Inhomogeneous Density Distribution Research Articles

Inhomogeneous Effects of Number Density on Polarization Relaxation of a Polar Solvent around an Ion

We studied the effects of the inhomogeneous density distribution of polar solvent molecules around an ion on the polarization relaxation by using linearized time-dependent density functional theory...

Analysis of Echo Characteristics of Spatially Inhomogeneous and Time-Varying Plasma Sheath

The “black barrier” phenomenon caused by plasma sheath during the reentry of a high-speed target will seriously affect the detection of radar echo by a ground monitoring station, which is not conducive to making corresponding adjustments to different situations in real-time. The echo reflects the information of the target’s position and speed, which is significant for real-time monitoring and effective strike. In this work, the effect of a high-speed target plasma sheath on electromagnetic wave echo characteristics was investigated by using the proposed spatially inhomogeneous and time-varying plasma sheath model. The Z-transform finite-difference time domain (Z-FDTD) method was adopted to calculate the time domain and frequency domain results of the echo in plasma sheaths with spatially inhomogeneous, uniform and spatially inhomogeneous and time-varying electron density distributions. The effects of different time-varying parameters and plasma parameters on the echo characteristics of the plasma sheath were analyzed. The results show that the reflected echo in spatially inhomogeneous and time-varying plasma is less than that of the other two. The magnitude of the reflected echo is proportional to the time-varying parameter peak coefficient and Gaussian width, and inversely proportional to the collision frequency and plasma thickness. The theoretical results provide important guidance for ground monitoring and anti-stealth.

Protein gel shrinkage during solvent exchange: Quantification of gel compaction, mass transfer and compressive strength

In this work, we experimentally investigate into one step solvent exchange of cylindrical shaped whey protein isolate (WPI) gels, where the pore fluid water inside the pores is replaced by ethanol. Solvent exchange has three major impacts on macroscopic gel properties, leading to an increase in compressive strength, shrinkage of the gel cylinders diameter and consequently a change in the spatial protein distribution. 1D-Raman spectroscopy was employed for in-situ investigation of pore fluid composition and protein density distribution, together with a conventional camera to track the diameter change of the gel during solvent exchange. Compressive strength of the gel cylinders is measured ex-situ using a universal testing machine. For gels with three different protein concentrations, we report comparable shrinkage of the diameter to about 85% of the initial size, with most shrinkage taking place during the first stages of solvent exchange, when ethanol is detected in the outermost layers only. Further shrinkage is prevented by a sharp increase in mechanical stiffness of the gels. Solvent exchange leads to an inhomogeneous protein density distribution for most gels, with a compaction towards the radial center. We showed a complex interaction of solvent exchange kinetics and pore fluid dependent stiffness of the gel matrix controls the final size and density distribution of the gel.

Spatially Inhomogeneous Distribution of the Quantum Mechanical Current Density at the Sub-Barrier Reflection of the Electron Wave from the Potential Step in the 2D Nanostructure

In this paper, the influence of the interference of electron waves during their reflection from a rectangular semi-infinite potential barrier to the spatial distribution of quantum mechanical current density еjx(x, z) (e is the electron charge and jx(x, z) is the flow of probability density) in a semiconductor 2D nanostructure is theoretically studied. The structure consists of narrow and wide rectangular quantum wells (QWs) arranged successively along the direction of electron wave propagation. It is assumed that the wave falls from narrow QW1 on a potential barrier of height V0 in wide QW2. It is shown that when a wave with energy less than V0 falls on the barrier, then, in certain conditions, spatially inhomogeneous distribution $$ej_{x}^{{\left( 1 \right)}}(x,z)$$ oscillating in a complicated way exists in QW1. An exponential attenuation and coordinate dependent leakage $$ej_{x}^{{\left( 2 \right)}}(x,z)$$ under the barrier may occur in QW2. It is shown that this behavior of $$ej_{x}^{{\left( 1 \right)}}(x,z)$$ and $$ej_{x}^{{\left( 2 \right)}}(x,z)$$ is caused by the interference of electron waves propagating through different quantum-dimensional subbands in the considered nanostructure.

