Strong Nonequilibrium Conditions Research Articles

Extensive analysis of the applicability range of the linear kinetic approaches in the case of the pressure driven gas mixture flows

The study of gas mixture flows through micro- and macro-channels remains a very attractive area for theorists and experimentalists worldwide, mainly due to their great practical applicability in several aspects of science and industry. The present work includes a comparative study between the linear (McCormack model) and the nonlinear (DSMC method) kinetic theories of binary gas mixture flows through channels over a wide range of the involved parameters. The results show that the McCormack model is a reliable kinetic model for predicting the gas mixture flow behavior. Specific criteria with respect to the applicability range of the linear kinetic theory of short and long capillaries are proposed. The analysis shows that the separation phenomenon remains strong even in the case of small pressure drops. The applicability range of the linear kinetic theory for binary gas mixture flows driven by large pressure drops is obtained to be smaller compared to that of single gases but still wide enough covering a wide range of pressure ratios. Furthermore, the present work shows that the long capillary theory remains a very powerful tool for studying the gas mixture flow behavior under weak and strong nonequilibrium conditions. This work provides a kinetic database of the linear kinetic data (as supplementary material), which until today is still missing from the literature, and as it is shown throughout this work, it can be used far beyond the restrictions defined by the linear kinetic theory.

The connection between dissipative chaos and nonequilibrium

Strong nonequilibrium conditions often give rise to dissipative chaos. However, the underlying connection between dissipative chaos and nonequilibrium remains unclear. Herein, we study this connection from the framework of the nonequilibrium potential-flux landscape. The nonequilibrium system dynamics are determined in part by the potential landscape associated with the steady-state probability distribution. The other part of the driving force for nonequilibrium dynamics comes from the steady-state probability flow, leading to the breaking of detailed balance. We find that dissipative chaos is driven mainly by the nonequilibrium probability flow. Using the Lorenz strange attractor model, we quantitatively demonstrate the close connection between the kinematic measures of the appearance of dissipative chaos and the degree of nonequilibrium. This work reveals the dynamic and thermodynamic origins of dissipative chaos beyond the interpretation based on nonlinearity. This study also provides a new understanding of the phase transitions in nonequilibrium systems.

Numerical Simulations of Shock Propagation Under Strong Nonequilibrium Conditions

A computational approach is developed to simulate high-enthalpy shock propagation under strong nonequilibrium conditions, such as reentry experiments at NASA’s Electric Arc Shock Tube (EAST) facility. Two-dimensional axisymmetric simulations of EAST flow are performed for Earth reentry at . An 11-species weakly ionized air model is used to capture the real-gas behavior with reaction rates from the literature. Species mass-diffusion fluxes are calculated assuming a self-consistent effective binary diffusion in conjunction with Gupta–Yos collision integral data. The unsteady, linearized system of conservation laws is solved in a moving frame of reference with active shock tracking to reduce the computational cost by three orders of magnitude relative to that of fixed-frame calculations on a uniform grid. This approach enables time-accurate yet computationally feasible predictions of gas behavior at the test section. Computational fluid dynamics predictions can be compared with the experimental data to gain an improved understanding of the flow physics. Additionally, useful quantities such as boundary-layer thickness and growth rate, and radial variation of gas properties can be computed, which are difficult to measure in experiments.

Luminescence and optical properties of cerium oxide nanoparticles

Cerium oxide (CeO2) nanoparticles present an interesting material for various applications in biotechnology and medicine. The mechanism of CeO2 nanoparticles activity is supposed to be related to the presence both Ce3+ and Ce4+ ions on the surface of the nanocrystal. Due to the Ce3+ ions, the charge deficiency of a crystal is compensated by oxygen vacancies in the lattice. The present work is devoted to study the cerium oxide defects by optical and luminescence spectroscopy methods. The CeO2 nanoparticles was produced by the method of pulsed electron evaporation of ceramic oxide targets with condensation of the vapors in a vacuum. The strong non-equilibrium conditions of this method could lead to higher oxygen non-stoichiometry of cerium oxide. The results of work show the cerium nanocrystal manifests an emission provided by intracenter transitions in the Ce3+ ions, F0-centers and Ce4+–O2- complexes with charge transfer. The band gap Eg of the CeO2 nanoparticles for direct and indirect transitions was estimated. The cerium oxide was found to demonstrate catalase-mimic activity, as well as differential cytotoxicity in relation to tumor and healthy cell cultures.

