Nonequilibrium Rate Research Articles

Photon production and elliptic flow from a momentum-anisotropic quark-gluon plasma

The emission of real photons from a momentum-anisotropic quark-gluon plasma (QGP) is affected by both the collective flow of the radiating medium and the modification of local rest frame emission rate due to the anisotropic momentum distribution of partonic degrees of freedom. In this paper, we first calculate the photon production rate from an ellipsoidally momentum-anisotropic QGP including hard contributions from Compton scattering and quark pair annihilation and soft contribution calculated using the hard thermal loop (HTL) approximation. We introduce a parametrization of the nonequilibrium rate in order to facilitate its further application in yield and flow calculations. We convolve the anisotropic photon rate with the space-time evolution of QGP provided by 3+1d anisotropic hydrodynamics (aHydro) to obtain the yield and the elliptic flow coefficient $v_2$ of photons from QGP generated at Pb-Pb collisions at LHC at 2.76 TeV and Au-Au collisions at RHIC at 200 GeV. We investigate the effects of various parameters on the results. In particular we analyze the sensitivity of results to initial momentum anisotropy.

Direct visualization of out-of-equilibrium structural transformations in atomically thin chalcogenides

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have been the subject of sustained research interest due to their extraordinary electronic and optical properties. They also exhibit a wide range of structural phases because of the different orientations that the atoms can have within a single layer, or due to the ways that different layers can stack. Here we report a unique study involving direct visualization of structural transformations in atomically thin layers under highly non-equilibrium thermodynamic conditions. We probe these transformations at the atomic scale using real-time, aberration-corrected scanning transmission electron microscopy and observe strong dependence of the resulting structures and phases on both heating rate and temperature. A fast heating rate (25 °C/sec) yields highly ordered crystalline hexagonal islands of sizes of less than 20 nm which are composed of a mixture of 2H and 3R phases. However, a slow heating rate (25 °C/min) yields nanocrystalline and sub-stoichiometric amorphous regions. These differences are explained by different rates of sulfur evaporation and redeposition. The use of non-equilibrium heating rates to achieve highly crystalline and quantum-confined features from 2D atomic layers present a new route to synthesize atomically thin, laterally confined nanostructures and opens new avenues for investigating fundamental electronic phenomena in confined dimensions.

From the Kinetic Theory of Gases to the Kinetics of Rate Processes: On the Verge of the Thermodynamic and Kinetic Limits.

A variety of current experiments and molecular dynamics computations are expanding our understanding of rate processes occurring in extreme environments, especially at low temperatures, where deviations from linearity of Arrhenius plots are revealed. The thermodynamic behavior of molecular systems is determined at a specific temperature within conditions on large volume and number of particles at a given density (the thermodynamic limit): on the other side, kinetic features are intuitively perceived as defined in a range between the extreme temperatures, which limit the existence of each specific phase. In this paper, extending the statistical mechanics approach due to Fowler and collaborators, ensembles and partition functions are defined to evaluate initial state averages and activation energies involved in the kinetics of rate processes. A key step is delayed access to the thermodynamic limit when conditions on a large volume and number of particles are not fulfilled: the involved mathematical analysis requires consideration of the role of the succession for the exponential function due to Euler, precursor to the Poisson and Boltzmann classical distributions, recently discussed. Arguments are presented to demonstrate that a universal feature emerges: Convex Arrhenius plots (super-Arrhenius behavior) as temperature decreases are amply documented in progressively wider contexts, such as viscosity and glass transitions, biological processes, enzymatic catalysis, plasma catalysis, geochemical fluidity, and chemical reactions involving collective phenomena. The treatment expands the classical Tolman’s theorem formulated quantally by Fowler and Guggenheim: the activation energy of processes is related to the averages of microscopic energies. We previously introduced the concept of “transitivity”, a function that compactly accounts for the development of heuristic formulas and suggests the search for universal behavior. The velocity distribution function far from the thermodynamic limit is illustrated; the fraction of molecules with energy in excess of a certain threshold for the description of the kinetics of low-temperature transitions and of non-equilibrium reaction rates is derived. Uniform extension beyond the classical case to include quantum tunneling (leading to the concavity of plots, sub-Arrhenius behavior) and to Fermi and Bose statistics has been considered elsewhere. A companion paper presents a computational code permitting applications to a variety of phenomena and provides further examples.

Efficient conversion of chemical energy into mechanical work by Hsp70 chaperones.

