bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical elucidation of effect of drag force and translation of bubble on weakly nonlinear pressure waves in bubbly flows
Authors: Yatabe Takahiro , Kanagawa Tetsuya , Ayukai Takahiro
Published: March 2021

Theoretical investigation of the effects of a translation of bubbles and a drag force acting on bubbles on the wave propagation in bubbly flows has long been lacking. In this study, we theoretically and numerically investigate the weakly nonlinear (i.e., finite but small amplitude) propagation of plane progressive pressure waves in compressible water flows that contain uniformly distributed spherical gas bubbles with translation and drag forces. First, we assume that the gas and liquid phases flow at independent velocities. Then, the drag force and virtual mass force are introduced in an interfacial transport across the bubble–liquid interface in the momentum conservation equations. Furthermore, we consider the translation and spherically symmetric oscillations as bubble dynamics and deploy a two-fluid model to introduce the translation and drag forces. Bubbles do not coalesce, break up, extinct, or appear. For simplicity, the gas viscosity, thermal conductivities of the gas and liquid, and phase change and mass transport across the bubble–liquid interface are ignored. The following results are then obtained: (i) Using the method of multiple scales, two types of Korteweg–de Vries–Burgers equations with a correction term due to the drag force are derived. (ii) The translation of bubbles enhances the nonlinear effect of waves, and the drag force acting on bubbles contributes the nonlinear and dissipation effects of waves. (iii) The results of long-period numerical analysis verify that the temporal evolution of the wave (not flow) dissipation due to the drag force differs from that caused by the acoustic radiation.
Abnormal in-plane permittivity and ferroelectricity of confined water: From sub-nanometer channels to bulk
Authors: Hamid Ilyar , Jalali Hossein , Peeters Francois. M. , Neek-Amal Mehdi
Published: March 2021

Dielectric properties of nano-confined water are important in several areas of science, i.e., it is relevant in the dielectric double layer that exists in practically all heterogeneous fluid-based systems. Molecular dynamics simulations are used to predict the in-plane dielectric properties of confined water in planar channels of width ranging from sub-nanometer to bulk. Because of suppressed rotational degrees of freedom near the confining walls, the dipole of the water molecules tends to be aligned parallel to the walls, which results in a strongly enhanced in-plane dielectric constant (ε∥) reaching values of about 120 for channels with height 8 Å < h < 10 Å. With the increase in the width of the channel, we predict that ε∥ decreases nonlinearly and reaches the bulk value for h > 70 Å. A stratified continuum model is proposed that reproduces the h > 10 Å dependence of ε∥. For sub-nanometer height channels, abnormal behavior of ε∥ is found with two orders of magnitude reduction of ε∥ around h ∼ 7.5 Å, which is attributed to the formation of a particular ice phase that exhibits long-time (∼μs) stable ferroelectricity. This is of particular importance for the understanding of the influence of confined water on the functioning of biological systems.
Evidence of improved tolerance to electronic excitation in nanostructured Nd2Zr2O7
Authors: Sharma S. K. , Grover V. , Shukla R. , Hussain A. , Mishra A. , Meena R. C. , Kulriya P. K.
Published: March 2021

Grain size driven effects on electronic excitation-induced structural modifications have been investigated in nanocrystalline (NC) Nd2Zr2O7 on irradiation with 100 MeV iodine ions. Characterizations have been performed with in situ x-ray diffraction, Raman spectroscopy, and plane-view high-resolution transmission electron microscopy techniques. NC-powders of Nd2Zr2O7 were synthesized by auto gel-combustion and sintered at different temperatures to obtain different grain-sized samples. XRD analysis of the smallest grain-sized sample reveals the highest order–disorder transition (from pyrochlore to a more radiation-resistant phase; anion-deficient fluorite) rate at initial ion fluences followed by least amorphization at higher ion fluences. A strong correlation of the transformation build-up with the double ion impact model confirms the two step amorphization process in NC-Nd2Zr2O7 with the disordered anion-deficient fluorite structure as an intermediate phase. TEM result supports the formation of circular ion track consisting of randomly distributed regions (anion-deficient fluorite structure and amorphous regions), surrounded by a microstrain induced defect-rich pyrochlore superstructure. Lesser ordering at cationic sites and a relatively larger number of grain boundaries are responsible for the highest radiation tolerance exhibited by the smallest grain-sized sample. The present study reports a relatively higher radiation stability of NC-ternary pyrochlore oxide, Nd2Zr2O7, with a grain size of a few tens of nm, which establishes its application as a potential inert matrix for nuclear applications.
Thermoacoustic wave generation in multilayered thermophones with cylindrical and spherical geometries
Authors: Guiraud Pierre , Giordano Stefano , Bou Matar Olivier , Pernod Philippe , Lardat Raphael
Published: March 2021