Hall effect and symmetry breaking in the nonmagnetic metal LuB12 with dynamic charge stripes

A comprehensive study of magnetoresistance and Hall effect has been performed for the set of the single crystals of non-magnetic metal LuB12 with the Jahn-Teller instability of the boron cage and dynamic charge stripes forming along <110> direction. Anomalous positive contribution to Hall effect for particular direction of magnetic field H//[001] is found in the single crystals of LuB12 of the highest quality. This contribution arising at T~ 150 K is shown to increase drastically when approaching the disordered ground state below 60 K. The Hall effect anomaly is shown to appear in combination with the peak of magnetoresistance. The various scenarios allowing for the topology of Fermi surface, anisotropy of relaxation time for charge carriers and the filamentary structure of fluctuating charge stripes are proposed to explain the features of magnetotransport in this metal with inhomogeneous distribution of electron density. The origin of SdH oscillations, which are observed in this non-equilibrium metal with electron phase separation and strong charge carrier scattering, is discussed.

Density Functional Theory Studies of the Electronic Structure and Muon Hyperfine Interaction in [Au25(SR)18]0 and [Au25(SeR)18]0 Nanoclusters.

Density functional theory computational investigation was performed to study the electronic structures, muon sites, and the associated hyperfine interactions in [Au25(SR)18]0 and [Au25(SeR)18]0 where R is phenylethane. The calculated electronic structures show inhomogeneous spin density distribution and are also affected by different ligands. The two most stable muon sites near Au atoms in the thiolated system are MAu11 and MAu6. When the thiolate ligands were replaced by selenolate ligands, the lowest energy positions of muons moved to MAu6 and MAu5. Muons prefer to stop inside the Au12 icosahedral shell, away from the central Au and the staple motifs region. Muonium states at phenyl ring and S/Se atoms in the ligand were found to be stable and the Fermi contact fields are much larger as compared to the field experienced by muons near Au atoms.

Hierarchical thermal transport in nanoconfined water.

The structure of nanoconfined fluids is particularly non-uniform owing to the wall interaction, resulting in the distinctive characteristic of thermal transport compared to bulk fluids. We present the molecular simulations on the thermal transport of water confined in nanochannels with a major investigation of its spatial distribution under the effects of wall interaction. The results show that the thermal conductivity of nanoconfined water is inhomogeneous and its layered distribution is very similar to the density profile. The layered thermal conductivity is the coupling result of inhomogeneous density and energy distributions that are generally diametrical, and their contributions to the thermal conductivity compensate with each other. However, the accumulative effect of water molecules is really dominating, resulting in a high thermal conductivity in the high-density layers with the low-energy molecules, and vice versa. Moreover, it is found that the adsorptive and repulsive interactions from solid walls have different roles in the hierarchical thermal transport in nanoconfined water. The adsorptive interaction is only responsible for the layered distribution of thermal conductivity, while the repulsive interaction is responsible for the overall thermal conductivity; accordingly, the thermal conductivity is independent of the strength of water-solid interactions. The identified hierarchical thermal transport in nanoconfined water and its underlying mechanisms have a great significance for the understanding of nanoscale thermal transport and even the mass and energy transport of nanoconfined fluids.

Electrical Access to Ising Anyons in Kitaev Spin Liquids.

We show that spin-spin correlations in a non-Abelian Kitaev spin liquid are associated with a characteristic inhomogeneous charge density distribution in the vicinity of Z_{2} vortices. This density profile and the corresponding local electric fields are observable, e.g., by means of surface probe techniques. Conversely, by applying bias voltages to several probe tips, one can stabilize Ising anyons (Z_{2} vortices harboring a Majorana zero mode) at designated positions, where we predict a clear Majorana signature in energy absorption spectroscopy.