Hsp70 molecular chaperones are abundant ATP-dependent nanomachines that actively reshape non-native, misfolded proteins and assist a wide variety of essential cellular processes. Here, we combine complementary theoretical approaches to elucidate the structural and thermodynamic details of the chaperone-induced expansion of a substrate protein, with a particular emphasis on the critical role played by ATP hydrolysis. We first determine the conformational free-energy cost of the substrate expansion due to the binding of multiple chaperones using coarse-grained molecular simulations. We then exploit this result to implement a non-equilibrium rate model which estimates the degree of expansion as a function of the free energy provided by ATP hydrolysis. Our results are in quantitative agreement with recent single-molecule FRET experiments and highlight the stark non-equilibrium nature of the process, showing that Hsp70s are optimized to effectively convert chemical energy into mechanical work close to physiological conditions.

A numerical study on the effect of cavitation erosion in a diesel injector

The consequences of geometry alterations in a diesel injector caused by cavitation erosion are investigated with numerical simulations. The differences in the results between the nominal design geometry and the eroded one are analyzed for the internal injector flow and spray formation. The flow in the injector is modeled with a three-phase Eulerian approach using a compressible pressure-based multiphase flow solver. Cavitation is simulated with a nonequilibrium mass transfer rate model based on the simplified form of the Rayleigh–Plesset equation. Slip velocity between the liquid-vapor mixture and air is included in the model by solving two separate momentum conservation equations. The eroded injector is found to result in a loss in the rate of injection but also lower cavitation volume fraction inside the nozzle. The injected sprays are then simulated with a Lagrangian method considering as initial conditions the predicted flow characteristics at the exit of the nozzle. The results obtained show wider spray dispersion for the eroded injector and shorter spray tip penetration.

The influence of polyethyleneimine dosages and molecular weight on sedimentation and rheology behavior of copper tailings

Polyethyleneimine (PEI) is a kind of modifier and flocculant with good adsorption to heavy metal ions, but there is a lack of literature investigate the influence of polyethyleneimine (PEI) dosages and molecular weight on sedimentation and rheology behavior of copper tailing suspension. In this research, the influence of PEI dosages and molecular weight on sedimentation and rheology behavior were characterized which is supported by measurements and calculation of initial sedimentation rate, turbidity, zeta potential, the contents of heavy metal ions in the supernatant, FT-IR analysis, shear stress and yield stress. With the application of lower molecular weight PEI or PEI_1800, the lower yield stress and inter-aggregate bridging force are obtained, the re-conformation rate is higher than the non-equilibrium flocculation rate between copper tailing particles which results in more charged PEI segments propagating into the suspension. The calculation of interaction energy shows that the smaller floc size is obtained when PEI_1800 or lower molecular weight PEI is used, which is also confirmed by the sedimentation behavior and rheology behavior. In 29 wt% copper tailing suspensions at pH∼9, PEI has a better adsorption performance on Cu2+ and Pb2+. The correlations between PEI molecular weight, polyethyleneimine dosages, yield stress, sedimentation behavior, interaction energy and mechanisms are analyzed of copper tailing suspensions with PEI.

Vibrational Energy Transfer and Collision-Induced Dissociation in Collisions

This paper presents molecular dynamics calculations of vibrational energy transfer and nonequilibrium dissociation in collisions. The interactions are modeled using nine potential energy surfaces corresponding to the , , , , , , , , and states, which govern electronically adiabatic collisions of ground-electronic-state collisions of diatomic oxygen with atomic oxygen. Characteristic vibrational excitation times are calculated over a temperature range of to , and nonequilibrium dissociation rate coefficients are calculated over a temperature range of to . Vibrational relaxation rates, specific to each PES, are found to vary by over an order of magnitude, indicating that all spin couplings and spatial degeneracies must be considered for accurate predictions of collisions. It has been observed that the characteristic vibrational excitation time for interactions is weakly dependent on temperature and increases slightly with increasing temperature. Predicted nonequilibrium dissociation rates, during quasi-steady state. Agree well with available experimental data, and the coupling between vibrational energy and dissociation is characterized.

Hydrogen-/propane-hydrate decomposition: thermodynamic and kinetic analysis

ABSTRACTThe dissociation of planar mixed hydrogen-/propane-hydrate in contact with water has been studied by molecular dynamics between 260 and 320 K. Two modes of termination of the hydrate interface were chosen: direct cleavage to produce an 001-surface, as well as one with completed cages. Substantial differences in melting points and initial dissociation kinetics were evident between both kinds of surface. These break-up rates were significantly dependent on temperature. A kinetic model was applied to fit dissociation profiles, clarifying crisply two hydrate dissociation regions; after a temperature-sensitive decomposition stage, a well-characterised lattice-collapse process is, by and large, insensitive to temperature. For hydrate-liquid systems at thermal equilibrium, consideration of relaxation times of auto-correlation functions of clathrate-molecule-number fluctuations shows significant differences between the two modes of interface: a unifying relationship was established between the non-equilibrium break-up rate from outright melting and that inferred from fluctuation-dissipation at equilibrium.