A thermoacoustic sound generation model, based on the classical balance equations of the continuum mechanics, is here developed for the cylindrical and the spherical thermoacoustic wave generation. In both geometries, the model considers an arbitrary multilayered structure, where each layer can be fluid or solid and it is characterized by the fully coupled thermo-visco-acoustic response. It means that the viscous behavior and the thermal conduction are considered in each layer. The model is based on a unified representation of cylindrical or spherical thermoacoustic waves, which is valid for both fluid and solid phases. Thanks to the continuity of temperature, particle velocity, normal stress, and heat flux between adjacent layers, the model can be implemented by means of a versatile matrix approach, allowing flexible analysis and design of cylindrical or spherical thermophones. Any thermoacoustic variable can be determined at any position, any frequency, and for any input power. The results are compared with the models already existing in the literature, and the underlying physics is thoroughly discussed. The analysis is focused on a better understanding of the thermoacoustic generation with application to the state of the art of the thermophone technology.
Study of band alignment at MoS2/SiO2 interfaces grown by pulsed laser deposition method
Authors: Sinha Sneha , Kumar Sujit , Arora Sunil K. , Jha S. N. , Kumar Yogesh , Gupta Vinay , Tomar Monika
Published: March 2021

We report on the large-area and high-quality growth of single- to few-monolayer thick MoS2 thin films on oxidized Si (100) substrates via the pulsed laser deposition method. Our Raman, x-ray photoelectron spectroscopic, and FE-SEM measurements confirmed that atomically thin MoS2 layers are highly uniform and are stoichiometric. We found a type-I band alignment at the MoS2/SiO2 heterointerfaces through photoemission spectroscopic valence-band measurements. The valence- and conduction band offset (VBO and CBO) at the 1L MoS2/SiO2 interface was observed to be 3.91 and 2.96 ± 0.05 eV, respectively. The values of VBO and CBO increase up to 4.15 and 3.56 ± 0.05 eV, respectively, with an increase in the MoS2 layer number. This observation can be attributed to the shift of the Mo-4dz2 orbitals due to interlayer coupling for thicker MoS2 films, reducing its bandgap, resulting in an increment in VBO and CBO values.
Carrier trapping and recombination at carbon defects in bulk GaN crystals grown by HVPE
Authors: Fang Yu , Wu Xingzhi , Yang Junyi , Wang Jianping , Wu Quanying , Song Yinglin
Published: March 2021

We investigate the impact of carbon defects on carrier trapping and recombination processes in high-quality GaN crystals using transient absorption spectroscopy (TAS). The different wavelengths for probing carrier dynamics enable discrimination of electron- and hole-capture processes by carbon acceptors (CN), and two charge states (−1 and 0) of the CN are demonstrated from TAS by tuning the pump fluence. The hole-capture coefficients for the CN− and CN0 states are at least ten times larger than that for Fe centers. On the other hand, electrons can also recombine efficiently with holes at the 0/+ level of the CN defect. The resulting carrier trapping on carbon in GaN would mediate carrier recombination and significantly reduce the efficiency of optoelectronic devices even for a low defect concentration (∼1015 cm−3).
The action-integral and energy to explode short gold wires in ambient air
Authors: Rae P. J.
Published: March 2021

This paper is about electrically induced explosions of short (1 mm) and thin (38 μm diameter) gold bridgewires in ambient air and contrasts this behavior with that observed for longer wires in a vacuum used in applications such as Z-pinch systems. The action-integral (the time integral of the wire current squared) and energy-to-burst are measured using high current discharge systems. It is found that in contrast to some reports, the action-integral to burst is not a constant for a specific wire geometry and material but, instead, is a function of the rate of current change in the wire around the time of burst and the related metric of the current flux at burst. Furthermore, it is found that the accuracy of the constant action-integral to burst approximation is dependent on the discharge system used. Short gold wires such as these have relevancy to exploding bridgewire detonators.
Heat transfer characteristics of successive oil droplet impingement under minimum quantity lubrication
Authors: Zheng Nan , Tong Baohong , Zhang Guotao , Hu Xiaolei , Liang He , Wang Wei , Liu Kun
Published: March 2021

During oil–gas minimum lubrication, lubricating oil droplets are easily formed into hollow oil droplets containing bubbles when disturbed by a high-speed airflow. Microbubbles have an important influence on the heat transfer characteristics and movement of multiple oil droplets successively impinging on an oil film. In this work, the behavior of multiple oil droplets successively impacting an oil film is numerically simulated on the basis of the coupled level set-volume fraction method, and the influences of different bubble distributions on the heat transfer characteristics of double oil droplets successively impinging on the oil film are investigated. The formation mechanism of some unique heat transfer phenomena in the impingement process is discussed, and the influences of different bubble distribution forms on the geometric size of the thermal wake and cooling effect of the impingement area are analyzed. Results showed that a “cicada wing-like” thermal wake appears during the falling process of high-temperature oil droplets. The combined effects of heat transfer, flow field, and air flow separation behavior are the main reasons behind this wake. During the falling and spreading process of solid and hollow oil droplets, the velocity gradient difference at the tail of the oil droplet affects the geometric size of the wake. In the later stages of the impingement process, a vortex is formed in the impingement pit under the combined action of the space flow field and the pressure field. This vortex strongly improves the heat flux density in the impingement area. Different bubble distribution forms have different effects on the cooling and heat dissipation effect during impingement, and hollow oil droplets are unfavorable for cooling and heat dissipation.
Probing pure and mixed neon ECR plasma—An effort for understanding the mechanism of isotope anomaly and gas mixing effect
Authors: Tripathi Puneeta , Singh Shushant Kumar , Kumar Pravin
Published: March 2021

