Nonequilibrium Surface Research Articles

Unstable Invasion of Sedimenting Granular Suspensions.

We investigate the development of mobility inversion and fingering when a granular suspension is injected radially between horizontal parallel plates of a cell filled with a miscible fluid. While the suspension spreads uniformly when the suspension and the displaced fluid densities are exactly matched, even a small density difference is found to result in a dense granular front which develops fingers with angular spacing that increase with granular volume fraction and decrease with injection rate. We show that the timescale over which the instability develops is given by the volume fraction dependent settling timescale of the grains in the cell. We then show that the mobility inversion and the nonequilibrium Korteweg surface tension due to granular volume fraction gradients determine the number of fingers at the onset of the instability in these miscible suspensions.

Reacting gas-surface interaction and heat transfer characteristics for high-enthalpy and hypersonic dissociated carbon dioxide flow

Hypersonic carbon dioxide reacting gas-surface interaction occurs on the heat shield of high-speed Mars entry capsules, which has great influences on the heat transfer characteristics. Based on hypersonic reacting flow solver with complex surface thermochemistry, the hypersonic chemical non-equilibrium flow with oxygen/carbon gas mixture was numerically simulated, and the interaction between non-equilibrium flow and surface reactions was numerically analyzed to reveal the mechanism of aerodynamic heating from surface thermochemistry. Numerical results of catalytic effects show that the near-wall thermochemical behaviors are essentially dominated by the near-wall diffusion and chemical reactions. Various surface reactions alter the flow structure in the boundary layer, and generate different aerodynamic heating patterns. Results from ablating simulations indicate that the ablating surface can induce additional near-wall diffusion and injection energy though their influence on the thermal boundary layer structure is neglectably small. The modeling of reacting gas-surface interaction can be of benefit to promoting the fidelity and precision of aerodynamic heating with complex surface thermochemistry.

Investigation of Spatial Effects Accompanying Local Boiling of Coolant in VVER Core on the Basis of Neutron Flux Noise Analysis

Situations with local boiling of the coolant become more likely when VVER-type power units are transferred to a heightened power level. In a VVER core, neutron flux detectors are not present in all FA. An important problem for evaluating the possibility of detecting local boiling in FA which are not equipped with detectors is the study of the spatial distribution of the change brought about in the neutron flux noise by local boiling of the coolant. The results of an investigation of the spatial distribution of the neutron flux noise initiated by fluctuations of the coolant parameters at the ingress into the core with non-equilibrium local surface boiling are reported. The sensitivity of the neutron flux noise in FA with no boiling to local boiling in neighboring FA is analyzed. The results could be helpful in analyzing the data recorded by the in-reactor noise diagnostics system in commercial operation in operating power-generating units.

Modeling of nonequilibrium surface growth by a limited-mobility model with distributed diffusion length.

Kinetic Monte Carlo (KMC) simulations are a well-established numerical tool to investigate the time-dependent surface morphology in molecular beam epitaxy experiments. In parallel, simplified approaches such as limited mobility (LM) models characterized by a fixed diffusion length have been studied. Here we investigate an extended LM model to gain deeper insight into the role of diffusional processes concerning the growth morphology. Our model is based on the stochastic transition rules of the Das Sarma-Tamborena model but differs from the latter via a variable diffusion length. A first guess for this length can be extracted from the saturation value of the mean-squared displacement calculated from short KMC simulations. Comparing the resulting surface morphologies in the sub- and multilayer growth regime to those obtained from KMC simulations, we find deviations which can be cured by adding fluctuations to the diffusion length. This mimics the stochastic nature of particle diffusion on a substrate, an aspect which is usually neglected in LM models. We propose to add fluctuations to the diffusion length by choosing this quantity for each adsorbed particle from a Gaussian distribution, where the variance of the distribution serves as a fitting parameter. We show that the diffusional fluctuations have a huge impact on cluster properties during submonolayer growth as well as on the surface profile in the high coverage regime. The analysis of the surface morphologies on one- and two-dimensional substrates during sub- and multilayer growth shows that the LM model can produce structures that are indistinguishable to the ones from KMC simulations at arbitrary growth conditions.