Nonequilibrium electron dynamics in pump-probe spectroscopy: Role of excited phonon populations

We study the role of excited phonon populations in the relaxation rates of nonequilibrium electrons using a nonequilibrium Green's function formalism. The transient modifications in the phononic properties are accounted for by self-consistently solving the Dyson equation for the electron and phonon Green's functions. The pump induced changes manifest in both the electronic and phononic spectral functions. We find that the excited phonon populations suppress the decay rates of nonequilibrium electrons due to enhanced phonon absorption. The increased phonon occupation also sets the nonequilibrium decay rates and the equilibrium scattering rates apart. The decay rates are found to be time-dependent, and this is illustrated in the experimentally observed population decay of photoexcited $\mathrm{Bi}_{1.5}\mathrm{Sb}_{0.5} \mathrm{Te}_{1.7}\mathrm{Se}_{1.3}$.

KAPPA: Kinetic approach to physical processes in atmospheres library in C++

KAPPA, an open-source software library for the computation of thermodynamic and transport properties in non-equilibrium gas flows and species production rates, is presented. Thermodynamic properties rely on custom databases which describe the characteristics of atoms and molecules and interaction features. Mixture thermodynamic properties are formulated from species quantities. Species production rates are based on elementary chemical reactions and vibrational energy transitions. Transport properties are derived from kinetic theory which provides relationships for transport coefficients depending on the flow model. Several levels of non-equilibrium flow description are implemented: the detailed state-to-state (STS) approach, multi-temperature and one-temperature models. Sets of transport coefficients depend on the deviation from equilibrium; in the state-to-state model, they include state-resolved diffusion coefficients for each pair of vibrational states. A highly modular, object-oriented application program interface (API) has been developed. The adopted object-oriented programming paradigm (OOP), along with the implemented models of state-resolved physico-chemical relaxation rates, is presented. The design of the library allows it to be easily included in existing CFD codes and is aimed at code reusability and readability.Program Title: KAPPAProgram Files doi:http://dx.doi.org/10.17632/mrx9n35bvs.1Licensing provisions: GNU General Public Licence, version 3.Programming language: C++; developed and tested with Intel Compiler v. 18.x and GNU.External routines/libraries: KAPPA must be linked to yaml-cpp [1] in order to read human-readable database files and to Armadillo [2] for solving algebraic linear systems.Nature of problem: Numerical computation of transport properties and rate exchange coefficients for non-equilibrium reacting flows.Solution method: Non-equilibrium reacting flow is treated by a rigorous state-to-state (STS) kinetic theory approach.Restrictions: At present, KAPPA is validated for computing transport properties and rate exchange coefficients using the state-to-state (STS) approach; 1- and 2-temperature models have still to be validated, but they are already implemented. C++ compiler is mandatory.Unusual features: KAPPA is an open-source object-oriented highly modular and easy-to-maintain C++ software library which implements rigorous mathematical kinetic theory models.Additional comments: KAPPA project adopts Git [3], a free and open source distributed version control system. A public repository dedicated to KAPPA project [4] has been created on github, a web-based hosting service for software development projects using git versioning system. Finally, a comprehensive documentation [5] is provided parsing source code comments by means of doxygen software [6].

Kinetic Selection of Template Polymer with Complex Sequences.

The emergence and maintenance of polymers with complex sequences pose a major question in the study of the origin of life. To answer this, we study a model polymerization reaction, where polymers are synthesized by stepwise ligation from two types of monomers, catalyzed by a long polymer as a template. Direct stochastic simulation and dynamical systems analysis reveal that the most dominant polymer sequence in a population successively changes, depending on the flow rate of monomers to the system, with more complex sequences selected at a lower flow rate. We discuss the relevance of this kinetic sequence selection through nonequilibrium flow to the origin of complex polymers.

