Branches Of Modes Research Articles

Droplet motions directed by an expansion section in the T-junctions

The controlled motion of droplets in microfluidic chips is a preliminary requirement to realize their functions. The influence of the expansion section on the droplet motion is mainly investigated in the T-junction. The droplet dynamic characteristics are analyzed at the junction and the applicable flow rate of the expansion section is explored. The expansion section can reduce the entered length and motion time of the droplet when droplets flow into the channel with it, and finally avoid the possibility of droplet splitting. Even under a large difference of the branch flow rate, the expansion section can direct the droplet into its located channel. It is found that with the increase in continuous phase flow rate, the effect of the expansion section on the droplet motion behavior is gradually weakened until it disappears. Moreover, the critical conditions of it can be obtained by theoretical calculation. The expansion section can direct droplet motion in both symmetric and asymmetric junctions. However, it is mainly achieved by influencing the interfacial tension of the droplets in the symmetric junction, while the key force is related to the droplet motion in the asymmetric junction. Specifically, the expansion section influences the differential pressure force to direct the droplet in the flow into the side branch (with expansion section) mode, but it varies the interfacial tension of the droplet in the flow into the main branch mode.

Hydrodynamics of Borromean Counterfluids.

Counterflow superfluidity in a system with N≥3 components is distinctively different from the N=2 case. The key feature is the difference between the number (N) of elementary vortex excitations and the number (N-1) of independent branches of phonon modes, that is, the number of superfluid modes is larger than the number of ordered phase variables. We formulate a hydrodynamic theory of this state. We show how all the dynamical and statistical aspects of this ("Borromean") type of ordering are naturally described by effective N-component theory featuring compact-gauge invariance. We also discuss how off diagonal intercomponent couplings convert the Borromean supercounterfluid into a Borromean insulator, with an emphasis on the properties of a nontrivial state with broken time-reversal symmetry.

Waveguide modes of exchange spin waves in epitaxial YIG films

It has been shown that exchange spin wave (ESW) waveguide modes can be excited in epitaxial ferrite films. This is facilitated by the presence of a thin transition layer at the interface between the ferrite film and the non-magnetic substrate. A method for calculating the dispersion and energy characteristics of ESW waveguide modes is proposed. It is shown that the propagation of ESW modes is accompanied by the transfer of exchange power in the plane of the film. As the wave numbers increase indefinitely, the dispersion of the ESW modes asymptotically approaches the quadratic dispersion law. At zero wave numbers, the frequencies of the ESW modes coincide with the frequencies of uniform spin wave resonances. The possibility of crossing the dispersion branches of different ESW modes is demonstrated, which indicates the possibility of intermodal magnon-magnon hybridization.

Impurity modes in two-dimensional strongly coupled complex plasma crystals

Complex plasma fluctuation processes have been extensively studied in many aspects, especially lattice waves in strongly coupled plasma crystals, which are of great significance for understanding fundamental physical phenomena. A challenge of experimental investigations in two-dimensional strongly coupled complex plasma crystals is to keep the main body and foreign particles of different masses on the same horizontal plane. To solve the problem, we have proposed a potential well formed by two negatively biased grids to bind the negatively charged particles in a two-dimensional (2D) plane, thus achieving a 2D plasma crystal in the microgravity environment. The study of such phenomena in complex plasma crystals under microgravity environment then becomes possible. In this paper, we focus on the continuum spectrum, including both phonon and optic branches of the impurity mode in a 2D system in microgravity environments. The results show the dispersion relation of the longitudinal and transverse impurity oscillation modes and their properties. Considering the macroscopic visibility of complex mesoscopic particle lattices, theoretical and experimental studies on this kind of complex plasma systems will help us further understand the physical nature of a wide range of condensed matters.