We report the charge state distributions of the pure and mixed (25%, 50%, and 75% with oxygen and helium gases) neon electron cyclotron resonance (ECR) plasma to probe the hitherto unrevealed mechanism of the gas mixing effect and the isotope anomaly [A. G. Drentje, Rev. Sci. Instrum. 63, 2875 (1992) and A. G. Drentje, Rev. Sci. Instrum. 74, 2631 (2003)]. The multiply charged (up to +7) neon plasma was produced using a 10 GHz all-permanent-magnet NANOGAN ECR ion source. The intensities of highly abundant two isotopes, viz., 20Ne (∼90.48%) and 22Ne (∼9.25%), have been measured by extracting the ions from the plasma, accelerating with different potentials and, finally, analyzing them in mass and energy using a high resolution, large acceptance analyzer-cum-switching dipole magnet. The gas mixing studies show that the addition of oxygen with neon is more effective than that of the helium, particularly on the ion intensities of 20Ne. The higher the fractions of mixing gases, the better are the intensities of highly charged neon ions in the plasma. With and without gas mixing, the intensity ratio of 22Ne to 20Ne shows clear evidence of the isotope anomaly. However, the anomalous effect tends to disappear with increasing oxygen and helium gas mixing. Contrary to the behavior of recently studied pure and oxygen mixed krypton plasma [Kumar et al., J. Mass. Spectrom. 51, 1090 (2016)], the present results are in good agreement with the prediction of linear Landau wave damping theory for selective ion heating in a two-component plasma.
The RED scheme: Rate-constant estimation from pre-steady state weighted ensemble simulations
Authors: DeGrave Alex J. , Bogetti Anthony T. , Chong Lillian T.
Published: March 2021

We present the Rate from Event Durations (RED) scheme, a new scheme that more efficiently calculates rate constants using the weighted ensemble path sampling strategy. This scheme enables rate-constant estimation from shorter trajectories by incorporating the probability distribution of event durations, or barrier-crossing times, from a simulation. We have applied the RED scheme to weighted ensemble simulations of a variety of rare-event processes that range in complexity: residue-level simulations of protein conformational switching, atomistic simulations of Na+/Cl− association in explicit solvent, and atomistic simulations of protein–protein association in explicit solvent. Rate constants were estimated with up to 50% greater efficiency than the original weighted ensemble scheme. Importantly, our scheme accounts for the systematic error that results from statistical bias toward the observation of events with short durations and reweights the event duration distribution accordingly. The RED scheme is relevant to any simulation strategy that involves unbiased trajectories of similar length to the most probable event duration, including weighted ensemble, milestoning, and standard simulations as well as the construction of Markov state models.
Thermoacoustically driven liquid-metal-based triboelectric nanogenerator: A thermal power generator without solid moving parts
Authors: Zhu Shunmin , Yu Guoyao , Tang Wei , Hu Jianying , Luo Ercang
Published: March 2021

Liquid metal has exhibited great potential in escalating power density in both triboelectric nanogenerator and thermoacoustic systems. An innovative endeavor of coupling these two systems through liquid metal enables a thermal power generator without solid moving parts and shows prospects in the recovery of waste heat. In this configuration, liquid metal functions as both triboelectric and resonant stuff. Based on the understanding, an experimental setup involving a standing-wave thermoacoustic heat engine and a liquid metal-based triboelectric nanogenerator is fabricated to validate the concept. So far, the highest open-circuit voltage amplitude of 15 V has been achieved, outweighing our previous work by 50%. Allowing for the reduction in size, the preliminary results well demonstrate the viability of future applications.
A model of micro lubrication between two walls with unequal temperature distribution based on kinetic theory
Authors: Doi Toshiyuki
Published: March 2021

Micro lubrication of a gas between two walls with arbitrary and independent temperature distributions is studied on the basis of the Bhatnagar–Gross–Krook–Welander (BGKW) model of the Boltzmann equation. The BGKW equation is studied analytically using the slowly varying approximation. Following the author's previous study [T. Doi, “A model of micro lubrication between two walls with an arbitrary temperature difference based on kinetic theory,” Phys. Fluids 32, 052005 (2020)], the leading-order approximation, which ought to be the solution of the nonlinear heat transfer problem, is replaced by its free molecular solution. A lubrication model of the Reynolds-type equation is derived in closed form. A direct numerical analysis of the lubrication flow subject to localized heating or cooling of the walls is conducted for an assessment of the lubrication model. The lubrication lift calculated using the model agrees with that of the direct numerical solution within an error of 5% when the Knudsen number based on the gap size lies between 0.1 and 10. The result of the lubrication model agrees also with that of the Boltzmann equation for a variable hard sphere gas. A sharp peak arises in the pressure distribution for large Knudsen numbers owing to the effect of thermal creep flows induced by localized heating.
A lattice Boltzmann modeling of the bubble velocity discontinuity (BVD) in shear-thinning viscoelastic fluids
Authors: Wang Di , Tan Danielle S. , Cheong Khoo Boo , Ouyang Zhenyu , Phan-Thien Nhan
Published: March 2021