Unraveling the controversy over a catalytic reaction mechanism using a new theoretical methodology: One probe and non-equilibrium surface Green's function

We propose a new methodology called “one probe and non-equilibrium surface Green's function (OPNS)” to elucidate the longstanding controversial issue of a catalytic reaction mechanism, especially iodine reduction reaction (IRR) mechanisms. With aid of OPNS overcoming the limitations of conventional approach based on the free energy diagram and theoretical slab model, we clearly elucidate that the IRR follows a consecutive mechanism where the configurational preference of approaching I2 molecules depending on the external electric field governs the IRR mechanism. Under reductive potential, I2 molecules prefer a vertical configuration (I2V) rather than a parallel configuration (I2P), which leads to the consecutive mechanism where I atoms are sequentially reduced due to asymmetric charge accumulation on a single I atom that is subsequently desorbed. In addition, we provide new convincing descriptors for catalytic activity evaluation, the slope of the linear relation between the reductive process and the electric field strength representing the ability of the partial reduction and the threshold electric field of minimum required potential for the complete reduction.

Неустойчивость термодинамически неравновесной поверхности жидкого металла в электрическом поле

Неустойчивость термодинамически неравновесной поверхности жидкого металла в электрическом поле

Modeling of Relationships Between the Mechanoelectrochemical Parameters of the Metal Surface

From the viewpoint of nonequilibrium thermodynamics and surface physics, we propose a mathematical model for the determination of the electrochemical parameters on the boundary of a metal with an inert medium depending on the concentration of conductivity electrons and the surface energy with regard for the internal mechanical stresses caused by the redistribution of electrons. On the basis of the experimental values of surface tension and surface energy, we theoretically estimate the capacitance and the potential of double electric layer for the diagnostics of the quality of corrosion protection of steel.

Study on the 3D fluid-solid coupling model with multi-pressure system of shale

The study of fluid solid coupling process in fractured shale reservoir, with massive hydraulic and natural fracture networks, during production is a key scientific issue for accurately understanding the production performance of multi-stage fractured wells. In view of the complicated flow mechanism of shale gas flow, multi-scale media and multiple pressure systems of organic matter/matrix pore/natural fractures and discrete artificial fractures are established to accurately describe the gas flow in shale reservoir. The mathematic model considers mechanisms of non-equilibrium desorption and surface diffusion of the adsorbed gas, the viscous flow and Knudsen diffusion of the free gas, and the influence of matrix pore, natural fracture and artificial fracture pressure system on shale deformation. A fully coupled dual effective stress fluid-solid coupling is established, which considers different pressure system respectively. The influence of rock deformation on shale gas flow in the fractured well production is analyzed. It is found that the natural fracture pressure system is the main influence factor that causes stress sensitivity during the shale gas production. The impact of solid deformation on the permeability of matrix pores continuum is smaller than that of Knudsen diffusion. The apparent permeability of matrix pores continuum is still larger than its original permeability, thus there is no stress sensitivity of matrix pore permeability. With the increase of natural fracture density, the effective stress coefficient of natural fracture pressure system increases rapidly, and the stress sensitivity effect during shale gas production process is stronger.

Relaxation processes at liquid-gas interfaces in one- and two-component Lennard-Jones systems: Molecular dynamics simulation