Element Stratification in the Middle-aged SN Ia Remnant G344.7–0.1

Abstract Despite their importance, a detailed understanding of Type Ia supernovae (SNe Ia) remains elusive. X-ray measurements of the element distributions in supernova remnants (SNRs) offer important clues for understanding the explosion and nucleosynthesis mechanisms for SNe Ia. However, it is challenging to observe the entire ejecta mass in X-rays for young SNRs, because the central ejecta may not have been heated by the reverse shock yet. Here we present over 200 kilosecond Chandra observations of the Type Ia SNR G344.7–0.1, whose age is old enough for the reverse shock to have reached the SNR center, providing an opportunity to investigate the distribution of the entire ejecta mass. We reveal a clear stratification of heavy elements with a centrally peaked distribution of the Fe ejecta surrounded by intermediate-mass elements (IMEs: Si, S, Ar Ca) with an arc-like structure. The centroid energy of the Fe K emission is marginally lower in the central Fe-rich region than in the outer IME-rich regions, suggesting that the Fe ejecta were shock-heated more recently. These results are consistent with the prediction for standard SN Ia models, where the heavier elements are synthesized in the interior of an exploding white dwarf. We find, however, that the peak location of the Fe K emission is slightly offset to the west with respect to the geometric center of the SNR. This apparent asymmetry is likely due to the inhomogeneous density distribution of the ambient medium, consistent with our radio observations of the ambient molecular and neutral gas.

Direct electron acceleration by chirped laser pulse in a cylindrical plasma channel**Project supported by the National Natural Science Foundation of China (Grant Nos. 11865014, 11765017, 11764039, 11475027, 11274255, and 11305132),the Natural Science Foundation of Gansu Province of China (Grant No. 17JR5RA076), the Scientific Research Project of Gansu Higher Education of China (Grant No. 2016A-005), the Natural Science Foundation of Education

We study the dynamics of single electron in an inhomogeneous cylindrical plasma channel during the direct acceleration by linearly polarized chirped laser pulse. By adjusting the parameters of the chirped laser pulse and the plasma channel, we obtain the energy gain, trajectory, dephasing rate and unstable threshold of electron oscillation in the channel. The influences of the chirped factor and inhomogeneous plasma density distribution on the electron dynamics are discussed in depth. We find that the nonlinearly chirped laser pulse and the inhomogeneous plasma channel have strong coupled influence on the electron dynamics. The electron energy gain can be enhanced, the instability threshold of the electron oscillation can be lowered, and the acceleration length can be shortened by chirped laser, while the inhomogeneity of the plasma channel can reduce the amplitude of the chirped laser.

Coherence properties of high-frequency wave field propagating through inhomogeneous ionosphere with anisotropic random irregularities of electron density: 1. Theoretical background

Rigorous analytic solution is constructed for the first of 2 s-order space position and frequency coherence function (which is alternatively also termed as symmetric) of the high-frequency wave field propagating through the stochastic transionospheric channel with the inhomogeneous layer of the background ionosphere and anisotropic fluctuations of the electron density in the approximation of the appropriate Markov equation. It is obtained employing the anisotropic parabolic model of the effective structure function of the fractional electron density fluctuations with two different scales along and across the Earth's magnetic field. The explicit analytic solution is presented, which is capable of describing the coherence properties of the field for any given inhomogeneous distribution of the electron density in the background ionosphere along a path of propagation. The procedure of calibrating the spatial scales of the anisotropic parabolic model of the electron density fluctuations along and across the Earth's magnetic field lines in order to provide the adequate solution to correspond to the realistic anisotropic inverse power law spectrum of the electron density fluctuations is presented in the companion paper.

The Application of the Filtered Backprojection Algorithm to Solar Rotational Tomography

Abstract Solar rotational tomography (SRT) is an important method to reconstruct the physical parameters of the three-dimensional solar corona. Here we propose an approach to apply the filtered backprojection (FBP) algorithm to the SRT. The FBP algorithm is generally not suitable for SRT due to the several issues with solar extreme ultraviolet (EUV) observations—in particular, a problem caused by missing data because of the unobserved back side of corona hidden behind the Sun. We developed a method to generate a modified sinogram that resolves the blocking problem. The modified sinogram is generated by combining the EUV data at two opposite sites observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory (SDO). We generated the modified sinogram for about one month in 2019 February and reconstructed the three-dimensional corona under the static state assumption. In order to obtain the physical parameters of the corona, we employed a differential emission measure inversion method. We tested the performance of the FBP algorithm with the modified sinogram by comparing the reconstructed data with the observed EUV image, electron density models, previous studies of electron temperature, and an observed coronagraph image. The results illustrate that the FBP algorithm reasonably reconstructs the bright regions and the coronal holes and can reproduce their physical parameters. The main advantage of the FBP algorithm is that it is easy to understand and computationally efficient. Thus, it enables us to easily probe the inhomogeneous coronal electron density and temperature distribution of the solar corona.