A novel method for stable thermoacoustic system design based on exceptional points - Part I: Conceptualization

In this conceptual study, we introduce the novel EPTD (Exceptional Point-based Thermoacoustic Design)-method for the design of stable thermoacoustic systems: dampen the entire thermoacoustic spectrum by shifting the EP towards smaller growth rates. For this, we first study a generic, linear parametric eigenvalue problem (LPEP) of a non-hermitian matrix featuring an EP. We derive analytical solutions for the eigenvalues and the EP as a function of arbitrary system parameters in the LPEP. Subsequently, we formulate a method for shifting the eigenvalue spectrum by relocating the EP in the complex plane. For this method, we show that not only the location of the EP in the complex plane is of relevance, but also the exceptional parameters, as they enforce the relations of the EP and the eigenvalues in the spectrum. When fixing these while shifting the EP, we demonstrate that the entire eigenvalue spectrum preserves its original shape, such that all eigenvalues only shift with the exact amount the EP was shifted. The derivation and the analysis of the relation between EP and spectrum generally holds for arbitrary system parameter combinations. Additionally, we showcase results for an exemplary configuration, i.e., a configuration with arbitrarily chosen and fixed parameters. We subsequently introduce a generic, thermoacoustic system, a Rijke tube, and study it by means of (semi-) analytical methods to confirm that the findings from the analysis of the LPEP also transfer to thermoacoustic systems. The configuration is modeled as a thermoacoustic network that allows to trace back the full system modes to their respective origins, which are either of acoustic or intrinsic thermoacoustic (ITA) nature. After identifying an EP that is formed by acoustic and ITA driven mode branches, we study its impact on the eigenfrequency spectrum and how this shift impacts the EP’s relation to the mode origins and the remaining eigenvalues. Similar to the analysis of the LPEP, the results show that not only the EP’s location in the CP is of relevance, but also its exceptional parameters: shifting the EP without consideration of the exceptional parameters leads to unsatisfactory shifts of the associated spectrum that becomes distorted. With this, we formulate the EPTD-method: shift the EP towards smaller growth rates while enforcing a well-defined relation between the mode origins and the EP by keeping the exceptional parameters constant. We show that this constraint preserves the relation between EP and mode origins when shifting the EP and finally demonstrate the practicality of the proposed method. For this, we shift the EP of the reference Rijke tube configuration towards smaller growth rates and simultaneously show that the resulting spectrum preserves its overall shape. While this paper covers the derivation and demonstration of the proposed EPTD-method based on a generic configuration, we consider a more complex combustion test rig, featuring a turbulent confined swirl flame, in part II of this study (M. Casel & A. Ghani, Combust. Flame, 2024), where we elaborate on consequences regarding the larger system parameter dimension as well as the existence of two EPs in the spectrum.Novelty and Significance statement:Thermoacoustic instabilities are actively researched for more than half a century. Despite this effort, scientifically sound strategies to design thermoacoustically stable combustors do not exist today. We present a novel method on a conceptual level that takes into account the latest discoveries in our field (e.g. ITA modes between 2014/2015 and Exceptional Points in 2018), connects them into one fast and robust numerical framework and opens the path to design the thermoacoustic stability map. This framework does not require tuning parameters or the like, which translates to an interpretable, parsimonious and computationally cheap design strategy. We demonstrate the Exceptional Point-based Thermoacoustic Design method (called EPTD-method) on two well-known experimental setups with increasing complexity and carefully perform parametric tests to unveil the key aspects of this novel design strategy. Finally, we turn the initially unstable setups into stable ones and explain why and how this was achieved.

