The flow rate of granular materials through an orifice

The flow rate of grains through large orifices is known to be dependent on its diameter to a 5/2 power law. This relationship has been checked for big outlet sizes, whereas an empirical fitting parameter is needed to reproduce the behavior for small openings. In this work, we provide experimental data and numerical simulations covering a wide span of outlet sizes, both in three- and two-dimensions. This allows us to show that the laws that are usually employed are satisfactory only if a small range of openings is considered. We propose a new law for the mass flow rate of grains that correctly reproduces the data for all the orifice sizes, including the behaviors for very large and very small outlet sizes.     

Enhanced Dynamic Adhesion in Nematic Liquid Crystal Elastomers

Smart adhesives that undergo reversible detachment in response to external stimuli enable a wide range of applications in household products, medical devices, or manufacturing. Here, a new model system for the design of smart soft adhesives that dynamically respond to their environment is presented. By exploiting the effect of dynamic soft elasticity in nematic liquid crystal elastomers (LCE), the temperature‐dependent control of adhesion to a solid glass surface is demonstrated. The adhesion strength of LCE is more than double in the nematic phase, in comparison to the isotropic phase, further increasing at higher detachment rates. The static work of adhesion, related to the interfacial energy of adhesive contact, is shown to change very little within the explored temperature range. Accordingly, the observed enhanced adhesion in the nematic phase is primarily attributable to the increased internal energy dissipation during the detachment process. This adhesion effect is correlated with the inherent bulk dynamic‐mechanical response in the nematic LCE. The reported enhanced dynamic adhesion can lead to the development of a new class of stimuli‐responsive adhesives.     

Mechanical response of a soft rubber compound compressed at various strain rates

Rate-dependent material constitutive behavior models are needed in numerical simulations of shock-mitigation structures. In this research, compressive stress–strain response of a soft rubber compound is obtained experimentally at quasi-static, intermediate and high strain rates under uniaxial-stress and uniaxial-strain loading states. Kolsky bars with modifications for characterizing soft materials and a long Kolsky bar are used to conduct the dynamic experiments, while an MTS load frame is used for conducting experiments at quasi-static rates. Compression experiments are conducted at each decade in the strain-rate scale without any gap typically seen in the intermediate range. The experimental results show significant strain-rate effects on the mechanical behavior of this soft material, which are summarized via a rate-dependent constitutive model.     

Characterization of CuInS2 thin films for solar cells prepared by spray pyrolysis

We have made a study of the chemical composition, the electrical, the optical and the structural properties of polycrystalline CuInS₂ thin films prepared by spray pyrolysis to be used for thin film solar cells. These films were deposited starting from aqueous solutions with different chemical compositions ([Cu]/[In] and [S]/[Cu] ratios) and at different substrate temperatures. In all cases, the material is p-type with grains preferentially oriented in the (112) direction of the sphalerite structure. The electro-optical properties show a very strong dependence on the [Cu]/[In] ratio in the solution. Films with copper excess have smaller resistivity and better crystallinity than those which are stoichiometric or have indium excess. The results obtained in this work show the possibility of having CuInS₂ thin films with a wide range of resistivity, a fact that could be important for making solar cells based on this material.     

Acoustic scattering in a waveguide with a height discontinuity bridged by a membrane: a tailored Galerkin approach

This article is concerned with the reflection and the transmission of fluid–structure-coupled waves at the junction between two flexible waveguides of different heights. Unlike previous studies, in which the waveguides are joined at the height discontinuity by a rigid or soft strip, in this study, the height discontinuity is bridged by a membrane. The aims are first to develop a solution method that enables a wide range of conditions to be applied at the edges of the bridging membrane and then to ascertain the effects that these have on the reflected and the transmitted fields. Numerical results are presented which confirm that that the conditions applied at the edges of the bridging membrane do have significant effects on the reflected and the transmitted components of power.     

Relaxation of perturbed axial segregation bands

The mixture of grains inside a rotating horizontal cylinder segregate into alternating bands of big and small grains along the rotation axis at appropriate conditions. However, the response of these bands to perturbations is largely unexplored. Here, we report that, when deformed by a weak localized perturbation from one end of the cylinder, the axial bands relax to their original positions after several rotations of the cylinder. Considering that relaxation in other physical systems is related to their structure and dynamics, this robust phenomenon could have interesting implications on our understanding of granular materials.     

