arXiv subjects

V. V. Brazhkin

Publications and source records attributed to V. V. Brazhkin.

At least 55 records · Page 3Linked to original sources

arXiv2015

Frenkel Line and Solubility Maximum in Supercritical Fluids

A new dynamic line, the Frenkel line, has recently been proposed to separate the supercritical state into rigid-liquid and non-rigid gas-like uid. The location of Frenkel line on the phase diagram is unknown for real uids. Here, we map the Frenkel line for three important systems: CO2, H2O and CH4. This provides an important demarcation on the phase diagram of these systems, the demarcation that separates two distinct physical states with liquid-like and gas-like properties. We find that the Frenkel line can have similar trend as the melting line above the critical pressure. Moreover, we discuss the relationship between unexplained solubility maxima and Frenkel line, and propose that the Frenkel line corresponds to the optimal conditions for solubility.

cond-mat.soft↗

arXiv2015

Dynamical Crossover in Supercritical Water

Dynamical crossover in water is studied by means of computer simulation. The crossover temperature is calculated from the behavior of velocity autocorrelation functions. The results are compared with experimental data. It is shown that the qualitative behavior of the dynamical crossover line is similar to the melting curve behavior. Importantly, the crossover line belongs to experimentally achievable $(P,T)$ region which stimulates the experimental investigation in this field.

cond-mat.soft↗

arXiv2014

Thermodynamics and Widom lines in supercritical carbon dioxide

Behavior of supercritical fluids attracts a lot of attention nowadays. It is important both from the point of view of fundamental science and technological applications. However, up to now the progress in the field is rather moderate. In this article we report a computational study of supercritical carbon dioxide which is one of the most important fluids for chemical industry. We study the response functions of $CO_2$ in supercritical regime and calculate the locations of their maxima (Widom lines). We also make preliminary calculations of the line of crossover of microscopic dynamics of particles (Frenkel line). The conclusions on the Frenkel line location can be applied to study of the atmosphere of Venus.

cond-mat.stat-mech↗

arXiv2014

Collective excitations and thermodynamics of disordered state: new insights into an old problem

Disorder has been long considered as a formidable foe of theoretical physicists in their attempts to understand system's behavior. Here, we review recently accumulated data and propose that from the point of view of calculating thermodynamic properties, the problem of disorder may not be as severe as has been hitherto assumed. We particularly emphasize that contrary to the long-held view, collective excitations do not decay in disordered systems. We subsequently discuss recent experimental, theoretical and modelling results related to collective excitations in disordered media, and show how these results pave the way to understanding thermodynamics of disordered systems: glasses, liquids, supercritical fluids and spin glasses. An interesting insight from the recent work is the realization that most important changes of thermodynamic properties of the disordered system are governed only by its fundamental length, the interatomic separation. We discuss how the proposed theory relates to the previous approaches based on general many-body statistical mechanics framework.

cond-mat.soft↗

arXiv2014

Dynamic transition in supercritical iron

Recent advance in understanding the supercritical state posits the existence of a new line above the critical point separating two physically distinct states of matter: rigid liquid and non-rigid gas-like fluid. The location of this line, the Frenkel line, remains unknown for important real systems. Here, we map the Frenkel line on the phase diagram of supercritical iron using molecular dynamics simulations. On the basis of our data, we propose a general recipe to locate the Frenkel line for any system, the recipe that importantly does not involve system-specific detailed calculations and relies on the knowledge of the melting line only. We further discuss the relationship between the Frenkel line and the metal-insulator transition in supercritical liquid metals. Our results enable predicting the state of supercritical iron in several conditions of interest. In particular, we predict that liquid iron in the Jupiter core is in the "rigid liquid" state and is highly conducting. We finally analyse the evolution of iron conductivity in the core of smaller planets such as Earth and Venus as well as exoplanets: as planets cool off, the supercritical core undergoes the transition to the rigid-liquid conducting state at the Frenkel line.

cond-mat.stat-mech↗

arXiv2014

Non-perturbative treatment of strongly-interacting fields: insights from liquid theory