A novel method for stable thermoacoustic system design based on exceptional points - Part II: Application to laminar and turbulent flame configurations

In this paper, we apply the novel EPTD (Exceptional Point-based Thermoacoustic Design)-method derived in part I of this study (M. Casel & A. Ghani, Combust. Flame, 2024) to two lab-scale combostor test rigs that exhbit thermoacoustic instabilities and demonstrate the method’s capability and practicality for stabilizing the originally unstable systems. Furthermore, we study, and refine, certain characteristics of the proposed EPTD method when applied to realistic combustor configurations. While the first configuration is laminar and unconfined, the second features a confined turbulent swirl flame. Both combustor test rigs are modeled by thermoacoustic networks incorporating measured flame responses. The models allow to trace back full system modes to their respective mode origins, which are either of intrinsic thermoacoustic (ITA) or acoustic nature, and are validated by means of experimental data. In both configurations, we identify exceptional points (EPs) that are formed by these respective mode branches. The unconfined, laminar configuration features only one EP, that we first investigate with respect to the system parameter sensitivity, and subsequently analyze its relation to the configuration’s spectrum, confirming the characteristics that were previously identified in part I of this study by means of generic (thermoacoustic) systems: not only the EP’s position in the complex plane, but also the exceptional parameters have a significant impact on the resulting thermoacoustic spectrum as they incorporate a relation between the respective mode origins and the full system modes. In contrast to the first configuration of this paper, the confined turbulent combustor allows for some further analysis of the EPTD application to realistic combustors as it features a significantly larger system parameter space as well as two EPs. Besides, it exhibits two EPs, for which we show how their respective sensitivities with respect to system parameter variations may already be utilized to identify which system parameters impact on which mode origins in the spectrum. In addition to this, we describe which of the two EPs may be chosen when applying the EPTD method. Finally, in both cases, we adopt the EPTD method derived in part I of this study for stabilizing thermoacoustic systems: Shift the EP towards smaller growth rates while constraining the exceptional parameters, i.e., shift the entire spectrum towards smaller growth rates while enforcing specific relations between the respective mode origins. Subsequently we showcase that this method in fact successfully dampens the entire spectrum of both combustion experiments and that the entire thermoacoustic spectrum shifts in growth rate with the same magnitude of the prescribed shift in EP growth rate. While the relatively small system parameter dimension of the unconfined laminar configuration does not allow for multiple system parameter combinations that realize the same EP, we identify multiple system parameter combinations that feature the same EP in the confined turbulent case. For this, we subsequently show that, as already shown for the generic configurations in part I of this study, different system parameter combinations, realizing the same EP, in fact result in the same thermoacoustic spectrum. This finally demonstrates that our proposed method is especially suited for the design of thermoacoustic systems that feature a large number of system parameters.Novelty and Significance statement:Thermoacoustic instabilities are actively researched for more than half a century. Despite this effort, scientifically sound strategies to design thermoacoustically stable combustors do not exist today. We present a novel method on a conceptual level that takes into account the latest discoveries in our field (e.g. ITA modes between 2014/2015 and Exceptional Points in 2018), connects them into one fast and robust numerical framework and opens the path to design the thermoacoustic stability map. This framework does not require tuning parameters or the like, which translates to an interpretable, parsimonious and computationally cheap design strategy. We demonstrate the Exceptional Point-based Thermoacoustic Design method (called EPTD-method) on two well-known experimental setups with increasing complexity and carefully perform parametric tests to unveil the key aspects of this novel design strategy. Finally, we turn the initially unstable setups into stable ones and explain why and how this was achieved.

A replaceable-component method to construct single-degree-of-freedom multi-mode planar mechanisms with up to eight links

Abstract. The multi-mode planar mechanisms (MMPMs) are excellent-performance reconfigurable mechanisms, which not only inherit structural characteristics of planar mechanisms but also have the multi-task, multi-working-condition application advantages of multi-mode mechanisms. However, lacking common bifurcation analysis and construction methods, their industrial application and development are seriously hindered. This paper presents a replaceable-component method to construct a set of single-degree-of-freedom (single-DOF) MMPMs based on the branch graphs of the corresponding planar mechanisms and the proposed multi-mode modules (MMMs). First, according to the established loop equations, all the kinematic information of the original planar mechanism is obtained by the branch graphs and singularity points using Maple. Then, compared to the relationship between the concepts of the branch and motion mode, the number and continuity of branches are taken as the index to identify the potential bifurcation and mode conversion ability for the corresponding planar mechanisms. Subsequently, the MMM is presented to help the planar mechanisms break the singularity positions to form the corresponding MMPMs, and the steps of constructing single-DOF MMPMs are summarized. Finally, a single-DOF Stephenson six-bar three-mode planar mechanism, a Watt six-bar three-mode planar mechanism, and an eight-bar four-mode planar mechanism are constructed for the first time, and the corresponding multi-mode motion analyses are made. The results can give the available configuration for the design of corresponding MMPMs. The proposed method will provide strong guidance for the configuration design of MMPMs.