Film formation process of natural rubber latex particles: roles of the particle size and distribution of non-rubber species on film microstructure

Natural rubber (NR) films from various mixing ratios of large- (LRP) and small rubber particles (SRP) were prepared through latex and solution casting methods. Film-forming behaviours of the resulting films were investigated by monitoring their surface characteristics using atomic force microscopy (AFM). When the SRP portion was higher, the surface of the latex-cast films was found to be smoother as smaller particles protruded out of the surface less than larger ones did. AFM phase micrographs revealed a hexagonal shape of packed rubber particles (RPs) of the LRP film, while the RPs in the other samples were sphere-like. After aging under ambient conditions, the packed RPs were flattened while retaining their RP boundaries, creating a supporting framework within the rubber matrix. On the contrary, when the rubber film was cast from a toluene solution, the characteristic of RP boundaries disappeared and became aggregates of the membrane components on the film surface. The latex-cast films performed much better in mechanical strength than the solution-cast films did due to the destruction of the supporting framework arisen from the non-rubber components at the RP boundaries of the solution-cast film. An inhomogeneous density distribution produced by the non-rubber aggregates in the latex-cast films was observed in small angle X-ray scattering measurements.

Phase coexistence of active Brownian particles.

We investigate motility-induced phase separation of active Brownian particles, which are modeled as purely repulsive spheres that move due to a constant swim force with freely diffusing orientation. We develop on the basis of power functional concepts an analytical theory for nonequilibrium phase coexistence and interfacial structure. Theoretical predictions are validated against Brownian dynamics computer simulations. We show that the internal one-body force field has four nonequilibrium contributions: (i) isotropic drag and (ii) interfacial drag forces against the forward motion, (iii) a superadiabatic spherical pressure gradient, and (iv) the quiet life gradient force. The intrinsic spherical pressure is balanced by the swim pressure, which arises from the polarization of the free interface. The quiet life force opposes the adiabatic force, which is due to the inhomogeneous density distribution. The balance of quiet life and adiabatic forces determines bulk coexistence via equality of two bulk state functions, which are independent of interfacial contributions. The internal force fields are kinematic functionals which depend on density and current but are independent of external and swim forces, consistent with power functional theory. The phase transition originates from nonequilibrium repulsion, with the agile gas being more repulsive than the quiet liquid.

Absorption of ultrashort electromagnetic pulses in plasma with inhomogeneous distribution of density and temperature

AbstractThe analysis of influence of the inhomogeneous distribution of temperature and density of atoms on the probability of absorption of ultrashort electromagnetic pulses in plasma is carried out. A specific example of divertor plasma of tokamaks and absorption in the Lyman series (at the Ly‐α line) is considered. It is shown that the absorption probability may exceed relative populations of excited levels in divertor plasma by several orders of magnitude. The influence of the pulse duration on the probability of excitation of an atomic level is considered. The results suggest a possibility of a sharp increase in a fluorescence signal under the action of an ultrashort pulse.