We outline a new programme of solving the problem of treating strong interactions in field theories. The programme does not involve perturbation theories and associated problems of divergences. We apply our recent idea of treating strongly interacting liquids to field theories by showing the equivalence of Hamiltonians of liquids and interacting fields. In this approach, the motion of the field results in the disappearance of $n-1$ transverse modes with frequency smaller than the Frenkel frequency $ω_{\rm F}$, similar to the loss of two transverse modes in a liquid with frequency $ω<ω_{\rm F}$. We illustrate the proposed programme with the calculation of the energy and propagator, and show that the results can not be obtained in perturbation theory to any finite order. Importantly, the Frenkel energy gap $E_{\rm F}=\hbarω_{\rm F}$ and the associated massive Frenkel particle naturally appear in our consideration, the result that is relevant for current efforts to demonstrate a mass gap in interacting field theories such as Yang-Mills theory. Notably, our mechanism involves a physically sensible starting point in terms of real masses (frequencies) in the harmonic non-interacting field, in contrast to the Higgs effect involving the imaginary mass as a starting point. We further note that the longitudinal mode in our approach remains gapless, implying that both short-range and long-range forces with massive and massless particles naturally emerge and unify in a single interacting field, a result not hitherto anticipated. Finally, we comment on the relationship between our results and hydrodynamic description of the quark-gluon plasma.

hep-th↗

arXiv2014

Measurement of bitumen viscosity in the room-temperature drop experiment: student education, public outreach and modern science in one

Slow flow of the viscous liquid is a thought-provoking experiment that challenges students, academics and public to think about some fundamental questions in modern science. In the Queensland demonstration, the world-longest running experiment earning the Ig Nobel prize, one drop of pitch takes about 10 years to fall, leading to problems of actually observing the drops. Here, we describe our recent demonstration of slowly-flowing bitumen where appreciable flow is observed on the time scale of months. The experiment is free from dissipative heating effects and has the potential to improve the accuracy of measurement. Bitumen viscosity was calculated by undergraduate students during the summer project. The worldwide access to the running experiment is provided by webcams uploading the images to a dedicated website, enhancing student education experience and promotion of science. This demonstration serves as an attractive student education exercise and stimulates the discussion of fundamental concepts and hotly debated ideas in modern physics research: difference between solids and liquids, the nature of liquid-glass transition, emergence of long time scales in a physical process, and the conflict between human intuition and physical reality.

cond-mat.soft↗

arXiv2014

Dynamic transition of supercritical hydrogen in gas giants: defining the boundary between interior and atmosphere

Understanding physics of gas giants requires the knowledge about the behavior of hydrogen at extreme pressures and temperatures. Molecular hydrogen in these planets is supercritical, and has been considered as a physically homogeneous state where no differences can be made between a liquid and a gas and where all properties undergo no marked or distinct changes with pressure and temperature, the picture believed to hold below the dissociation and metallization transition. Here, we show that in Jupiter and Saturn, supercritical molecular hydrogen undergoes a dynamic transition around 10 GPa and 3000 K from the "rigid" liquid state to the "non-rigid" gas-like fluid state at the Frenkel line recently proposed, with accompanying qualitative changes of all major physical properties. The consequences of this finding are discussed, including a physically justified way to demarcate the interior and the atmosphere in gas giants.

astro-ph.EP↗

arXiv2014

The "True" Widom Line for a Square-Well System

In the present paper we propose the van der Waals-like model, which allows a purely analytical study of fluid properties including the equation of state, phase behavior and supercritical fluctuations. We take a square-well system as an example and calculate its liquid - gas transition line and supercritical fluctuations. Employing this model allows us to calculate not only the thermodynamic response functions (isothermal compressibility $β_T$, isobaric heat capacity $C_P$, density fluctuations $ζ_T$, and thermal expansion coefficient $α_T$), but also the correlation length in the fluid $ξ$. It is shown that the bunch of extrema widens rapidly upon departure from the critical point. It seems that the Widom line defined in this way cannot be considered as a real boundary that divides the supercritical region into the gaslike and liquidlike regions. As it has been shown recently, the new dynamic line on the phase diagram in the supercritical region, namely the Frenkel line, can be used for this purpose.