Heart failure prognosis prediction: Let’s start with the MDL-HFP model

Heart failure, as a critical symptom or terminal stage of assorted heart diseases, is a world-class public health problem. Establishing a prognostic model can help identify high dangerous patients, save their lives promptly, and reduce medical burden. Although integrating structured indicators and unstructured text for complementary information has been proven effective in disease prediction tasks, there are still certain limitations. Firstly, the processing of single branch modes is easily overlooked, which can affect the final fusion result. Secondly, simple fusion will lose complementary information between modalities, limiting the network’s learning ability. Thirdly, incomplete interpretability can affect the practical application and development of the model. To overcome these challenges, this paper proposes the MDL-HFP multimodal model for predicting patient prognosis using the MIMIC-III public database. Firstly, the ADASYN algorithm is used to handle the imbalance of data categories. Then, the proposed improved Deep&Cross Network is used for automatic feature selection to encode structured sparse features, and implicit graph structure information is introduced to encode unstructured clinical notes based on the HR-BGCN model. Finally, the information of the two modalities is fused through a cross-modal dynamic interaction layer. By comparing multiple advanced multimodal deep learning models, the model’s effectiveness is verified, with an average F1 score of 90.42% and an average accuracy of 90.70%. The model proposed in this paper can accurately classify the readmission status of patients, thereby assisting doctors in making judgments and improving the patient’s prognosis. Further visual analysis demonstrates the usability of the model, providing a comprehensive explanation for clinical decision-making.

Phonon spectra split in vertically aligned bilayer crystal of binary complex plasma

Structural and dynamical properties of a bilayer binary strongly coupled dusty plasma system in a sheath of an rf discharge are experimentally investigated. Microparticles of two different sizes are aligned into vertical pairs due to ion focusing (plasma wakes) downstream from the upper particles in the ion flow toward the bottom electrode. The pairs are ordered into a 2D hexagonal crystalline structure. The collective mode dispersion relations are measured and analyzed. Splitting of the modes in the bilayer lattice is observed and theoretically described. The relative intensities of the mode branches reflect non-reciprocal features of the wake-mediated interaction between the microparticles. The conditions of stability of vertically aligned bilayer structure are discussed, and relevant plasma parameters are estimated.

The Pacific water flow branches in the eastern Chukchi Sea

The flow of Pacific-origin water across the Chukchi Sea shelf impacts the regional ecosystem in profound ways, yet the two current branches on the eastern shelf that carry the water from Bering Strait to Barrow Canyon – the Alaskan Coastal Current (ACC) and Central Channel (CC) Branch – have not been clearly distinguished or quantified. In this study we use an extensive collection of repeat hydrographic sections occupied at three locations on the Chukchi shelf, together with data from a climatology of shipboard velocity data, to accomplish this. The data were collected predominantly between 2010 and 2020 during the warm months of the year as part of the Distributed Biological Observatory and Arctic Observing Network. The mean sections show that mass is balanced for both currents at the three locations: Bering Strait, Point Hope, and Barrow Canyon. The overall mean ACC transport is 0.34 ± 0.04 Sv, and that of the CC Branch is 0.86 ± 0.11 Sv. The dominant hydrographic variability at Bering Strait is seasonal, but this becomes less evident to the north. At Barrow Canyon, the dominant hydrographic signal is associated with year-to-year variations in sea-ice melt. Farther south there is pronounced mesoscale variability: an empirical orthogonal function analysis at Bering Strait and Point Hope reveals a distinct ACC mode and CC Branch mode in hydrography and baroclinic transport, where the former is wind-driven. Finally, the northward evolution in properties of the two currents is investigated. The poleward increase in salinity of the ACC can be explained by lateral mixing alone, but solar heating together with wind mixing play a large role in the temperature evolution. This same atmospheric forcing also impacts the northward evolution of the CC Branch.