Backscattered electron imaging of cementitious microstructures: understanding and quantification

During the last 20 years, backscattered electron imaging of polished surfaces has become well established as a method for the study of cement and concrete microstructures. The technique has many advantages, including the visualisation of representative cross-sections over a wide range of magnifications and reproducible contrast dependent on atomic number. Nevertheless the limitations of observing a two-dimensional section of a three-dimensional structure must be borne in mind.

In this paper, the general microstructural features of hydrated cement pastes are described. Although the amount of aluminate phase (“C₃A”) in cement is comparatively minor, it plays an important role in determining many of the microstructural features of cement paste microstructure, for example in the formation of “Hadley” grains.

Despite the very heterogeneous nature of cement paste, it is important to be able to derive quantitative measures if the relationships between microstructure and properties are to be understood. The possibilities to quantify BSE images are described. The interface between paste and aggregates in concrete is particularly variable, but average features can be measured, which help to understand the processes of packing cement grains, which gives rise to this region. Finally an example of the potential for BSE images to study concrete durability is given.     

Grain refinement and mechanical behavior of a magnesium alloy processed by ECAP

Equal-channel angular pressing (ECAP) is an effective tool for refining the grain structure of magnesium alloys and improving the ductility at moderate temperatures. However, grain refinement in these alloys differs from other metals because new grains are formed along the boundaries of the initial structure and these newly formed grains slowly spread to consume the interiors of the larger grains in subsequent passes. A model is presented for grain refinement in magnesium alloys processed by ECAP based on the principles of dynamic recrystallization where new fine grains are formed along the initial boundaries and along twin boundaries. This model provides an explanation for a wide range of experimental data and introduces the concept of grain size engineering for achieving selected material properties in magnesium alloys.     

Interfacial Superconductivity on the Topological Semimetal Tungsten Carbide Induced by Metal Deposition

Interfaces between materials with different electronic ground states have become powerful platforms for creating and controlling novel quantum states of matter, in which inversion symmetry breaking and other effects at the interface may introduce additional electronic states. Among the emergent phenomena, superconductivity is of particular interest. Here, by depositing metal films on a newly identified topological semimetal tungsten carbide (WC) single crystal, interfacial superconductivity is obtained, evidenced from soft point-contact spectroscopy. This very robust phenomenon is demonstrated for a wide range of metal/WC interfaces, involving both nonmagnetic and ferromagnetic films, and the superconducting transition temperatures are surprisingly insensitive to the magnetism of thin films. This method offers an opportunity to explore the long-sought topological superconductivity and has potential applications in topological-state-based spin devices.     

Transformation properties and microstructure of sputter-deposited Ni-Ti shape memory alloy thin films

The influence of annealing parameters on the martensitic phase transformation in sputter-deposited Ti rich Ni-Ti films is systematically studied by differential scanning calorimetry and by transmission electron microscopy. The annealing temperature range extends from the crystallization temperature of the films up to 900°C. For increasing temperature, multiple phase transformations, transformations via an R-phase or direct martensite/austenite transformations are observed. A similar behavior is found for increasing annealing time. Related changes of the film microstructure, such as the strongly varying distribution of round Ti2Ni precipitates in the grains, are analyzed. Transformation temperatures could be shifted over a wide range by adjusting the film composition from 48 to 54 at.% Ti. The corresponding transformation curves, grain structure as well as nature and amount of precipitates were investigated. No subsequent annealing process is required for films deposited on substrates heated above about 500°C. In this case, the as-deposited films have a very fine-grained and homogeneous microstructure.     

Carbon Dot-Based Hydrogels: Preparations,Properties, and Applications (Small 17/2023)

Hydrogels have extremely high moisture content, which makes it very soft and excellently biocompatible. They have become an important soft material and have a wide range of applications in various fields such as biomedicine, bionic smart material, and electrochemistry. Carbon dot (CD)-based hydrogels are based on carbon dots (CDs) and auxiliary substances, forming a gel material with comprehensive properties of individual components. CDs embedding in hydrogels could not only solve their aggregation-caused quenching (ACQ) effect, but also manipulate the properties of hydrogels and even bring some novel properties, achieving a win–win situation. In this review, the preparation methods, formation mechanism, and properties of CD-based hydrogels, and their applications in biomedicine, sensing, adsorption, energy storage, and catalysis -are summarized. Finally, a brief discussion on future research directions of CD-based hydrogels will be given.     