The bubble velocity discontinuity (BVD), when single bubble rising in shear-thinning viscoelastic fluids, is studied numerically. Our three-dimensional numerical scheme employs a phase-field lattice Boltzmann method together with a lattice Boltzmann advection-diffusion scheme, the former to model the macroscopic hydrodynamic equations for multiphase fluids, and the latter to describe the polymer dynamics modeled by the exponential Phan–Thien–Tanner (ePTT) constitutive model. An adaptive mesh refinement technique is implemented to reduce computational cost. The multiphase solver is validated by simulating the buoyant rise of single bubble in a Newtonian fluid. The critical bubble size for the existence of the BVD and the velocity-increasing factor of the BVD are accurately predicted, and the results are consistent with the available experiments. Bubbles of different sizes are characterized as subcritical (smaller than critical size) and supercritical (larger than critical size) according to their transient rising velocity behaviors, and the polymeric stress evolution affecting the local flow pattern and bubble deformation is discussed. Pseudo-supercritical bubbles are observed with transition behaviors in bubble velocity, and their sizes are smaller than the critical value. The formation of bubble cusp and the existence of negative wake are observed for both the pseudo-supercritical and the supercritical bubbles. For the supercritical bubble, the trailing edge cusp and the negative wake arise much earlier. The link between the BVD, the bubble cusp, and the negative wake is discussed, and the mechanism of the BVD is explained.
A mathematical model of the electric field perturbation about an array of conductive wires and its impact on the passing charged particle distribution
Authors: Shadman K.
Published: March 2021

An analytic model of the electric field perturbation about an array of wires, such as a grid or a mesh, is presented to assess the impact of using such arrays in charged particle analyzers. The analysis begins with an exact solution for the electric field about a grid of flat wires. The result is connected to the Fourier analysis of a grid with high transparency, which is extended to give a solution for the field perturbation about a mesh of wires. The Fourier decomposition of the electric field profile is utilized to calculate the effective barrier that a grid or a mesh provides to charged particles and to calculate the diffusion of the particle energies that pass through such arrays of wires. The results are applied to the measurements of the temperature of a Maxwellian particle distribution by these arrays and by a collector placed downstream.
On the peristaltic pumping
Authors: Floryan J. M. , Panday S. , Faisal Kh. Md.
Published: March 2021

Peristaltic pumping in a two-dimensional conduit using vibrations in the form of traveling waves has been investigated. Two qualitatively different responses producing vastly different flow rates have been identified, with a transition occurring at wavelengths of the order of the conduit opening. The flow rate is always proportional to the wave phase speed and the second power of the amplitude. Long waves produce sloshing which extends across the whole conduit producing a small, nearly wave-number-independent flow rate. The use of such in-phase waves on both walls nearly eliminates this flow while the use of out-of-phase waves maximizes it. Short waves affect the near-wall regions, which appear to the bulk of the fluid as moving walls. Such waves produce an order of magnitude larger flow rate, with its magnitude increasing proportionally to the second power of the wavenumber. Each vibrating wall produces its own wall boundary layer with an unmodulated core flow in the central zone of the conduit. The core flow looks like a Couette flow and reduces to a plug flow when both waves have identical amplitudes. The phase difference between such waves does not affect the flow rate. Wave tilting increases the flow rate similarly to the increase in distance between these waves. The use of waves characterized by a combination of wavenumbers increases the flow rate but only when the commensurability index is greater than one. The best performance is achieved by concentrating all wave energy in a single and largest achievable wavenumber.
Microstructure and rheology of shear-thickening colloidal suspensions with varying interparticle friction: Comparison of experiment with theory and simulation models
Authors: Lee Yu-Fan , Luo Yimin , Bai Tianyi , Velez Carlos , Brown Scott C. , Wagner Norman J.
Published: March 2021

Two colloidal suspensions of paucidisperse, spherical silica particles with different surface chemistries leading to extreme limits of surface contact friction are studied to identify experimental differences in shear rheology and microstructure and quantitatively test theory and simulation models. The nonequilibrium microstructure in the plane of shear is measured by flow-small angle neutron scattering for steady shear states spanning the shear thinning and shear thickening regimes. The shear rheology and microstructure are compared against predictions from theory for Brownian hard sphere suspensions and state-of-the-art simulation methods that incorporate either contact friction or enhanced lubrication hydrodynamics. The first normal stress differences are confirmed to distinguish between these micromechanical mechanisms for stress enhancement in the shear thickened regime. The nonequilibrium microstructure in the plane of shear shows more anisotropy for the suspension with higher interparticle friction. A significant fourfold symmetry is confirmed and found to be amplified with increasing surface contact friction in the shear thickened state. The differences in shear-induced microstructures between suspensions with varying contact friction demonstrate that the nonequilibrium microstructure can distinguish between nanotribological interactions in the shear thickened state. Statistical comparison of experiments with simulations indicates that better resolution of microstructures in simulation models is required to be validated by the experimental data presented. Implications for the development of theories for colloidal suspension rheology are discussed.
Self-consistent hydrodynamic two-fluid model of vortex plasma
Authors: Mironov Victor L.
Published: March 2021