Plasmonic detection of the parity anomaly in a two-dimensional Chern insulator

In this paper, we present an analytic study on the surface plasmon polaritons in a two-dimensional parity anomaly Chern insulator. The two-dimensional conductivities derived from the BHZ model are antisymmetric, based on which two surface plasmon modes each contains two branches of dispersions have been found. In the absence of parity anomaly, the half-integer-valued Hall conductivities with positive and negative Dirac mass terms differ by a sign; two branches of each surface plasmon mode are exactly degenerate. However, the parity anomaly can lift such degeneracy and lead to significant modifications of these dispersion curves or even the occurrence of an extra branch of surface plasmons under particular condition. Our investigations have revealed the effects of the interplay of parity anomaly and topology on the dispersion relations of the surface plasmon polaritons, which may pave a possible way for the detection of the parity anomaly in a two-dimensional Chern insulator via plasmonic responses.

Suppression of filamentation in refraction index modulated Kerr media

In this work, the possibility of using periodic refractive index modulation – photonic crystals – to counteract the spatial Kerr effect, thus suppressing filamentation, is shown. Numerical simulations by beam propagation method indicate that fine control of the spatial dispersion can be achieved by tuning the photonic crystal geometry and projecting to the appropriate Bloch mode branch by chirping the longitudinal period. Controlling the spatial dispersion allows management of the strength of diffraction. Furthermore, it is shown that photonic crystals can be engineered with such geometry that the designed diffraction compensates for self-focusing, effectively delaying, or entirely eliminating beam filamentation.

Polarization and properties of low-frequency waves in warm magnetized two-fluid plasma

This paper presents the derivation of a general wave dispersion relation for warm magnetized plasma under the two-fluid formalism. The discussion is quite general except for the assumption of low frequency and slow phase speed, for which the displacement current is negligible, under the implicit assumption that the plasma is sufficiently dense to satisfy the condition ωpe>ωce, where ωpe and ωce denote the plasma oscillation frequency and electron gyro frequency, respectively. The present discussion does not invoke charge neutrality associated with the fluctuations although it is implicitly satisfied. The resulting dispersion relation that includes the fluid thermal effects shows that there are three eigen modes, which include those corresponding to ideal MHD, namely, fast, slow, and kinetic Alfvén waves, as well as higher-frequency modes including the ion and electron cyclotron and lower-hybrid resonances. The fluid effects in the ideal MHD wave branches are influenced by the finite Larmor radius scales, and when the wave number in the cross field direction is comparable to these values, the fluid effects become significant. It is found that the Larmor radius should be interpreted in the sense as ion-acoustic gyro-radius instead of ion thermal gyro radius only. That is, it is found that the electrons also contribute to the non-ideal effect associated with the kinetic Alfvén wave. A comprehensive explanation of the polarization of each mode is also presented. The present findings indicate that the polarity may change its sign only for the kinetic Alfvén mode branch and that such a transition is based on the propagation angle. When such a change does take place, it is found that the kinetic Alfvén wave transits to an ion-acoustic mode. For each branch, it is also found that the electric field along the ambient magnetic field is purely transverse.

Elastic fingering in a rotating Hele-Shaw cell.