cond-mat.soft↗

arXiv2013

Evidence for structural crossover in the supercritical state

The state of matter above the critical point is terra incognita, and is loosely discussed as a physically homogeneous flowing state where no differences can be made between a liquid and a gas and where properties undergo no marked or distinct changes with pressure and temperature. In particular, the structure of supercritical state is currently viewed to be the same everywhere on the phase diagram, and to change only gradually and in a featureless way while moving along any temperature and pressure path above the critical point. Here, we demonstrate that this is not the case, but that there is a well-defined structural crossover instead. Evidenced by the qualitative changes of distribution functions of interatomic distances and angles, the crossover demarcates liquid-like and gas-like configurations and the presence of medium-range structural correlations. Importantly, the discovered structural crossover is closely related to both dynamic and thermodynamic crossovers operating in the supercritical state, providing new unexpected fundamental interlinks between the supercritical structure, dynamics and thermodynamics.

cond-mat.stat-mech↗

arXiv2013

"Liquid-gas" transition in the supercritical region: Fundamental changes in the particle dynamics

Recently, we have proposed a new dynamic line on the phase diagram in the supercritical region. Crossing this line corresponds to the radical changes of the fluid properties. Here, we focus on the dynamics of model Lennard-Jones and Soft-Sphere fluids. We show that the change of the dynamics from the liquid-like to gas-like can be established on the basis of the velocity autocorrelation function and mean-square displacement. Using the rigorous criterion, we show that the crossover of particle dynamics and key liquid properties occurs at the same line. We further show that positive sound dispersion disappears in the vicinity of this line in both kinds of systems. The dynamic line bears no relationship to the existence of the critical point. We find that the region of existence of liquid-like dynamics narrows with the increase of the exponent of the repulsive part of inter-particle potential.

cond-mat.soft↗

arXiv2013

Thermodynamic behavior of supercritical matter

Since their discovery in 1822, supercritical fluids have been of enduring interest, and have started to be deployed in many important applications. Theoretical understanding of the supercritical state is lacking, and is seen to limit further industrial deployment. Here, we study thermodynamic properties of the supercritical state, and discover that specific heat shows a crossover between two different regimes, an unexpected result in view of currently perceived homogeneity of supercritical state in terms of physical properties. We subsequently formulate a theory of system thermodynamics above the crossover, and find good agreement between calculated and experimental specific heat with no free fitting parameters. We derive a power law and analyze supercritical scaling exponents in the system above the Frenkel line. In this theory, energy and heat capacity are governed by the minimal length of the longitudinal mode in the system only, and do not explicitly depend on system-specific structure and interactions.

physics.chem-ph↗

arXiv2013

Phase transformations in methanol at high pressure measured by dielectric spectroscopy technique

Dielectric response in methanol measured in wide pressure and temperature range ($P < 6.0$ GPa; 100 K $ $ 1.2 GPa $T < 270$ K. In the intermediate P-T region $P \approx 3.4-3.7$ GPa $T \approx 260-280$ K a set of complicated structural transformations occurs involving four methanol crystalline structures. At higher pressures within a narrow range $P \approx 4.3-4.5$ GPa methanol can be obtained in the form of fragile glass ($T_g \approx 200$ K, $m_p \approx 80$ at $P= 4.5$ GPa) by relatively slow cooling.

cond-mat.mtrl-sci↗

arXiv2013

Duality of liquids

Liquids flow, making them remarkably distinct from solids and close to gases. At the same time, interactions in liquids are strong as in solids. The combination of these two properties is believed to be the ultimate obstacle to constructing a general theory of liquids. Here, we adopt a new approach to liquids: instead of focusing on the problem of strong interactions, we zero in on the relative contributions of vibrational and diffusional motion in liquids. We subsequently show that from the point of view of thermodynamics, liquid energy and specific heat are given, to a very good approximation, by their vibrational contributions as in solids, for relaxation times spanning 15 orders of magnitude. We therefore find that liquids show an interesting {\it duality} not hitherto known: they are close to solids from the thermodynamical point of view and to gases from the point of view of flow. We discuss the experimental implications of this approach.