A nonlinear model of piezoelectric polycrystalline ceramics under quasi-static electromechanical loading

Nonlinear characteristics of tetragonal perovskite type polycrystalline piezoelectric ceramics under electromechanical loading are theoretically simulated using a threedimensional micromechanical model. The model consists of many differently oriented grains which form the bulk material. Uni-axial, quasi-static loading is applied in the simulations. The calculations which are based on a linear constitutive and nonlinear domain switching model are performed at each grain. All grains are assumed to be statistically random oriented at the virgin state.The behavior of piezoelectric ceramics under constant compressive stress which is applied in the same direction of the cyclic electric field is investigated. The macroscopic response of the bulk ceramics to the applied loading is predicted by averaging the response of individual grains. It is assumed that a domain or a microstructure switches if the reduction in potential energy of the polycrystal exceeds a threshold of critical energy per unit volume of the material. Due to intergranular effects domain switching may occur in reality even for those grains, for which the critical energy level is not reached. This effect is modeled by introducing a probability for domain switching as a function of the actual energy level related to the critical energy level. By use of the probability functions, it is possible to model the nonlinearity even in a small electromechanical loading range. The effect of different probability functions, and material parameters are also analysed. The results of simulations have been compared with experimental data from literature.     

Contributory Factors to the Spread of Sensitivities Among Grains of a Single Size

Critical consideration is given to the possibility that sensitivity spread among grains of a single size can be attributed to exposure fluctuations. It is concluded that in general the contribution of this effect to sensitivity spread is small.

In view of the success of the latent-image model of Bayer and Hamilton in accounting for a wide range of scnsitometric effects it must be accepted that a major contribution to spread arises from randomness in latent-image events. In such modelling it has been assumed that grains are identical, and comparisons between theory and experiment have been effected for emulsions in which differences between grains are likely to be small.

However, in the majority of emulsions used in practice, grains of similar absorptivity may be dillerent in shape and in the number and location of relevant features such as twin planes. Grains of similar size and shape may exhibit differences in detailed structure and it becomes reasonable to suppose that in the general case sensitivity spread may in part represent differences of inherent sensitivity between grains. Analysis of experimental data supports this conclusion.     

Recent progress in medium-Mn steels made with new designing strategies,a review

After summarizing the relevant researches on the medium Mn steels in references, two new targets on the tensile properties have been defined. One is that both transformation-induced(TRIP) and twinninginduced plasticity(TWIP) could be realized for the steel with a relatively low Mn content, which exhibits the similar tensile properties to the classical TWIP steels with higher Mn content. The other is to achieve ultrahigh ultimate tensile strength(1.5 GPa) without sacrificing formability. To achieve these goals,new designing strategies was put forward for compositions and the processing route. In particular, warm rolling was employed instead of the usual hot/cold rolling process because the former can produce a mixture of retained austenite grains with different morphologies and sizes via the partial recrystallization. Consequently, the retained austenite grains have a wide range of mechanic stability so that they can transform to martensite gradually during deformation, leading to enhanced TRIP effect and then improved mechanic properties. Finally, it is succeeded in manufacturing these targeted medium Mn steels in laboratory, some of them even exhibit better tensile properties than our expectation.     

A formulation to model the nonlinear viscoelastic properties of the vascular tissue

Nearly all soft tissues, including the vascular tissue, present a certain degree of viscoelastic material response, which becomes apparent performing multiple relaxation tests over a wide range of strain levels and plotting the resulting stress relaxation curves, nonlinear viscoelasticity of the tissue. Changes in relaxation rate at each strain may occur at multiple strain levels. A constitutive formulation considering the particular features of the vascular tissue, such as anisotropy, together with these nonlinear viscoelastic phenomena is here presented and used to fit stress?Cstretch curves from experimental relaxation tests. This constitutive model was used to fit several data set of in vitro experimental stress relaxation tests performed on ovine and porcine aorta. The good fitting of the experimental data shows the capability of the model to reproduce the viscoelastic response of the vascular tissue.     