The process of formation of an equilibrium liquid-gas interface, beginning with a sharp stepwise interfacial surface between phases with equal pressures, temperatures and chemical potentials, has been reproduced in molecular dynamics (MD) simulation of one- and two-component Lennard-Jones systems. In binary mixtures, the second component was a volatile impurity that is adsorbed in the interfacial layer. The tasks of this study are quantitative estimations of the establishment time of mechanical equilibrium between phases, the magnitude of the non-equilibrium surface tension and the time of its relaxation, the characteristic times of achieving equilibrium values of the composition, the shape and the thickness of the interface, as well as the effect of the volatile component on the relaxation processes in liquid-gas interfaces and nucleation in a superheated solution. Time dependences of the surface tension, the thickness of the interfacial layer and the relative adsorption of the volatile component have been obtained, and the times of their relaxation have been determined for a temperature close to the temperature of the solvent triple point, at the concentration range of the volatile component in the liquid phase cl from 0 to 0.25 mol fraction.It is shown that the maximum value of the non-equilibrium surface tension exceeds the equilibrium value 1.2 to 1.6 times. The relaxation time of the surface tension to equilibrium increases from 10 to 100 ps with an increase in cl. In a two-component system with a limited volume of the gas phase an equilibrium interfacial layer forms in two stages. At the first stage the volatile component is transferred into the interfacial layer from the near-surface regions of the gas phase and liquid phase. While achieving the equilibrium partial density of the volatile component in the gas phase, a transition to the second stage takes place, when particles are mainly transferred from the liquid phase, which leads to a considerable increase in the relaxation time of the relative adsorption and surface tension.Comparison of the data obtained with the results of an MD study of nucleation in solution shows that the time for establishment of adsorption equilibrium in a liquid-gas interfacial layer exceeds by an order of magnitude the time in which the equilibrium composition and the size of a vapor-gas bubble are established. This can lead to the fact that when the top of the activation barrier of nucleation is passed, the surface tension at the bubble-solution interface can exceed its equilibrium value.

Impact of anisotropic interactions on nonequilibrium cluster growth at surfaces

Using event-driven kinetic Monte-Carlo simulations we investigate the early stage of non-equilibrium surface growth in a generic model with anisotropic interactions among the adsorbed particles. Specifically, we consider a two-dimensional lattice model of spherical particles where the interaction anisotropy is characterized by a control parameter $\eta$ measuring the ratio of interaction energy along the two lattice directions. The simplicity of the model allows us to study systematically the effect and interplay between $\eta$, the nearest-neighbor interaction energy $E_{n}$, and the flux rate $F$, on the shapes and the fractal dimension $D_{f}$ of clusters before coalescence. At finite particle flux $F$ we observe the emergence of rod-like and needle-shaped clusters whose aspect ratio $R$ depends on $\eta$, $E_{n}$ and $F$. In the regime of strong interaction anisotropy, the cluster aspect ratio shows power-law scaling as function of particle flux, $R \sim F^{- \alpha}$. Furthermore, the evolution of the cluster length and width also exhibit power-law scaling with universal growth exponents for all considered values of $F$. We identify a critical cluster length $L_{c}$ that marks a transition from one-dimensional to self-similar two-dimensional cluster growth. Moreover, we find that the cluster properties depend markedly on the critical cluster size $i^{*}$ of the isotropically interacting reference system ($\eta = 1$).

Counterintuitive wetting route of droplet on patterned hydrophilic surface

The wetting transition of a droplet on a patterned hydrophilic surface can occur spontaneously and may further lead to superwetting that has the potential to develop novel technologies in the field of anti-fogging, printing and heat transfer. However, it is still unknown how the wetting transition occurs on such a patterned surface. In contrast to the conventional view that wetting occurs immediately in the vertical direction upon the contact of the droplet with the solid surface due to the capillary force, we find that the droplet spreads first in the horizontal direction if the patterned surface has a large enough roughness. Then, the wetting transition occurs at the periphery rather than in the middle part of droplet, which is termed as “one-dimensional wetting”. We ascribe such an interesting phenomenon to the competition between the horizontal force arising from the non-equilibrium surface tension and the vertical capillary force as well as to the different pressure under the droplet, which lead to three different wetting routes. Thus, we hope that this new point of view can be helpful to the understanding of the wetting transition of the droplet on the patterned hydrophilic surface.