We propose the system of self-consistent equations for vortex plasma in the framework of hydrodynamic two-fluid model. These equations describe both longitudinal flows and the rotation and twisting of vortex tubes taking into account internal electric and magnetic fields generated by fluctuations of plasma parameters. The main peculiarities of the proposed equations are illustrated with the analysis of electron and ion sound waves.
Scaling of the mean transverse flow and Reynolds shear stress in turbulent plane jet
Authors: Wei Tie , Livescu Daniel
Published: March 2021

Proper scaling for the mean transverse flow and Reynolds shear stress in a turbulent plane jet is determined using a scaling patch approach. By seeking an admissible scaling, a key concept in the scaling patch approach, for the mean continuity equation, a proper scale for the mean transverse flow in a turbulent plane jet is found as Vref=−δdUctr/dx, where δ is the jet half width and dUctr/dx is the decay rate of the mean axial velocity at the jet centerline. By seeking an admissible scaling for the mean axial momentum equation, a proper scale for the kinematic Reynolds shear stress is found as Ruv,ref=Uctr Vref, which is a mix of the velocity scales in the axial and transverse directions. Approximation functions for the scaled mean transverse flow and Reynolds shear stress are developed and found to agree well with experimental and numerical data. Similarities and differences between the scales of the mean transverse flow and Reynolds shear stress in turbulent plane jets and zero-pressure-gradient turbulent boundary layer flows are clarified.
Displacement field measurements in traditional and rotational diamond anvil cells
Authors: Pandey K. K. , Levitas Valery I.
Published: March 2021

A digital image correlation-based method has been developed to measure the displacement field during compression in a traditional diamond anvil cell (DAC) and torsion in rotational DAC (RDAC) employing ruby fluorescence microscopy imaging. The optical arrangements for these measurements are adaptable at any commercial or customized micro-confocal system used for in situ high-pressure Raman or ruby fluorescence spectroscopy. In this paper, we describe details of the setup developed at Iowa State University along with a few demonstrative measurements for a zirconium sample. In particular, under compression in DAC, no adhesion zone is found, and relative sliding increases almost linearly along the radius. During torsion in RDAC, actual angular displacement of the material is found to be 5 times smaller than the rotation angle of an anvil, which is routinely used in the definition of the plastic shear for the determination of stress–strain curves and plastic strain-induced kinetics of phase transformations and grain refinement in materials. Obtained displacements can be used as the boundary conditions for finite element method (FEM) simulations of processes in DAC and RDAC instead of hypothetical friction conditions. After iterative fitting of FEM simulations and all measured fields from x-ray diffraction and absorption experiments, this will allow us to more precisely determine contact friction conditions and material parameters in the constitutive equations for elastoplastic flow and strain-induced phase transformations.
Spin transport properties in Dirac spin gapless semiconductors Cr2X3 with high Curie temperature and large magnetic anisotropic energy
Authors: Feng Yulin , Liu Na , Gao Guoying
Published: March 2021

2D honeycomb-Kagome (HK) lattices have attracted extensive attention in recent years due to the peculiar electronic and magnetic properties such as the Dirac band, the half-metallicity, and the high Curie temperature. In this Letter, we theoretically investigate the spin transport properties of a recently proposed 2D Dirac spin gapless semiconductor (also known as a Dirac half-metal with zero energy gap in one spin channel) of the Cr2S3 monolayer with the HK lattice. The excellent spin filtering effect and negative differential resistance effect are found at a bias voltage, and interestingly, a temperature difference can also drive the spin filtering effect. These peculiar transport properties can be understood from the Dirac spin gapless semiconductivity and the spin-dependent transmission spectrum. In addition, we predict that, similar to Cr2S3 and Cr2Se3, 2D Cr2Te3 is also a Dirac spin gapless semiconductor with the above room-temperature Curie temperature and a large magneto-crystalline anisotropic energy (MAE). Under a tensile biaxial strain, the MAE can be greatly increased, and the easy magnetization axis is still along the in-plane. All these results are achieved by the first-principles combined with nonequilibrium Green's function method. The present work will stimulate theoretical and experimental studies on spintronic devices and spin caloritronic devices based on more 2D Dirac HK lattices.
Flow and heat transfer for a two-phase slug flow in horizontal pipes: A mechanistic model
Authors: Dong Chuanshuai , Qi Ronghui , Zhang Lizhi
Published: March 2021

In view of the practical importance of gas–liquid two-phase flow in many applications, such as chemical engineering, petroleum engineering, nuclear engineering, etc., a reliable model of flow and heat transfer for two-phase flow is of practical importance in the two-phase flow analysis. Among various two-phase flow regimes, slug flow is most complicated due to the intrinsic randomness and intermittency. This paper aims at developing a novel mechanistic model of flow and heat transfer for two-phase slug flow in horizontal pipes. First, a hydrodynamic model of two-phase slug flow is developed using the concept of slug unit cell. Then, a heat transfer model is deduced based on the hydrodynamic model. The overall heat transfer coefficient is integrated by the local heat transfer coefficients of liquid slug, liquid film, and elongated bubble. The newly developed mechanistic model is well validated by the experimental results. Finally, the dependence of the heat transfer performance on the overall flow parameters, such as superficial liquid velocity and superficial gas velocity, and the local flow parameters, such as slug frequency, pressure drop, void fraction, and ratio of slug length to unit cell length, is comprehensively investigated. The heat transfer enhancement of two-phase slug flow compared with single-phase flow is mainly attributed to the turbulence increase in liquid by the injection of air and the decrease in thermal boundary layer by the frequent alternation between the liquid slug and the elongated bubble.
Fluctuating magnetism of Co- and Cu-doped NaFeAs
Authors: Pelliciari Jonathan , Ishii Kenji , Xing Lingyi , Wang Xiancheng , Jin Changqing , Schmitt Thorsten
Published: March 2021