cond-mat.soft↗

arXiv2013

Dielectric Spectroscopy and Ultrasonic Study of Propylene Carbonate under Ultra-high Pressures

We present the high pressure dielectric spectroscopy (up to 4.2 GPa) and ultrasonic study (up to 1.7 GPa) of liquid and glassy propylene carbonate (PC). Both of the methods provide complementary pictures of the glass transition in PC under pressure. No other relaxation processes except $α$-relaxation have been found in the studied pressure interval. The propylene carbonate liquid is a glassformer where simple relaxation and the absence of $β$-relaxation are registered in the record-breaking ranges of pressures and densities. The equation of state of liquid PC was extended up to 1 GPa from ultrasonic measurements of bulk modulus and is in good accordance with the previous equations developed from volumetric data. We measured the bulk and shear moduli and Poisson's ratio of glassy PC up to 1.7 GPa. Many relaxation and elastic properties of PC can be qualitatively described by the soft-sphere or Lennard-Jones model. However, for the quantitative description of entire set of the experimental data, these models are insufficient. Moreover, the Poisson coefficient value for glassy PC indicates a significant contribution of non-central forces to the intermolecular potential. The well-known correlation between Poisson's ratio and fragility index (obtained from dielectric relaxation) is confirmed for PC at ambient pressure, but it is violated with pressure increase. This indicates that different features of the potential energy landscape are responsible for the evolution of dielectric response and elasticity with pressure increase.

cond-mat.soft↗

arXiv2013

Heat capacity of matter beyond the Dulong-Petit value

We propose a new simple way to evaluate the effect of anharmonicity on a system's thermodynamic functions such as heat capacity. In this approach, the contribution of all potentially complicated anharmonic effects to constant-volume heat capacity is evaluated by one parameter only, the coefficient of thermal expansion. Importantly, this approach is applicable not only to crystals but also to glasses and viscous liquids. To support this proposal, we perform molecular dynamics simulations of several crystalline and amorphous solids as well as liquids, and find a good agreement between results from theory and simulations. We observe an interesting non-monotonic behavior of liquid heat capacity with a maximum, and explain this effect as a result of competition between anharmonicity at low temperature and decreasing number of transverse modes at high temperature.

cond-mat.stat-mech↗

arXiv2013

Helium at elevated pressures: Quantum liquid with non-static shear rigidity

The properties of liquid helium have always been a fascinating subject to scientists. The phonon theory of liquids taking into account liquid non-static shear rigidity is employed here for studying internal energy and heat capacity of compressed liquid 4-He. We demonstrate good agreement of calculated and experimental heat capacity of liquid helium at elevated pressures and supercritical temperatures. Unexpectedly helium remains a quantum liquid at elevated pressures for a wide range of temperature supporting both longitudinal and transverse-like phonon excitations. We have found that in the very wide pressure range 5 MPa-500 MPa liquid helium near melting temperature is both solid-like and quantum.

cond-mat.stat-mech↗

arXiv2013

Properties of Liquid Iron along the Melting Line up to the Earth-core Pressures

We report a molecular dynamics study of transport coefficients and infinite frequency shear mod- ulus of liquid iron at high temperatures and high pressures. We observe a simultaneous rise of both shear viscosity and diffusion coefficient along the melting line and estimate if liquid iron can vitrify under Earth-core conditions. We show that in frames of the model studied in our work iron demonstrates a moderate increase of viscosity along the melting line. It is also demonstrated that in the limit of high temperatures and high pressures the liquid iron behaves similar to the soft spheres system with exponent n=4.6.

cond-mat.mtrl-sci↗