Estimation of the Maximum Nonequilibrium Charge-Carrier Concentration in GaN Under Electron-Beam Irradiation

An empirical expression describing the lateral distribution of the nonequilibrium charge-carrier generation rate in GaN by a focused electron beam is obtained. The maximum nonequilibrium charge-carrier concentration as a function of the beam energy and diffusion length is calculated using this expression. Comparison of the results with values obtained using previously proposed approximate expressions provides an opportunity to evaluate the applicability of these approximations. Within the study of GaN by the cathodoluminescence and electron-beam-induced-current methods, the possibility to achieve a low injection level is analyzed considering the fact that the diffusion lengths of nonequilibrium charge carriers can be measured correctly only at low injection levels. It is shown that, in spite of the submicron values of the diffusion lengths in GaN, rather low beam currents are required to achieve a low injection level.

Assessment of Classical Impulsive Models of Dissociation in Thermochemical Nonequilibrium

In strong shock waves in rarefied gases, chemical processes such as dissociation and exchange reactions occur in conditions of thermodynamic nonequilibrium, and the rate coefficients are functions of both translational–rotational and vibrational temperatures. In this work, we reexamine and validate the physical model for nonequilibrium dissociation rates proposed earlier by Macheret et al. The model is based on the classical impulsive approximation and enables analytical formulas for reaction thresholds, probabilities, and multitemperature rate coefficients. The recent quasi-classical trajectory calculations for , , , , and are used to evaluate the Macheret–Fridman dissociation model. The full version of the model that incorporates the reduction of dissociation energy due to rotation of the dissociating molecule as well as the appearance of the centrifugal barrier is found to be in good agreement with quasi-classical trajectory data, whereas the simplified version of the model that ignores the rotational effect significantly underpredicts the dissociation rates in vibrationally cold conditions. The good agreement with quasi-classical trajectory data validates the classical impulsive approximation and implies that details of the potential energy surface are not important for dissociation at high temperatures. Preliminary one-dimensional computational fluid dynamics calculations with the validated dissociation model were conducted for conditions of the shock-tube experiments in pure oxygen by Ibraguimova et al. Generally reasonable agreement was found between the calculations and the experimental data; however, some underprediction of the peak vibrational temperature suggests that some improvement in the model would be desirable.

High Initial Sputter Rate Found for Vaccinia Virions Using Isotopic Labeling, NanoSIMS, and AFM.

High-lateral-resolution secondary ion mass spectrometry (SIMS) has the potential to provide functional and depth resolved information from small biological structures, such as viral particles (virions) and phage, but sputter rate and sensitivity are not characterized at shallow depths relevant to these structures. Here we combine stable isotope labeling of the DNA of vaccinia virions with correlated SIMS imaging depth profiling and atomic force microscopy (AFM) to develop a nonlinear, nonequilibrium sputter rate model for the virions and validate the model on the basis of reconstructing the location of the DNA within individual virions. Our experiments with a Cs+ beam show an unexpectedly high initial sputter rate (∼100 um2·nm·pA-1·s-1) with a rapid decline to an asymptotic rate of 0.7 um2·nm·pA-1·s-1 at an approximate depth of 70 nm. Correlated experiments were also conducted with glutaraldehyde-fixed virions, as well as O- and Ga+ beams, yielding similar results. Based on our Cs+ sputter rate model, the labeled DNA in the virion was between 50 and 90 nm depth in the virion core, consistent with expectations, supporting our conclusions. Virion densification was found to be a secondary effect. Accurate isotopic ratios were obtained from the initiation of sputtering, suggesting that isotopic tracers could be successfully used for smaller virions and phage.

The Enhancement of Interfacial Exciton Dissociation by Energetic Disorder Is a Nonequilibrium Effect.

The dissociation of excited electron–hole pairs is a microscopic process that is fundamental to the performance of photovoltaic systems. For this process to be successful, the oppositely charged electron and hole must overcome an electrostatic binding energy before they undergo ground state recombination. It has been observed previously that the presence of energetic disorder can lead to a reduction in recombination losses. Here we investigate this effect using a simple model of charge dynamics at a donor–acceptor interface. We consider the effect of spatial variations in electronic energy levels, such as those that arise in disordered molecular systems, on dissociation yield and demonstrate that it is maximized with a finite amount of disorder. We demonstrate that this is a nonequilibrium effect that is mediated by the dissipation driven formation of partially dissociated intermediate states that are long-lived because they cannot easily recombine. We present a kinetic model that incorporates these states and show that it is capable of reproducing similar behavior when it is parametrized with nonequilibrium rates.