We report an x-ray emission spectroscopy study of the local fluctuating magnetic moment (μbare) in NaFe1−xCoxAs and NaFe1−xCuxAs. In NaFeAs, the reduced height of the As ions induces a local magnetic moment higher than BaFe2As2 despite lower TN and ordered magnetic moment. As NaFeAs is doped with Co, μbare is slightly reduced, whereas Cu doping leaves it unaffected, indicating a different doping mechanism: based on electron counting for Co, whereas impurity scattering dominates in the case of Cu. Finally, we observe an increase in μbare with temperature in all samples as observed in electron- and hole-doped BaFe2As2. Since both Co and Cu doping display superconductivity, our findings demonstrate that the formation of Cooper pairs is not connected with the complete loss of fluctuating paramagnetic moments.
Corrigendum to: A taxonomic revision of the Australasian genera Dracophyllum and Richea (Richeeae: Styphelioideae: Ericaceae)
Authors: Venter Stephanus
Published: March 2021

The genus Dracophyllum Labill. is revised, with a total of 61 species being recognised in four subgenera and two species (Dracophyllum minimum F.Muell. and D. strictum Hook.f.) are listed as incertae sedis. The genus Richea R.Br. is reduced to synonymy under Dracophyllum where it is divided into two new subgenera, namely, Dracophyllum subgenus Cystanthe (R.Br.) S.Venter and D. subgenus Richea (R.Br.) S.Venter. Replacement names published here are Dracophyllum laciniatum S.Venter, D. persistentifolium S.Venter and D. tasmanicum S.Venter, and new combinations published here are Dracophyllum alpinum (Menadue) S.Venter, D. continentis (B.L.Burtt) S.Venter, D. desgrazii (Hombr. ex Decne.) S.Venter, D. gunnii (Hook.f.) S.Venter, D. pandanifolia (Hook.f.) S.Venter, D. procerum (F.Muell.) S.Venter, D. sprengelioides (R.Br.) S.Venter and D. victorianum (Menadue) S.Venter. Nomenclature, descriptions, illustrations, photographs and distribution maps are provided for each species and lectotypes are designated where necessary. A key to the subgenera and keys to species within these are provided.
Phylogeographic patterns of the Australian grass trees (
Authors: McLay Todd G. B. , Ladiges Pauline Y. , Doyle Stephen R. , Bayly Michael J.
Published: March 2021

Despite the prevalence of sclerophyllous vegetation in Australia, there are few phylogeographic studies with continent-wide sampling addressing questions about the expansion of this vegetation post-aridification, and what physical barriers are associated with breaks in distribution or patterns of genetic structure. We used amplicon sequencing of plastid and nuclear DNA to investigate phylogeography of Xanthorrhoea. Despite extremely low genetic variation and widespread sharing of identical DNA sequence among multiple species and across significant geographic distance, haplotype analyses showed phylogeographic structure. Network analysis of six plastid loci and 137 samples identified 54 haplotypes (two common, 35 unique to single samples) in three geographic groups, south west and central Australia, northern–eastern Australia, and southern–eastern Australia, indicating restricted gene flow among regions. A nrDNA haplotype network of 152 samples showed less variation, with one haplotype being widespread, found in all three plastid geographic groups and 70% of species. nrDNA indicated four groups in a pattern, suggestive of at least two east–west divergences. A geographic area of a high haplotype diversity and divergence was identified in southern New South Wales, near the Southern Transition Zone. Our results showed the value of targeted amplicon sequencing and using multiple samples per species to identify both broad-scale and fine-scale biogeographic patterns.
Contrasting drivers of diversity in hosts and parasites across the tropical Andes
Authors: McNew Sabrina M. , Barrow Lisa N. , Williamson Jessie L. , Galen Spencer C. , Skeen Heather R. , DuBay Shane G. , Gaffney Ariel M. , Johnson Andrew B. , Bautista Emil , Ordoñez Paloma , Schmitt C. Jonathan , Smiley Ashley , Valqui Thomas , Bates John M. , Hackett Shannon J. , Witt Christopher C.
Published: March 2021

Significance

Interactions between hosts and parasites can reciprocally limit the ranges of individual species. But to what extent do they affect composition of communities? In the Andean biodiversity hot spot, we tested why host and parasite communities vary over spatial and temporal scales. Variation in rainfall was the predominant predictor of turnover for both hosts and parasites. However, the effects of species interactions were asymmetric: Parasite communities were strongly predicted by host communities, but not vice versa. Against conventional wisdom, climatic effects on parasite turnover were primarily local while biotic effects predominated regionally. Using our predictive models, we mapped faunal dissimilarity, illustrating hot spots of turnover that can guide conservation of host and parasite biodiversity.
Better outcomes for pest pressure, insecticide use, and yield in less intensive agricultural landscapes
Authors: Gagic Vesna , Holding Matthew , Venables William N. , Hulthen Andrew D. , Schellhorn Nancy A.
Published: March 2021