Suction of splash after impact on dry quick sand

It is well known that a splash occurs when an object impacts at high velocity on a liquid??s surface. If the impact is fast enough, surface tension and air pressure gradients pull the crown-shape splash all the way towards the axis of symmetry, making it to collapse and seal the surface. In this paper we show that splash and surface sealing are also observed in impacts on soft, dry sand. We observe influence of air pressure and grains size on the shape of the splash. By tracking individual grains using high-speed imaging we calculate their acceleration, which results from gravity and drag forces. Assuming friction drag parallel, and pressure drag perpendicular to the direction of motion of grains we estimate the friction and pressure drag contributions to the drag force. Our results support the idea that pressure drag from Bernoulli effect is at the origin of the surface seal.     

A Fourier-based algorithm for modelling aberrations in HETE-2''s imaging system

The High-Energy Transient Explorer, launched in October 2000, is a satellite experiment dedicated to the study of γ-ray bursts in a very wide energy range from soft X-ray to γ-ray wavelengths. The intermediate X-ray range (2–30 keV) is covered by the Wide-field X-ray Monitor (WXM), a coded aperture imager. In this article, an algorithm for reconstructing the positions of γ-ray bursts is described, which is capable of correcting systematic aberrations to approximately 1′ throughout the field of view. Functionality and performance of this algorithm have been validated using data from Monte-Carlo simulations as well as from astrometric observations of the X-ray source Scorpius X-1.     

Linear elasticity of planar Delaunay networks. III: Self-consistent approximations

Summary Two-phase Delaunay and regular triangular networks, with randomness per vertex, provide generic models of granular media consisting of two types of grains — soft and stiff. We investigate effective macroscopic moduli of such networks for the whole range of area fractions of both phases and for a very wide range of stiffnesses of both phases. Results of computer simulations of such networks under periodic boundary conditions are used to determine which of several different self-consistent models can provide the best possible approximation to effective Hooke's law. The main objective is to find the effective moduli of a Delaunay network as if it was a field of inclusions, rather than vertices connected by elastic edges, without conducting the computer-intensive calculations of large windows. First, we report on the dependence of effective Poisson's ratio onp for a single-phase Delaunay network with all the spring constantsk assigned according tok=l ^p. In case of two-phase media, it is found that the Delaunay network is best approximated by a system of ellipses perfectly bonded to a matrix in a symmetric self-consistent formulation, while the regular network is best approximated by a circular inclusion-matrix model. These two models continue to be adequate up to the point of percolation of holes, but the reverse situation of percolation of rigid inclusions is better approximated by the ellipses model in an asymmetric formulation. Additionally, we give results of calculation of Voigt and Reuss bounds of two-dimensional matrix-inclusion composites with springy interfaces.     

Sol-gel transition in agar-gelatin mixtures studied with transient elastography

Using the shear wave propagation in solids, the transient elastography technique has been developed to assess the elastic properties of soft tissues. Here, a new approach of transient elastography allows assessing the viscoelastic properties of soft tissues. In this paper, the method is used to follow-up the sol-gel transition of an agar-gelatin mixture noninvasively. The shear wave velocity and shear wave attenuation through the mixture were continuously monitored in the audible range of frequencies (from 50 Hz to 200 Hz). The observed changes in velocities and attenuations as a function of frequency confirmed the validity of the Voigt's model to describe the gel at its stable mechanical state. By a simple inverse problem approach, based on the one-dimensional (1-D) Helmholtz equation, the elasticity and the viscosity of such a mixture were recovered as a function of time. The results obtained are in good agreement with the literature and theoretical predictions. Overall, they demonstrate the high sensitivity of the transient elastography measurements to the rheological parameter changes in agar-gelatin mixtures during gelation.     

二维应力对PbTiO_3多晶薄膜相变的影响

用热力学唯象埋论研究了二维应力对PbTiO3多晶铁电薄膜相转变的影响。理论分析表明，在二维应力的作用下，薄膜的相转变温度Tc将发生移动，该移动量△T的大小和正负不仅与应力的性质有关，而且与晶粒的取向有关。经推导给出了△T与晶粒取向之间的关系式。对晶粒呈随机取向的多晶薄膜，在二维张应力的作用下，平均Tc将向高温移动，并且相转变温区将展宽。