Study on the nonlinear coupling seepage model of shale reservoir with discrete networks

Gas shale is rich in natural fractures, in which the percolation mechanisms are complicated. Consequently, it’s the key scientific problem of accurately evaluating shale gas production to figure out the law of flow in fractured shale reservoirs. In this paper, a multi-continuum/discrete fracture model was built to describe the shale reservoir space after hydraulic fracturing, and a nonlinear coupled seepage flow mathematical model is established based on the adsorbed gas and free gas coupling, including the nonlinear non-equilibrium desorption and surface diffusion of adsorbed gas, viscous flow and Knudsen diffusion mechanism of free gas. The model believes that the surface diffusion of adsorbed gas occurs on the surface of the organic matter, and non-equilibrium desorption occurs between the adsorbed gas and the pores of the inorganic matrix. There are viscous flow and Knudsen diffusion in the pores of the inorganic matrix. But there is only viscous flow in natural and hydraulic fractures. Through the numerical simulation results of a fractured horizontal well, the time-space evolution process of the net desorption rate of the adsorbed gas in the hydraulic fractured shale reservoir was revealed. It was found that the region of the maximum net desorption rate gradually spread from the artificial network to the matrix. Through the analysis of multiple pressure systems, it was found that the diffusion of free gas in natural fractures, free gas in inorganic matrix pores and adsorbed gas in organic matter lagged successively. Through the flow analysis of natural fracture field, it is found that the flow patterns, outside the envelope of discrete fracture network, appear in succession with time: elliptical radial flow, linear flow and non-linear flow caused by primary fracture interference. This study reveals the percolation mechanism of fractured shale reservoirs and provides a scientific basis for the exploitation of the shale gas reservoir.

Modelling the physical multiphase interactions of HNO<sub>3</sub> between snow and air on the Antarctic Plateau (Dome C) and coast (Halley)

Abstract. Emissions of nitrogen oxide (NOx = NO + NO2) from the photolysis of nitrate (NO3−) in snow affect the oxidising capacity of the lower troposphere especially in remote regions of high latitudes with little pollution. Current air–snow exchange models are limited by poor understanding of processes and often require unphysical tuning parameters. Here, two multiphase models were developed from physically based parameterisations to describe the interaction of nitrate between the surface layer of the snowpack and the overlying atmosphere. The first model is similar to previous approaches and assumes that below a threshold temperature, To, the air–snow grain interface is pure ice and above To a disordered interface (DI) emerges covering the entire grain surface. The second model assumes that air–ice interactions dominate over all temperatures below melting of ice and that any liquid present above the eutectic temperature is concentrated in micropockets. The models are used to predict the nitrate in surface snow constrained by year-round observations of mixing ratios of nitric acid in air at a cold site on the Antarctic Plateau (Dome C; 75°06′ S, 123°33′ E; 3233 m a.s.l.) and at a relatively warm site on the Antarctic coast (Halley; 75°35′ S, 26°39′ E; 35 m a.s.l). The first model agrees reasonably well with observations at Dome C (Cv(RMSE) = 1.34) but performs poorly at Halley (Cv(RMSE) = 89.28) while the second model reproduces with good agreement observations at both sites (Cv(RMSE) = 0.84 at both sites). It is therefore suggested that in winter air–snow interactions of nitrate are determined by non-equilibrium surface adsorption and co-condensation on ice coupled with solid-state diffusion inside the grain, similar to Bock et al. (2016). In summer, however, the air–snow exchange of nitrate is mainly driven by solvation into liquid micropockets following Henry's law with contributions to total surface snow NO3− concentrations of 75 and 80 % at Dome C and Halley, respectively. It is also found that the liquid volume of the snow grain and air–micropocket partitioning of HNO3 are sensitive to both the total solute concentration of mineral ions within the snow and pH of the snow. The second model provides an alternative method to predict nitrate concentration in the surface snow layer which is applicable over the entire range of environmental conditions typical for Antarctica and forms a basis for a future full 1-D snowpack model as well as parameterisations in regional or global atmospheric chemistry models.