We consider the steady-state fingering instability of an elastic membrane separating two fluids of different density under external pressure in a rotating Hele-Shaw cell. Both inextensible and highly extensible membranes are considered, and the role of membrane tension is detailed in each case. Both systems exhibit a centrifugally driven Rayleigh-Taylor-like instability when the density of the inner fluid exceeds that of the outer one, and this instability competes with the restoring forces arising from curvature and tension, thereby setting the finger scale. Numerical continuation is used to compute not only strongly nonlinear primary finger states up to the point of self-contact, but also secondary branches of mixed modes and circumferentially localized folds as a function of the rotation rate and the externally imposed pressure. Both reflection-symmetric and symmetry-broken chiral states are computed. The results are presented in the form of bifurcation diagrams. The ratio of system scale to the natural length scale is found to determine the ordering of the primary bifurcations from the unperturbed circle state as well as the solution profiles and onset of secondary bifurcations.

Raman scattering study of photoexcited plasma in GaAsBi/GaAs heterostructures: Influence of carrier confinement on photoluminescence

GaAsBi alloys are potential candidates for near-infrared optoelectronic applications, such as light-emitting diodes, laser diodes, avalanche photodiodes, and solar cells. In this paper, photoexcited plasma in GaAs1-xBix/GaAs (1.7 ≤ x ≤ 8.6%) heterostructures grown by molecular beam epitaxy was investigated using Raman spectroscopy. The strong LO phonon-plasmon coupled mode (LOPC) was generated by continuous-wave laser excitation at room temperature. The upper branch of the LOPC mode (L+ mode) was clearly observed for Bi contents from x = 3.9–8.6%, which indicates that photoexcited carriers with relatively good carrier mobility were confined in the GaAsBi/GaAs heterostructure. The photoexcited carrier density that was obtained by line shape analysis of the Raman spectra based on a dielectric model increased as the Bi content increased from 1.7 to 6.4%. This indicates that carrier overflow from the GaAsBi layer was suppressed because of the larger conduction band offset at the GaAsBi/GaAs heterointerface. In addition, the optical mobility calculated using the damping constant of the photoexcited L+ mode gradually decreased with increasing Bi content. This may be attributed to an increased defect density with decreasing growth temperature. Our findings provide valuable insight into the development of high-performance optoelectronic devices based on GaAsBi alloys and show the potential of Raman spectroscopy for evaluation of carrier confinement in heterostructures.

Control Strategy for Array Buoy Type Wave Energy Converters' Power Conversion

In order to solve the problem of complex inner‐loop control structure, difficult debugging of control parameters and poor anti‐interference ability in the double PI power conversion control method of array buoy type wave energy converter, this article proposes a power conversion control strategy in which the one step delay compensation model predictive current control strategy is proposed for the machine side rectifier and the three closed‐loop control structure with PI voltage outer‐loop, quasi‐proportional resonant (QPR) current inner‐loop and proportional capacitor current inner‐loop is proposed for the grid‐side LCL inverter. In addition, this article proposes a graded power generation scheme for a single oscillating buoy and two power generation branches, which aims to address the issue of a restricted safe working pressure range for the accumulator in the single buoy and single power generation branch mode. Based on the mathematical model of the system's key components, the MATLAB/Simulink program is applied for modeling and simulation. The results indicate that the recommended graded power generation scheme and power conversion control mechanism may accomplish stable operation of the system under three diverse wave conditions, showing the scheme's practicality. © 2023 Institute of Electrical Engineer of Japan and Wiley Periodicals LLC.

Polar modes and isospectrality of Ellis-Bronnikov wormholes

We consider polar perturbations of static Ellis-Bronnikov wormholes and derive the coupled set of perturbation equations for the gravitational and the scalar field. For massless wormholes the perturbations decouple, and we obtain two identical master equations for the scalar and gravitational modes, which moreover agree with the master equation for the axial modes. Consequently there is isospectrality with threefold degenerate modes. For a finite mass of the background wormhole solutions, the equations are coupled. We then obtain two distinct branches of polar quasinormal modes for a given multipole number l, associated with the presence of the two types of fields. We calculate the quasi-normal mode frequencies and decay rates for the branches with l=2, 3 and 4. For a given l the real frequencies of the two branchesget the closer, the higher the multipole number gets.