Significance

Increasing crop field sizes and decreasing seminatural area is believed to lead to increased pest pressure and insecticide use due to earlier and frequent pest immigration and fewer natural enemies. However, after decades of research on how landscape simplification affects pest pressure in crops, there is conflicting evidence. We show that smaller fields surrounded by landscapes with 20 to 30% seminatural area had delayed and reduced pest immigration and spraying while producing the highest yields. These findings reveal a previously untested link sequence starting from agricultural intensification through pest immigration and dynamics, to insecticide use and yield. Moreover, fine temporal analyses provide unique understanding of how these effects change over time and potential explanation for the inconsistencies in the literature.
Mono- and bilayer smectic liquid crystal ordering in dense solutions of “gapped” DNA duplexes
Authors: Gyawali Prabesh , Saha Rony , Smith Gregory P. , Salamonczyk Miroslaw , Kharel Prakash , Basu Soumitra , Li Ruipeng , Fukuto Masafumi , Gleeson James T. , Clark Noel A. , Jákli Antal , Balci Hamza , Sprunt Samuel
Published: March 2021

Significance

Liquid crystalline behavior of DNA has been studied for decades, yet only recently has one of the most fundamental liquid crystal phases—the smectic phase in which rod-like objects assemble into stacks of fluid layers—been demonstrated in this system. Here, we describe distinct bilayer and monolayer smectic phases in solutions of “gapped” DNA (GDNA), consisting of two DNA duplexes connected by a single strand. The smectic order is amphotropic (sensitive to temperature as well as concentration), and the layer structure is stable against significant perturbations of the GDNA construct (e.g., “gap” length variation and asymmetry in duplex lengths). The temperature sensitivity may provide a means to compare end-to-end interaction strengths between DNA duplexes at biologically relevant concentrations.
Background-independent composite gravity
Authors: Batz Austin , Erlich Joshua , Mrini Luke
Published: March 2021

We explore a background-independent model of composite gravity. In a mean-field approximation, the vacuum expectation value of the composite metric satisfies Einstein’s equations (with corrections) as a consistency condition. A gravitational interaction then emerges in vacuum correlation functions. The action remains diffeomorphism invariant even as perturbation theory is organized about the dynamically selected vacuum spacetime. We discuss the role of nonphysical clock and rod fields in the analysis, the identification of physical observables, and the generalization to other theories including the standard model.
Thermal effect induced dynamically lasing mode tuning in GaN whispering gallery microcavities
Authors: Qin Feifei , Zhu Gangyi , Wang Ru , Wang Xiaoxuan , Lu Junfeng , Yang Ying , Qiu Guoqing
Published: March 2021

Thermal modulated real-time wavelength tuning of semiconductors has shown great potential for GaN-based sensors or photo-electricity modulators. Herein, we study the temperature mediated photoluminescence (PL) properties in GaN materials via PL and time-resolved PL measurement in situ and synchronously. We then broaden the phenomenon to lasing mode tuning of whispering gallery cavities. To understand the underling mechanism, time, and frequency domain properties of spontaneous emission from GaN film, amplified spontaneous emission and stimulated emission from floating GaN microdisks in a temperature region from 0 °C to 50 °C are compared. According to analysis of temperature-related changes in the central wavelength, peak intensity, full width at half maximum (FWHM), and carrier dynamics, the thermal controlled PL properties of various structures are well understood. Material structure-related changes in exciton combination channels and temperature-related changes in central wavelength, peak intensity, FWHM, and exciton combination times are observed. Finally, real-time lasing mode modulation in floating GaN microdisks is realized. Our work reveals the lasing tuning method in situ, implying a promising strategy for fabricating high performance thermal-optic modulation devices.
Higgs Inflation with four-form couplings
Authors: Ouseph C J , Cheung Kingman
Published: March 2021

We consider a new inflationary model in which an antisymmetric tensor field A

νρσ
and its four-form field strength F

μνρσ
= 4∂[μ

A

νρσ] are coupled to the scalar sector of the standard model and to the Ricci scalar

R

. The four-form field induces modifications to the Higgs self-coupling constant, the cosmological constant, and the non-minimal coupling constant, which results in the modification to the inflaton potential. We also show that there is no need for the Higgs-gravity coupling in the presence of four-form gravity interaction, but still can produce the right amount of density perturbation for inflation.
Electronic spectrum of Kekulé patterned graphene considering second neighbor-interactions
Authors: Andrade Elías , Naumis Gerardo G , Carrillo-Bastos R
Published: March 2021