Particle-Size-Dependent Triboelectric Charging in Single-Component Granular Materials: Role of Humidity

We carry out experiments to investigate the triboelectric charging due only to particle–particle collisions as a function of humidity. At low humidity, we find that large particles tend to charge positive and small particles tend to charge negative, in agreement with previous studies. However, at high humidity, we find no significant particle-size dependence for the particle charging. To explain these results, we apply a theoretical model based on charge carriers trapped in nonequilibrium surface states characteristic of electrically insulating materials. Monte Carlo simulations show that collisions between particles enable the charge carriers to reach lower energy states on other particles. These nonequilibrium dynamics lead to an accumulation of charge carriers on small particles, and if the charge carriers are negative (electrons or negative ions), the small particles would tend to charge negatively. We propose that humidity leads to conductive layers on the surface of particles that act as a sink for ...

Optimal detrended fluctuation analysis as a tool for the determination of the roughness exponent of the mounded surfaces

We present an optimal detrended fluctuation analysis (DFA) and apply it to evaluate the local roughness exponent in nonequilibrium surface growth models with mounded morphology. Our method consists in analyzing the height fluctuations computing the shortest distance of each point of the profile to a detrending curve that fits the surface within the investigated interval. We compare the optimal DFA (ODFA) with both the standard DFA and nondetrended analysis. We validate the ODFA method considering a one-dimensional model in the Kardar-Parisi-Zhang universality class starting from a mounded initial condition. We applied the methods to the Clarke-Vvedensky (CV) model in 2+1 dimensions with thermally activated surface diffusion and absence of step barriers. It is expected that this model belongs to the nonlinear molecular beam epitaxy (nMBE) universality class. However, an explicit observation of the roughness exponent in agreement with the nMBE class was still missing. The effective roughness exponent obtained with ODFA agrees with the value expected for the nMBE class, whereas using the other methods it does not agree. We also characterize the transient anomalous scaling of the CV model and obtained that the corresponding exponent is in agreement with the value reported for other nMBE models with weaker corrections to the scaling.

Optically excited structural transition in atomic wires on surfaces at the quantum limit.

Transient control over the atomic potential-energy landscapes of solids could lead to new states of matter and to quantum control of nuclear motion on the timescale of lattice vibrations. Recently developed ultrafast time-resolved diffraction techniques combine ultrafast temporal manipulation with atomic-scale spatial resolution and femtosecond temporal resolution. These advances have enabled investigations of photo-induced structural changes in bulk solids that often occur on timescales as short as a few hundred femtoseconds. In contrast, experiments at surfaces and on single atomic layers such as graphene report timescales of structural changes that are orders of magnitude longer. This raises the question of whether the structural response of low-dimensional materials to femtosecond laser excitation is, in general, limited. Here we show that a photo-induced transition from the low- to high-symmetry state of a charge density wave in atomic indium (In) wires supported by a silicon (Si) surface takes place within 350 femtoseconds. The optical excitation breaks and creates In-In bonds, leading to the non-thermal excitation of soft phonon modes, and drives the structural transition in the limit of critically damped nuclear motion through coupling of these soft phonon modes to a manifold of surface and interface phonons that arise from the symmetry breaking at the silicon surface. This finding demonstrates that carefully tuned electronic excitations can create non-equilibrium potential energy surfaces that drive structural dynamics at interfaces in the quantum limit (that is, in a regime in which the nuclear motion is directed and deterministic). This technique could potentially be used to tune the dynamic response of a solid to optical excitation, and has widespread potential application, for example in ultrafast detectors.

Subpicosecond spin dynamics of excited states in the topological insulator Bi2Te3