Relaxation processes in carbon dioxide

The present study is aimed in providing a framework for applying different continuum models of relaxation processes in carbon dioxide flows. Kinetic equations for the distribution function are written taking into account the CO2 structure and various mechanisms of vibrational relaxation; collision operators for different internal energy transitions are derived. For weak non-equilibrium conditions, a one-temperature model is developed with emphasis to the bulk viscosity phenomenon. For strong non-equilibrium conditions, multi-temperature models are introduced, and their advantages and limitations are discussed. A general algorithm for calculating vibrational relaxation time in polyatomic molecules is proposed. Bulk viscosity coefficients are studied in the temperature range 200–2500 K; it is shown that uncoupling rotational and vibrational modes results in essentially overpredicted values of the bulk viscosity coefficient at low temperatures. The shock wave structure in CO2 is studied using the continuum models and compared with the solution obtained in the frame of the model kinetic approach; the effect of bulk viscosity on the shock wave width and temperature profile is evaluated. It is concluded that well justified choice of the model extends considerably the range of applicability of the continuum approach for non-equilibrium flow simulations.

Monotropic polymorphism in a glass-forming metallic alloy

This study investigates the crystallization and phase transition behavior of the amorphous metallic alloy Au70Cu5.5Ag7.5Si17. This alloy has been recently shown to exhibit a transition of a metastable to a more stable crystalline state, occurring via metastable melting under strong non-equilibrium conditions. Such behavior had so far not been observed in other metallic alloys. In this investigation fast differential scanning calorimetry (FDSC) is used to explore crystallization and the solid–liquid–solid transition upon linear heating and during isothermal annealing, as a function of the conditions under which the metastable phase is formed. It is shown that the occurrence of the solid–liquid–solid transformation in FDSC depends on the initial conditions; this is explained by a history-dependent nucleation of the stable crystalline phase. The microstructure was investigated by scanning and transmission electron microscopy and x-ray diffraction. Chemical mapping was performed by energy dispersive x-ray spectrometry. The relationship between the microstructure and the phase transitions observed in FSDC is discussed with respect to the possible kinetic paths of the solid–liquid–solid transition, which is a typical phenomenon in monotropic polymorphism.

Principal component analysis acceleration of rovibrational coarse-grain models for internal energy excitation and dissociation.

The present work introduces a novel approach for obtaining reduced chemistry representations of large kinetic mechanisms in strong non-equilibrium conditions. The need for accurate reduced-order models arises from compression of large ab initio quantum chemistry databases for their use in fluid codes. The method presented in this paper builds on existing physics-based strategies and proposes a new approach based on the combination of a simple coarse grain model with Principal Component Analysis (PCA). The internal energy levels of the chemical species are regrouped in distinct energy groups with a uniform lumping technique. Following the philosophy of machine learning, PCA is applied on the training data provided by the coarse grain model to find an optimally reduced representation of the full kinetic mechanism. Compared to recently published complex lumping strategies, no expert judgment is required before the application of PCA. In this work, we will demonstrate the benefits of the combined approach, stressing its simplicity, reliability, and accuracy. The technique is demonstrated by reducing the complex quantum N2(Σg+1)-N(Su4) database for studying molecular dissociation and excitation in strong non-equilibrium. Starting from detailed kinetics, an accurate reduced model is developed and used to study non-equilibrium properties of the N2(Σg+1)-N(Su4) system in shock relaxation simulations.

OH radical kinetics in hydrogen-air mixtures at the conditions of strong vibrational nonequilibrium