Internal Kink Instabilities Driven by Poloidal Flow via Non-Resonant Excitation in Tokamaks

Non-resonant excitation due to plasma inertia may be dominant in inducing internal kink (IK) instabilities. Poloidal rotation can effectively modify plasma inertia and cause non-resonant excitation to occur. An extended dispersion relation including poloidal rotation is established to study the IK mode and the fishbone (FB) mode. It is found that in rotating plasmas, even for a stable IK mode (i.e., the perturbed potential energy of background plasma δWc is positive) and in the absence of energetic particles (EPs), poloidal rotation can drive the IK mode via non-resonant excitation. Moreover, the IK mode is easy to be driven by poloidal rotation in weak magnetic shear plasmas. Similar to toroidal rotation, when poloidal rotation frequency exceeds a threshold, the FB mode can transform into a branch of a non-resonant mode. The real frequency of the mode, being independent of the precessional frequency of EPs, is just equal to the poloidal rotation frequency. Thus, the non-resonant mode is characterized by the long-lived mode (LLM) observed in toroidal rotating plasmas. The critical gradient of the poloidal rotation profile plays a crucial role in causing the resonant mode to evolve into a non-resonant one; for instance, only for a very peaked poloidal rotation profile can the FB mode transform into the LLM. In addition, the diamagnetic drift frequency of thermal ions can stabilize the FB and the IK modes.

Structure and Dynamics in Liquid Iron at High Pressure and Temperature. A First Principles Study

AbstractWe have studied the evolution of structural and dynamic properties of liquid Fe as a function of pressure for 11 thermodynamic states close to the melting line. The pressure range considered goes from ambient pressure to 323 GPa, and the study has been carried out by using the ab‐initio molecular dynamics technique. The agreement between the calculated static structure and the available experimental data is very good, including details like an asymmetric second peak, which remains over most of the whole pressure range and suggests a significant local icosahedral short‐range order in the liquid. The dynamical structure is studied through the characteristics of the propagating density fluctuations and the associated longitudinal and transverse particle currents. The transverse dispersion relations expose two branches of modes for all pressures, whose range of appearance is analyzed and put in connection with the double‐peak structure of the Fourier spectra of velocity autocorrelation functions. We have also investigated the existence of fingerprints of transverse acoustic excitation modes in the dynamic structure factor for the high pressure states similar to those observed in the inelastic X‐ray scattering intensity data of liquid Fe at ambient pressure. The calculated electronic density of states shows that with increasing pressure there is a widening of the conduction band along with a decreasing significance of spin polarization. Finally, we also report results for transport coefficients like self‐diffusion, shear viscosity and adiabatic sound velocity, which are compared with the available experimental data.

In-situ observation of strain evolution and ratchetting of growing fatigue cracks

Crack-tip strain localization and accumulation has long been recognized while the mechanics and physics of fatigue are yet to be understood for complicated cracking modes, microstructural damage, and cross-scale fatigue crack growth modeling. In this work, near-tip strain was characterized at high-resolution and statistically analyzed to develop a strain-based yielding criterion for crack-tip plasticity evaluation which was found applicable to both tensile and fatigue loadings. The near-tip strain of face centered cubic 304 stainless steel satisfied Lognormal distribution whereas body centered cubic 25Cr steel obeyed Gauss distribution, and was found evolving Logistic-Gauss-Lognormal with fatigue crack growing and plasticity increasing. The relative strain accumulation was proposed and found well representing branch or stable growth mode, and drove fatigue crack growth when relative strain accumulation was beyond 1.3. The cracking speed parameter in terms of strain ratchetting per crack extension was established, which indicated that different cracking at macro-scale behaved similarly at the atomic scale for a given material. The microstructural damage analysis near crack-tip illustrated that fatigue crack growth of 25Cr steel was governed by breakdown of lathy martensite and formation of fine grains while the cracking of 304ss was accompanied with austenite-to-martensite transformation that was occurred at a near-tip strain of approximately 13%.