Using time-, spin- and angle-resolved photoemission, we investigate the ultrafast spin dynamics of hot electrons on the surface of the topological insulator Bi$_2$Te$_3$ following optical excitation by fs-infrared pulses. We observe two surface-resonance states above the Fermi level coexisting with a transient population of Dirac fermions that relax in about $\sim$2 ps. One state is located below $\sim$0.4 eV just above the bulk continuum, the other one at $\sim$0.8 eV inside a projected bulk band gap. At the onset of the excitation, both states exhibit a reversed spin texture with respect to that of the transient Dirac bands, in agreement with our one-step photoemission calculations. Our data reveal that the high-energy state undergoes spin relaxation within $\sim$0.5 ps, a process that triggers the subsequent spin dynamics of both the Dirac cone and the low-energy state, which behave as two dynamically-locked electron populations. We discuss the origin of this behavior by comparing the relaxation times observed for electrons with opposite spins to the ones obtained from a microscopic Boltzmann model of ultrafast band cooling introduced into the photoemission calculations. Our results demonstrate that the nonequilibrium surface dynamics is governed by electron-electron rather than electron-phonon scattering, with a characteristic time scale unambiguously determined by the complex spin texture of excited states above the Fermi level. Our findings reveal the critical importance of detecting momentum and energy-resolved spin textures with fs resolution to fully understand the sub-ps dynamics of transient electrons on the surface of topological insulators.

Single-crystalline ultrathin 2D TiO2 nanosheets: A bridge towards superior photovoltaic devices

Ultrathin two-dimensional (2D) TiO2 nanostructures with atomic-level thickness are urgently demanded to achieve high-performance photocatalytic, photovoltaic, and electrochemical devices, by taking advantage of their unique characteristics of large surface-to-volume ratio, distinctive electronic properties, and intriguing chemical reactivity. Here, we present a molecular self-assembly of single-crystalline ultrathin 2D TiO2 nanosheets from a micelle-assisted solution. Via this synthesis strategy, highly-homogeneous ultrathin 2D TiO2 nanosheets with a thickness around 3 nm were obtained with relatively large production. The ultrathin 2D TiO2 nanosheet composite has a dominant anatase phase and a trace of rutile phase. It is interesting that highly reactive, non-equilibrium surfaces, such as the anatase {010} and rutile {001} facets, were observed in the fabricated nanosheets, which are very difficult to obtain by conventional synthesis methods but extremely useful for improving the performance of TiO2 nanostructures. Owing to the unique features of single-crystallinity, atomic-level thickness, highly reactive exposed surfaces, etc., the ultrathin 2D TiO2 nanosheets possess superior photochemical properties. With photoanodes made from ultrathin 2D TiO2 nanosheets, dye-sensitized solar cells showed significantly enhanced efficiency (8.28%) compared to ones with commercial Degussa P25 TiO2 nanoparticle anode (5.12%). This new family of graphene-like TiO2 nanosheets thus paves the way to further improving the performance of TiO2-based energy conversion and storage devices.

Nonequilibrium surface growth in a hybrid inorganic-organic system

Using kinetic Monte Carlo simulations, we show that molecular morphologies found in non-equilibrium growth can be strongly different from those at equilibrium. We study the prototypical hybrid inorganic-organic system 6P on ZnO(10-10) during thin film adsorption, and find a wealth of phenomena including re-entrant growth, a critical adsorption rate and observables that are non-monotonous with the adsorption rate. We identify the transition from lying to standing molecules with a critical cluster size and discuss the competition of time scales during growth in terms of a rate equation approach. Our results form a basis for understanding and predicting collective orientational ordering during growth in hybrid material systems.

Dynamic Scaling and Island Growth Kinetics in Pulsed Laser Deposition of SrTiO_{3}.

We use real-time diffuse surface x-ray diffraction to probe the evolution of island size distributions and its effects on surface smoothing in pulsed laser deposition (PLD) of SrTiO_{3}. We show that the island size evolution obeys dynamic scaling and two distinct regimes of island growth kinetics. Our data show that PLD film growth can persist without roughening despite thermally driven Ostwald ripening, the main mechanism for surface smoothing, being shut down. The absence of roughening is concomitant with decreasing island density, contradicting the prevailing view that increasing island density is the key to surface smoothing in PLD. We also report a previously unobserved crossover from diffusion-limited to attachment-limited island growth that reveals the influence of nonequilibrium atomic level surface transport processes on the growth modes in PLD. We show by direct measurements that attachment-limited island growth is the dominant process in PLD that creates step flowlike behavior or quasistep flow as PLD "self-organizes" local step flow on a length scale consistent with the substrate temperature and PLD parameters.