This work presents results of time-resolved, absolute measurements of OH number density, nitrogen vibrational temperature, and translational-rotational temperature in air and lean hydrogen-air mixtures excited by a diffuse filament nanosecond pulse discharge, at a pressure of 100 Torr and high specific energy loading. The main objective of these measurements is to study kinetics of OH radicals at the conditions of strong vibrational excitation of nitrogen, below autoignition temperature. N2 vibrational temperature and gas temperature in the discharge and the afterglow are measured by ns broadband coherent anti-Stokes Raman scattering. Hydroxyl radical number density is measured by laser induced fluorescence, calibrated by Rayleigh scattering. The results show that the discharge generates strong vibrational nonequilibrium in air and H2-air mixtures for delay times after the discharge pulse of up to ~1 ms, with a peak vibrational temperature of Tv ≈ 1900 K at T ≈ 500 K. Nitrogen vibrational temperature peaks at 100–200 µs after the discharge pulse, before decreasing due to vibrational-translational relaxation by O atoms (on the time scale of several hundred µs) and diffusion (on ms time scale). OH number density increases gradually after the discharge pulse, peaking at t ~ 100–300 µs and decaying on a longer time scale, until t ~ 1 ms. Both OH rise time and decay time decrease as H2 fraction in the mixture is increased from 1% to 5%. Comparison of the experimental data with kinetic modeling predictions shows that OH kinetics is controlled primarily by reactions of H2 and O2 with O and H atoms generated during the discharge. At the present conditions, OH number density is not affected by N2 vibrational excitation directly, i.e. via vibrational energy transfer to HO2. The effect of a reaction between vibrationally excited H2 and O atoms on OH kinetics is also shown to be insignificant. As the discharge pulse coupled energy is increased, the model predicts transient OH number density overshoot due to the temperature rise caused by N2 vibrational relaxation by O atoms, which may well be a dominant effect in discharges with specific energy loading.

Diamond synthesis in aluminum matrix in molten alkali-halide at ambient pressure

The interaction of the molten aluminum with the carbon-containing admixture in a molten halide-alkali medium at atmospheric pressure with temperatures of 700–800°С under strong non-equilibrium conditions is studied. Cubic diamond crystals of 2–100μm and nanorods of 20–100nm diameter and 5μm lengths are first found forming inside the aluminum matrix under certain cooling regimes. The X-ray diffraction analysis, electron backscatter diffraction spectroscopy, electronic microscopy, Raman spectroscopy and microhardness detection register the perfect cubic diamond formation inside aluminum without applying of high temperature and high-pressure. Diamond formation mechanism is growing by carbon atom diffusion through an aluminum droplet. Diamond and nanodiamond synthesis conducted directly at the carbon ion interaction with the molten metal volume and subsequent cooling regimes. Prime novelty statementDiamond synthesis has been performed in molten aluminium via the interaction between molten aluminium and carbide-containing halide melt at 700–750°C with ambient pressure.The X-ray diffraction analysis, electron backscatter diffraction spectroscopy, electronic microscopy, Raman spectroscopy and microhardness detection register the perfect cubic diamond formation inside aluminium without high temperatures and high-pressure regimes.

Taming microwave plasma to beat thermodynamics in CO2 dissociation.

The strong non-equilibrium conditions provided by the plasma phase offer the opportunity to beat traditional thermal process energy efficiencies via preferential excitation of molecular vibrations. Simple molecular physics considerations are presented to explain potential dissociation pathways in plasma and their effect on energy efficiency. A common microwave reactor approach is evaluated experimentally with Rayleigh scattering and Fourier transform infrared spectroscopy to assess gas temperatures (exceeding 10(4) K) and conversion degrees (up to 30%), respectively. The results are interpreted on a basis of estimates of the plasma dynamics obtained with electron energy distribution functions calculated with a Boltzmann solver. It indicates that the intrinsic electron energies are higher than is favorable for preferential vibrational excitation due to dissociative excitation, which causes thermodynamic equilibrium chemistry to dominate. The highest observed energy efficiencies of 45% indicate that non-equilibrium dynamics had been at play. A novel approach involving additives of low ionization potential to tailor the electron energies to the vibrational excitation regime is proposed.

Modeling of molecular nitrogen collisions and dissociation processes for direct simulation Monte Carlo.

The Direct Simulation Monte Carlo (DSMC) method typically used for simulating hypersonic Earth re-entry flows requires accurate total collision cross sections and reaction probabilities. However, total cross sections are often determined from extrapolations of relatively low-temperature viscosity data, so their reliability is unknown for the high temperatures observed in hypersonic flows. Existing DSMC reaction models accurately reproduce experimental equilibrium reaction rates, but the applicability of these rates to the strong thermal nonequilibrium observed in hypersonic shocks is unknown. For hypersonic flows, these modeling issues are particularly relevant for nitrogen, the dominant species of air. To rectify this deficiency, the Molecular Dynamics/Quasi-Classical Trajectories (MD/QCT) method is used to accurately compute collision and reaction cross sections for the N2(Σg+1)-N2(Σg+1) collision pair for conditions expected in hypersonic shocks using a new potential energy surface developed using a ReaxFF fit to recent advanced ab initio calculations. The MD/QCT-computed reaction probabilities were found to exhibit better physical behavior and predict less dissociation than the baseline total collision energy reaction model for strong nonequilibrium conditions expected in a shock. The MD/QCT reaction model compared well with computed equilibrium reaction rates and shock-tube data. In addition, the MD/QCT-computed total cross sections were found to agree well with established variable hard sphere total cross sections.