Hydrocarbon-Phase Behaviors in Shale Nanopore/Fracture Model: Multiscale, Multicomponent, and Multiphase

Summary Hydrocarbon recovery from shale subformations has greatly contributed to the global energy supply and has been constantly reshaping the energy sector. Oil production from shale is a complex process in which multicomponent–fluid mixtures experience multiphase transitions in multiscale volumes (i.e., nanoscale pores are connected to fractures/macropores). Understanding such complicated phenomena plays a critical role in the estimation of ultimate oil recovery, well productivity, and reserves estimation, and ultimately in policy making. In this work, we use density–functional theory (DFT) to explicitly consider fluid/surface interactions, inhomogeneous–density distributions in nanopores, volume partitioning in nanopores, and connected macropores/natural fractures to study the complex multiphase transitions of multicomponent fluids in multiscale volumes. We found that vapor–like and liquid–like phases can coexist in nanopores when pressure is between the bubblepoint and dewpoint pressures of nanoconfined fluids, both of which are much lower than those of the originally injected hydrocarbon mixtures. As the volume ratio of the bulk at the initial condition to pores decreases, both the bubblepoint and the dewpoint in nanopores increase and the pore two–phase region expands. Within the pore two–phase region, both C1 and C3 are released from the nanopores to the bulk phase as pressure declines. Meanwhile, both liquid and vapor phases become denser as pressure drops. By further decreasing pressure below the dewpoint of confined fluids, C3 in the nanopore can be recovered. Throughout the process, the bulk–phase composition varies, which is in line with the field observation. Collectively, this work captures the coupled complexity of multicomponent and multiphase fluids in multiscale geometries that is inherent to shale reservoirs and provides a theoretical foundation for reservoir simulation, which is significant for the accurate prediction of well productivity and ultimate oil recovery in shale reservoirs.

A Hybrid Modeling Approach for Current Diffusion in Rectangular Conductors

This paper deals with modeling and simulation of devices where current-carrying conductors are located close to magnetic bodies. These configurations typically occur in electromechanical energy converters such as in slots of electrical machines. Additional losses due to the inhomogeneous current density distribution (“current” or “field” diffusion) at high excitation frequency are a well-known phenomenon. However, classical approaches reach their limits as soon as field diffusion is influenced by ferromagnetic material with non-linear properties. An accurate but time-consuming computation can be achieved by using time-transient quasi-stationary finite-element analysis (FEA). Current diffusion can be described more efficiently by separating the conductor into parallel filaments, following the partial element equivalent circuit (PEEC) approach. As long as linear material characteristics are prevailing, the problem can, thus, be solved efficiently in the frequency domain. In this paper, a hybrid FEA-PEEC modeling approach is presented, where the magnetization of the ferromagnetic material is represented by equivalent currents obtained by 2-D FEA. It is suitable for complex multi-conductor configurations even including non-linear ferromagnetic material. For the proposed method, the resulting linear system is efficiently solvable in the frequency domain by direct solvers. A numerical example from the field of electric traction motors is presented to emphasize the accuracy and benefit of the proposed method for practical electromagnetic computations.

Operando and Ex-Situ Investigation of PEMFC Degradation

One of the key requirements for fuel cell electric vehicles (FCEVs) is longevity with a targeted lifetime of the fuel cell of 5000 - 6000 h. This lifetime needs to be assured in dynamic PEMFC operation including critical events. To mitigate performance degradation a fundamental understanding of the degradation phenomena that occur during different operation conditions are required. Therefore operando electrochemical investigations including monitoring of local operation conditions have been carried out in a broad scope of operation conditions to i) study impact of operation conditions on degradation and ii) identify phenomena that accelerate performance decay (so called accelerated stress tests - AST). Our results show that dynamic load cycling leads to lower performance losses compared to operation at constant current [1]. Moreover, operation in the low current range (i.e. high potential range) exhibited highest irreversible and reversible degradation which is caused by a significantly increased ohmic and mass transport resistance. The observed performance losses are closely linked with inhomogeneous current density distributions. Ex-situ measurements show that MEA regions with particularly high local current densities exhibit more pronounced platinum band formation in the membrane, as shown in the Figure, which is correlated with irreversible cathode degradation. In order to understand the impact of specific operation conditions occurring during load cycling as required for automotive operation specific in-situ ASTs were performed focusing on high potential operation, mechanical stress, temperature cycling and high load operation. Although in-situ ASTs exist for the membrane and the catalyst layer, no in-situ ASTs are available for the GDL so far. Hence our current activity is focused on the development of GDL specific ASTs and quantification of their impact on degradation of individual MEA components. To quantify impact of these ASTs on degradation of individual MEA components aged components were analyzed not only by ex-situ methods, but also by electrochemical characterization in combination of with pristine MEA components. The results clearly show that, for instance, mechanical stress due to freezing water during freeze-thaw cycling has negative effect on mass transport capabilities of the MEA. In this context also mitigation strategies to avoid icing in the cell is proposed which mitigates performance decay [2]. In addition of studying degradation effects caused by operation conditions, we also provide an analysis of effects of different externally introduced contaminants. These include silicon (e.g. from gasket) and nickel (e.g. from defective coatings) which effects on local MEA degradation was studied with DLR’ segmented cell in different operation conditions [3]. Apart from understanding degradation on material level, our activities cover the development of methods for on-line fault diagnostics and state-of-health monitoring in order to identify critical operation conditions characterized by gradients in various parameters and uneven flow distribution between the single cells [4]. The presented work is focused on fault monitoring in PEMFC stacks by evaluation of local performance phenomena and impedance analysis. Combined studies of local current density measurements and impedance data for all single cells in the stack will be presented and links between impedance response, local effects within the cells, and stack fault modes will be highlighted. Acknowledgements: This project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 779565 (ID-Fast). This Joint Undertaking receives support from the European Union’s Horizon 2020 research and innovation programme.