The effects of second-neighbor interactions in Kekulé-Y patterned graphene electronic properties are studied starting from a tight-binding Hamiltonian. Thereafter, a low-energy effective Hamiltonian is obtained by projecting the high energy bands at the Γ point into the subspace defined by the Kekulé wave vector. The spectrum of the low energy Hamiltonian is in excellent agreement with the one obtained from a numerical diagonalization of the full tight-binding Hamiltonian. The main effect of the second-neighbour interaction is that a set of bands gains an effective mass and a shift in energy, thus lifting the degeneracy of the conduction bands at the Dirac point. This band structure is akin to a ‘pseudo spin-one Dirac cone’, a result expected for honeycomb lattices with a distinction between one third of the atoms in one sublattice. Finally, we present a study of Kekulé patterned graphene nanoribbons. This shows that the previous effects are enhanced as the width decreases. Moreover, edge states become dispersive, as expected due to second neighbors interaction, but here the Kek-Y bond texture results in an hybridization of both edge states. The present study shows the importance of second neighbors in realistic models of Kekulé patterned graphene, specially at surfaces.
How to study a persistent active glassy system
Authors: Mandal Rituparno , Sollich Peter
Published: March 2021

We explore glassy dynamics of dense assemblies of soft particles that are self-propelled by active forces. These forces have a fixed amplitude and a propulsion direction that varies on a timescale τ
p, the persistence timescale. Numerical simulations of such active glasses are computationally challenging when the dynamics is governed by large persistence times. We describe in detail a recently proposed scheme that allows one to study directly the dynamics in the large persistence time limit, on timescales around and well above the persistence time. We discuss the idea behind the proposed scheme, which we call ‘activity-driven dynamics’, as well as its numerical implementation. We establish that our prescription faithfully reproduces all dynamical quantities in the appropriate limit τ
p → ∞. We deploy the approach to explore in detail the statistics of Eshelby-like plastic events in the steady state dynamics of a dense and intermittent active glass.
Length-dependent carbon nanotube film structures and mechanical properties
Authors: Zhang Liwen , Ma Xiaolong , Zhang Yongyi , Bradford Philip D , Zhu Yuntian T
Published: March 2021

We investigated the microstructures of carbon nanotube (CNT) films and the effect of CNT length on their mechanical performance. 230 μm-, 300 μm-, and 360 μm- long CNTs were grown and used to fabricate CNT films by a winding process. Opposite from the length effect on CNT fibers, it has been found that the mechanical properties of the CNT films decrease with increasing CNT length. Without fiber twisting, short CNTs tend to bundle together tightly by themselves in the film structure, resulting in an enhanced packing density; meanwhile, they also provide a high degree of CNT alignment, which prominently contributes to high mechanical properties of the CNT films. When CNTs are long, they tend to be bent and entangled, which significantly reduce their packing density, impairing the film mechanical behaviors severely. It has also been unveiled that the determinant effect of the CNT alignment on the film mechanical properties is more significant than that of the film packing density. These findings provide guidance on the optimal CNT length when attempting to fabricate high-performance macroscopic CNT assemblies.
Temperature-dependent electrical resistivity of macroscopic graphene nanoplatelet strips
Authors: Sibilia S , Bertocchi F , Chiodini S , Cristiano F , Ferrigno L , Giovinco G , Maffucci A
Published: March 2021

This paper studies the temperature dependence of the electrical resistivity of low-cost commercial graphene-based strips, made from a mixture of epoxy and graphene nanoplatelets. An equivalent homogenous resistivity model is derived from the joint use of experimental data and simulation results obtained by means of a full three-dimensional (3D) numerical electrothermal model. Three different types of macroscopic strips (with surface dimensions of cm2) are analyzed, differing in their percentage of graphene nanoplatelets. The experimental results show a linear trend of resistivity in a wide temperature range (−60°C to +60°C), and a negative temperature coefficient . The derived analytical model of temperature-dependent resistivity follows the simple law commonly adopted for conventional conducting materials, such us copper. The model is then validated by using the graphene strips as heating elements by exploiting the Joule effect. These results suggest that such materials can be used as thermistors in sensing or heating applications.
Thermal expansion of water in the science lab—advantages and disadvantages of different experimental setups
Authors: Wagner S , Maut C , Priemer B
Published: March 2021

When the thermal expansion of water is discussed in school or university lessons, functional relationships are often inferred without considering measurement uncertainties. Moreover, in some learning materials, the goal of experimentation and the experimental setup do not match. This creates the wrong picture of scientific practice. In this article we compare five experimental setups with different complexity to investigate the relationship between heat and volume change of water. We take different goals of experimentation into account, as well as measurement uncertainties. We compare the different experimental setups, show advantages and limitations, and give advises for their implementation in the classroom. By doing so, we demonstrate that thermal expansion can be very well adopted for applying scientific practice in school. It is shown that the more complex the experimental setup, the more precise the quantitative significance of the functional relationship.
Shedding synchrotron light on teacher training
Authors: Acioly V , Paiva T , Azevedo G , Rocha T , Picoreti R , Santos A C F
Published: March 2021

This paper aims to introduce the initiative Escola Sirius para Professores do Ensino Médio (ESPEM, Sirius School for High School Teachers) hosted by the Centro Nacional de Pesquisa em Energia e Materiais (CNPEM, Brazilian Center for Research in Energy and Materials) and supported by the Sociedade Brasileira de Física (SBF, Brazilian Physical Society). The program covers the principles and applications of synchrotron radiation with expositive lectures, discussions, and visits to the CNPEM facilities. By using semi structured interviews, a remote focal group and surveys to find out the impact of ESPEM, we observed that the teachers shared their experience and disseminate the acquired knowledge with their peers and students.