Nonequilibrium dynamical mean-field theory and its applications

The study of nonequilibrium phenomena in correlated lattice systems has developed into an active and exciting branch of condensed matter physics. This research field provides rich new insights that could not be obtained from the study of equilibrium situations, and the theoretical understanding of the physics often requires the development of new concepts and methods. On the experimental side, ultra-fast pump-probe spectroscopies enable studies of excitation and relaxation phenomena in correlated electron systems, while ultra-cold atoms in optical lattices provide a new way to control and measure the time-evolution of interacting lattice systems with a vastly different characteristic timescale compared to electron systems. A theoretical description of these phenomena is challenging because, firstly, we have to compute the quantum-mechanical time-evolution of many-body systems out of equilibrium, and secondly, deal with strong-correlation effects which can be of nonperturbative nature. In this review, we discuss the nonequilibrium extension of the dynamical mean field theory (DMFT), which treats quantum fluctuations in the time domain and works directly in the thermodynamic limit. The method reduces the complexity of the calculation via a mapping to a self-consistent impurity problem. Particular emphasis is placed on a detailed derivation of the formalism, and on a discussion of numerical techniques, which enable solutions of the effective nonequilibrium DMFT impurity problem. We summarize the insights gained into the properties of the infinite-dimensional Hubbard model under strong non-equilibrium conditions. These examples illustrate the current ability of the theoretical framework to reproduce and understand fundamental nonequilibrium phenomena, such as the dielectric breakdown of Mott insulators, photo-doping, and collapse-and-revival oscillations in quenched systems.

Molecular-dynamics study of the mechanism of short-pulse laser ablation of single-crystal and polycrystalline metallic targets

Short-pulse laser radiation focused on the surface of a material can simultaneously cause a large number of interconnected nonequilibrium processes that occur in a submicron interval within several picoseconds. Under the implemented extremal conditions, the ablation mechanism induced in a solid substance is extremely complex and has elicited numerous contradictory interpretations. A possible way to investigate its mechanism under strong nonequilibrium conditions is by using molecular dynamics. This method is used in this paper as a basis for a hybrid atomistically continuous model to describe how a picosecond laser pulse interacts with single-crystal and polycrystalline targets made from gold. The kinetics and the mechanism of induced ablation are studied. Differences are detected, and their causes are determined.

Immune to local heating

Electronic transport in a nanodevice can be made insensitive to local heating by driving the device into strong non-equilibrium conditions.

State-resolved master equation analysis of thermochemical nonequilibrium of nitrogen

Using a NASA database of state-to-state transition rates for N+N2, master equation studies are performed for various nonequilibrium heat bath conditions. In these master equation studies, relaxation of the rotational and vibrational modes, time variation of chemical composition, reaction rate coefficients, and average rotational and vibrational energy losses due to dissociation are each considered in strong and weak nonequilibrium conditions. A system of master equations is coupled with one-dimensional flow equations to analyze the relaxation of N2 in post-normal shock flows. From the results of master equations and the post-normal shock calculations, it is recommended that the rotational nonequilibrium of N2 should be treated as a nonequilibrium mode in hypersonic re-entry calculations.

Role of electronic-excitation effects in the melting and ablation of laser-excited silicon

We present a model of laser–solid interactions in silicon based on a previously-developed interatomic potential for silicon where the parameters describing the interactions depend on the temperature of the electronic subsystem Te, which is directly related to the density of electron–hole pairs and hence the number of broken covalent bonds. For 25fs pulses, a wide range of fluence values are simulated resulting in heterogeneous melting, homogeneous melting, and ablation. The results presented here demonstrate that phase transitions can usually be described by ordinary thermal processes even when the electronic temperature Te is much greater than the lattice temperature TL during the transition. However, the evolution of the system and details of the phase transitions depend strongly on Te and corresponding density of broken bonds. Homogeneous melting appears to be an ordinary thermal phase transition, but occurring over very fast time scales (1–5ps) before TL reaches the ordinary melting temperature of silicon. For high enough laser fluence, homogeneous melting is followed by rapid expansion of the superheated liquid and ablation. Rapid expansion of the superheated liquid occurs partly due to high pressures generated by a high density of broken bonds. As a result of rapid expansion of the superheated liquid, the system is readily driven into the liquid–vapor coexistence region which initiates phase explosion. These results strongly indicate that phase explosion, generally thought of as an ordinary thermal process, can occur even under strong nonequilibrium conditions when Te≫TL. Thus, the results both for melting and ablation processes suggest that for many cases there is no clear separation between thermal and nonthermal processes. Instead, ordinary thermal mechanisms are found to apply when Te≫TL, with the high density of broken bonds playing an important role in the detailed evolution of the system.