Temperature-dependent XAFS study of the local lattice distortion of the CuO2 plane in Sr2CuO3+δ powder sample

In order to study the relationship between local lattice distortion and superconductivity, we performed temperature dependent x-ray absorption fine structure (XAFS) on Sr2CuO3+δ powder sample (single phase with Tc=48 K). The temperature dependent Debye-Waller factor of Cu-O pair appears an anomalous upturn at 90 K and becomes drop around the superconducting transition temperature of ∼50 K. It means that local lattice distortion appears at 90 K. Furthermore, the analysis of temperature dependent X ray Absorption Near Edge Structure (XANES) shows that the R factor, which denotes the inhomogeneous charge density distribution, shows a sharp drop at 90 K. According to previous reports on other doped 214 cuprates, the above experimental results reveal that the local lattice distortion at Cu site in CuO2 plane occurs with the inhomogeneous charge density distribution at 90 K. It indicates that the inhomogeneous charge density distribution may be caused by the local lattice distortion at Cu site before superconductive order formation, implying that electron-lattice interaction play a key role in superconductivity of Sr2CuO3+δ.

Physical transport processes affect the origins of harmful algal blooms in estuaries

The effects of physical transport processes on the initiation of harmful algal blooms (HABs) in estuaries were investigated through both mathematical model analysis and numerical model experiments. This study highlights the influence of the flushing effect due to physical transport processes on the location of bloom initiation, which is comparable to or even more important than local processes. The theoretical analysis suggests that the differential flushing effect at different waterbodies due to complex geometry is one of the dominant factors causing inhomogeneous distribution of algal density during HAB initiation. The ratio of residence time to volume is one of the key variables that determine the differential timing of HAB occurrence in estuary-subestuary systems with multiple interconnected waterbodies. As a result, a HAB tends to be observed first in those locations with relatively long residence time and small waterbodies, such as tributaries or areas with large eddies. Multiple unconnected originating locations can co-exist within an estuary.Two three-dimensional model experiments with realistic forcings were conducted to demonstrate the flushing effect on annual Cochlodinium polykrikoides bloom in the lower James River. The results show that while the environmental conditions that affect local processes, such as salinity and temperature, are important in determining the originating locations of HABs, the differential flushing effect is the dominant factor driving the spatial difference in the density of C. polykrikoides in this region during the bloom initiation. This explains why the occurrence of the first bloom in this region is frequently observed in the Lafayette River, a relatively small waterbody with long residence time. Because of the relatively low growth rate of C. polykrikoides and because of the high water-exchange between the mainstem and tributaries of the James River, initial cyst distribution is suggested to have a relatively small impact on originating locations of the bloom compared to flushing effect and salinity, and the HAB originating locations do not have to be in the waterbody with abundant cysts.