][16] and viscoelastic memory function approach [17][18][19] revealed features of the short-wavelength longitudinal collec- tive excitations, which can even exist outside the first pseudoBrillouin zone in liquids and were also obtained from the analysis of the inelastic neutron scattering experiments. 20,21 Only recently have experimental, [22][23][24][25][26] theoretical, [27][28][29] and simulation studies [30][31][32][33] started to focus on revealing the nonhydrodynamic modes in collective dynamics of liquids that can be observed in the shape of dynamic structure factors only outside the hydrodynamic region. The hydrodynamic regime of collective dynamics of liquids means relaxation and propagating processes that are constrained by local conservation laws valid on sufficiently large spatial and time scales.…”

Section: Introductionmentioning

confidence: 99%

Pressure-induced emergence of unusually high-frequency transverse excitations in a liquid alkali metal: Evidence of two types of collective excitations contributing to the transverse dynamics at high pressures

Bryk

Ruocco

Scopigno

et al. 2015

The Journal of Chemical Physics

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Unlike phonons in crystals, the collective excitations in liquids cannot be treated as propagation of harmonic displacements of atoms around stable local energy minima. The viscoelasticity of liquids, reflected in transition from the adiabatic to elastic high-frequency speed of sound and in absence of the long-wavelength transverse excitations, results in dispersions of longitudinal (L) and transverse (T) collective excitations essentially different from the typical phonon ones. Practically, nothing is known about the effect of high pressure on the dispersion of collective excitations in liquids, which causes strong changes in liquid structure. Here dispersions of L and T collective excitations in liquid Li in the range of pressures up to 186 GPa were studied by ab initio simulations. Two methodologies for dispersion calculations were used: direct estimation from the peak positions of the L/T current spectral functions and simulation-based calculations of wavenumber-dependent collective eigenmodes. It is found that at ambient pressure, the longitudinal and transverse dynamics are well separated, while at high pressures, the transverse current spectral functions, density of vibrational states, and dispersions of collective excitations yield evidence of two types of propagating modes that contribute strongly to transverse dynamics. Emergence of the unusually high-frequency transverse modes gives evidence of the breakdown of a regular viscoelastic theory of transverse dynamics, which is based on coupling of a single transverse propagating mode with shear relaxation. The explanation of the observed high-frequency shift above the viscoelastic value is given by the presence of another branch of collective excitations. With the pressure increasing, coupling between the two types of collective excitations is rationalized within a proposed extended viscoelastic model of transverse dynamics. Unlike phonons in crystals, the collective excitations in liquids cannot be treated as propagation of harmonic displacements of atoms around stable local energy minima. The viscoelasticity of liquids, reflected in transition from the adiabatic to elastic high-frequency speed of sound and in absence of the long-wavelength transverse excitations, results in dispersions of longitudinal (L) and transverse (T) collective excitations essentially different from the typical phonon ones. Practically, nothing is known about the effect of high pressure on the dispersion of collective excitations in liquids, which causes strong changes in liquid structure. Here dispersions of L and T collective excitations in liquid Li in the range of pressures up to 186 GPa were studied by ab initio simulations. Two methodologies for dispersion calculations were used: direct estimation from the peak positions of the L/T current spectral functions and simulation-based calculations of wavenumber-dependent collective eigenmodes. It is found that at ambient pressure, the longitudinal and transverse dynamics are well separated, while at high pressures, the...

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Dynamics and Thermodynamics beyond the critical point

Cited by 83 publications

References 28 publications

Widom line and dynamical crossovers as routes to understand supercritical water

Widom line and dynamical crossovers as routes to understand supercritical water

Density of states and dynamical crossover in a dense fluid revealed by exponential mode analysis of the velocity autocorrelation function

Pressure-induced emergence of unusually high-frequency transverse excitations in a liquid alkali metal: Evidence of two types of collective excitations contributing to the transverse dynamics at high pressures

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