State-resolved thermochemical nonequilibrium analysis of hydrogen mixture flows

The complete sets of state-to-state transition rate coefficients for both target and projectile molecules of hydrogen are derived from the predicted response surface designed by the ordinary Kriging model. A system of master equations is constructed for bound-bound and bound-free transitions with these designed transition rate coefficients, and the rovibrational number densities are numerically evaluated by implicitly integrating a system of master equations. In these master equation studies, relaxation of rotation and vibration modes, number density relaxation, reaction rate coefficients, and average rotational and vibrational energy losses due to dissociation are each considered in strong nonequilibrium conditions. A system of master equations is coupled with one-dimensional flow equations to analyze the relaxations of hydrogen in post-normal shock and nozzle expanding flows. In post-normal shock flows, at high temperature, the relaxation of the rotational mode is similar to the relaxation of the vibrational mode. In nozzle expanding flows, the relaxations of both rotational and vibrational modes appear to be frozen.

Ultrafast thermoelectric properties of gold under conditions of strong electron-phonon nonequilibrium

The electronic scattering rates in metals after ultrashort pulsed laser heating can be drastically different than those predicted from free electron theory. The large electron temperature achieved after ultrashort pulsed absorption and subsequent thermalization can lead to excitation of subconduction band thermal excitations of electron orbitals far below the Fermi energy. In the case of noble metals, which all have a characteristic flat d-band several electron volts well below the Fermi energy, the onset of d-band excitations has been shown to increase electron-phonon scattering rates by an order of magnitude. In this paper, we investigate the effects of these large electronic thermal excitations on the ultrafast thermoelectric transport properties of gold, a characteristic noble metal. Under conditions of strong electron-phonon nonequilibrium (relatively high electron temperatures and relatively low lattice temperatures, Te⪢TL), we find that the Wiedemann–Franz law breaks down and the Seebeck coefficient is massively enhanced. Although we perform representative calculations for Au, these results are expected to be similar for the other noble metals (Ag and Cu) due to the characteristic large d-band separation from the Fermi energy.

Peculiarities of superheated liquid discharging under strong and weak nonequilibrium conditions

Results obtained in experimental investigating the characteristics of superheated liquid jet discharging under different nonequilibrium conditions are presented. The cases of liquid discharge from a high-pressure chamber and under gravitational jet formation conditions are considered. Different mechanisms of liquid flash boiling with heating growing up to limit superheating are identified in the case of liquid discharging from the high-pressure chamber. Dependences of the reactive force and the jet shape on the value of liquid overheating in a flow are presented. It is shown that the rapidly decreasing value of the reactive force of the jet is connected with its complete disintegration caused by homogeneous nucleation and by the presence of a normal wall behind the outlet from the channel. The parameters of two-phase jet stability in the conditions of gravitational falling, the flow rates, and the size of droplet dispersion zones depend substantially on the shape of holes and the heat flux density. Pronounced effects of partial or full blocking of the holes as a result of soluble impurities deposition in the regimes of cryogenic liquid evaporation or boiling when vapor cavities are formed in the channel are observed in the conditions of long-term discharging under nonadiabatic conditions. This leads to complete disintegration of the jet and uncontrollable reduction of the flow rate through the hole up to full blocking of the latter.

Anomalous non-equilibrium electron transport in one-dimensional quantum nano wire at half-filling: time dependent density renormalization group study

We study non-equilibrium properties of one-dimensional Hubbard model by the density-matrix renormalization-group method. First, we demonstrate stability of "doublon", which characterized by double occupation on a site due to the integrability of the model. Next, we present a kind of anomalous transport caused by the doublons created under strong non-equilibrium conditions in an optical lattice system regarded as an ideal testbed to investigate fundamental properties of the Hubbard model. Finally, we give a result on development of the pair correlation function in a strong non-equilibrium condition. This can be understood as a development of coherence among many excited doublons.