Equivalent circuit-level model of quantum cascade lasers with integrated hot-electron and hot-phonon effects

We report a study of the effects of hot-electron and hot-phonon dynamics on the output characteristics of quantum cascade lasers (QCLs) using an equivalent circuit-level model. The model is developed from the energy balance equation to adopt the electron temperature in the active region levels, the heat transfer equation to include the lattice temperature, the nonequilibrium phonon rate to account for the hot phonon dynamics and simplified two-level rate equations to incorporate the carrier and photon dynamics in the active region. This technique simplifies the description of the electron–phonon interaction in QCLs far from the equilibrium condition. Using the presented model, the steady and transient responses of the QCLs for a wide range of sink temperatures (80 to 320 K) are investigated and analysed. The model enables us to explain the operating characteristics found in QCLs. This predictive model is expected to be applicable to all QCL material systems operating in pulsed and cw regimes.

Quantification of spatial lag effect on sediment transport around a hydraulic structure using Eulerian–Lagrangian model

The spatial lag effect on sediment transport around a hydraulic structure is quantified through numerical simulation using a Eulerian–Lagrangian model. The Eulerian–Lagrangian model uses a deterministic approach of bed load motion and stochastic approach of Einstein's concept, which allows for a reasonable quantification of both non-equilibrium and equilibrium bed load transport rates. Numerical simulations are conducted for local scour around a spur dyke and a weir-type structure. As pointed out in previous experimental studies on the one-dimensional problem, Eulerian–Lagrangian simulation reveals an unsaturated bed load in a scour hole around a hydraulic structure. Furthermore, an oversaturated bed load is also found in the deposition region, particularly behind the structure. The validity of the linear adaptation equation is discussed using numerical results, revealing the possibility that the linear adaptation equation does not appropriately represent the existence of the strong spatial lag effect. The present study finds there are two modes in the response of the adaptation length to the unsaturated bed load, which depend on the clear-water scour condition or the live-bed scour condition. Findings of this study will contribute to multi-dimensional sediment transport simulations that consider the spatial lag effect.

Green light activated hydrogen sensing of nanocrystalline composite ZnO-In2O3 films at room temperature

The possibility of reducing the operating temperature of H2 gas sensor based on ZnO-In2O3 down to room temperature under green illumination is shown. It is found that sensitivity of ZnO-In2O3 composite to H2 nonmonotonically depends on the oxides’ content. The optimal ratio between the components is chosen. The new mechanism of nanocrystalline ZnO-In2O3 sensor sensitivity to H2 under illumination by green light is proposed. The mechanism considers the illumination turns the composite into nonequilibrium state and the photoconductivity change in the H2 atmosphere is linked with alteration of nonequilibrium charge carriers recombination rate.

Electronic Excited State Lifetimes of Anionic Water Clusters: Dependence on Charge Solvation Motif.

An ongoing controversy about water cluster anions concerns the electron-binding motif, whether the charge center is localized at the surface or within the cluster interior. Here, mixed quantum-classical dynamics simulations have been carried out for a wide range of cluster sizes (n ≤ 1000) for (H2O)n- and (D2O)n-, based on a nonequilibrium first-order rate constant approach. The computed data are in good general agreement with time-resolved photoelectron imaging results (n ≤ 200). The analysis reveals that, for surface state electrons, the cluster size dependence of the excited state electronic energy gap and the magnitude of the nonadiabatic couplings have compensating influences on the excited state lifetimes: the excited state lifetime for surface states reaches a minimum for n ∼ 150 and then increases for larger clusters. It is concluded that the electron resides in a surface-localized motif in all of these measured clusters, dominating at least up to n = 200.

Assessment of vibration–dissociation coupling models for hypersonic nonequilibrium simulations

The fidelity of three widely-used two-temperature vibration–dissociation coupling models, including the Park model (1988), the Macheret–Fridman model (1994), and the coupled vibration–dissociation–vibration (CVDV) model (1963), is numerically investigated via a comparison with state-specific results and existing shock tube data for oxygen flows. Under the hypothetical condition where a Boltzmann distribution corresponding to a vibrational temperature is assumed, it is found that the CVDV model with a proper parameter is capable of providing the most accurate results, whereas the Park and Macheret–Fridman models present similar prediction accuracy for which the nonequilibrium dissociation rate coefficients could be as much as three orders of magnitude lower than state-specific values. However, for actual postshock flows with intensive vibrational excitation and dissociation processes, the CVDV model still shows significant discrepancies relative to state-specific results and experimental data, presenting a much lower vibrational temperature and a higher degree of dissociation. The essential cause for the disagreement lies in the existence of non-Boltzmann distributions observed when the incubation period is over and dissociation starts to dominate. Recommendations on further modification of two-temperature vibration–dissociation coupling models are presented.