Negative Bulk Research Articles

An efficient multiscale method for subwavelength transient analysis of acoustic metamaterials.

A reduced-order homogenization framework is proposed, providing a macro-scale-enriched continuum model for locally resonant acoustic metamaterials operating in the subwavelength regime, for both time and frequency domain analyses. The homogenized continuum has a non-standard constitutive model, capturing a metamaterial behaviour such as negative effective bulk modulus, negative effective density and Willis coupling. A suitable reduced space is constructed based on the unit cell response in a steady-state regime and the local resonance regime. A frequency domain numerical example demonstrates the efficiency and suitability of the proposed framework.This article is part of the theme issue 'Current developments in elastic and acoustic metamaterials science (Part 2)'.

Low-frequency noise attenuation through hybrid perforated hemispherical shells and membrane-type acoustic metamaterial

We propose a novel hybrid acoustic metamaterial that can achieve broadband sound insulation below 452 Hz and almost perfect sound absorption at 864 Hz. The metastructure was fabricated using additive manufacturing. Finite element method simulations were used to study the acoustic properties of the fabricated metamaterials. An equivalent electrical circuit was built using an electro-acoustic analogy to evaluate the sound absorption coefficient. The unique design of this meta-structure possesses two resonant frequencies. Low-frequency sound insulation is found due to the effective negative density at frequencies lower than the cutoff frequency of the elastic membrane. In contrast, a negative effective bulk modulus is the reason behind the broadband sound absorption. The theoretical and simulation results were validated through experiments. Experiments carried out showed an overall average sound transmission loss of 26.31 dB between 50 and 1600 Hz and 24.72 dB in the low-frequency zone (<400 Hz). Furthermore, over 69% of the sound intensity is absorbed in the 500–1000 Hz frequency range. The designed meta-structure exhibits broadband effective negative density below 452 Hz and effective negative bulk modulus from 864 to 1220 Hz. The design specifies a sample thickness of 3.8 cm, corresponding to a subwavelength thickness of approximately λ/10. As a result, the developed meta-structure can potentially be employed for sound insulation and absorption at low frequencies in the aerospace and automotive industries and in-room acoustic applications.

3D observations provide striking findings in rubber elasticity

The mechanical response of rubbers has been ubiquitously assumed to be only a function of the imposed strain. Using innovative X-ray measurements capturing the three-dimensional spatial volumetric strain fields, we demonstrate that rubbers and indeed many common engineering polymers undergo significant local volume changes. But remarkably, the overall specimen volume remains constant regardless of the imposed loading. This strange behavior which also leads to apparent negative local bulk moduli is due to the presence of a mobile phase within these materials. Combining X-ray tomographic observations with high-speed radiography to track the motion of the mobile phase, we have revised classical thermodynamic frameworks of rubber elasticity. The work opens broad avenues to understand not only the mechanical behavior of rubbers but a large class of widely used engineering polymers.

Fermions, quantum gravity, and holography in two dimensions

We study a model comprising N flavors of Kähler Dirac fermion propagating on a triangulated two-dimensional disk which is constrained to have a negative average bulk curvature. Dirichlet boundary conditions are chosen for the fermions. Quantum fluctuations of the geometry are included by summing over all possible triangulations consistent with these constraints. We show in the limit N→∞ that the partition function is dominated by a regular triangulation of two-dimensional hyperbolic space. We use strong coupling expansions and Monte Carlo simulation to show that in this limit boundary correlators of the fermions have a power law dependence on boundary separation as one expects from holography. However, we argue that this behavior breaks down for any finite number of massive fields in the thermodynamic limit and quantum fluctuations of the bulk geometry drive the theory into a nonholographic phase. In contrast, for massless fermions, we find evidence that the boundary is conformal even for finite N. This is consistent with theoretical results in quantum Liouville theory. Published by the American Physical Society 2024

Bulk viscous late acceleration under near equilibrium conditions in f(R, T) gravity with mixed dark matter

Numerous studies have tried to explain the observed late acceleration of the Universe as being caused by the bulk viscosity associated with the dark matter (DM) component. However, for driving the said accelerated expansion, all such models require a violation of near equilibrium conditions (NECs) associated with the background viscous theory. But recently, it was found that, with the aid of a cosmological constant, it is possible to maintain NEC for the bulk viscous warm DM during certain evolutionary epochs of the Universe. Nevertheless, this negated the possibility of having a ‘solely’ viscous-driven late acceleration in Einstein gravity within the NEC limit. In the present study, we investigate a model of the Universe composed of mixed DM components, with viscous DM, and inviscid cold DM as its constituents, in the context of R+2λTvm gravity, and show that the model predicts late acceleration by satisfying NEC, critical energy condition and second law of thermodynamics (SLT) throughout the evolution, even in the absence of a cosmological constant. One intriguing feature observed in this model is the possibility of having a negative bulk viscous coefficient and yet satisfying the SLT. Finally, by applying both theoretical and observational constraints on the model parameters, we determined the best-fit values of model parameters and thereby analyzed the evolutionary behavior of some relevant cosmological observables.

Lightweight soundproofing meta-panel for two separate broadband noises

We propose a lightweight soundproofing meta-panel, with a multi-scale lattice structure sandwiched between thin membranes, effectively suppressing two separate broadband noises. By adjusting the geometrical parameters, the meta-panel is designed to induce negative effective mass density and negative effective bulk modulus for two distinct target frequency bands: 100–500 Hz and 1000–1500 Hz, thereby achieving high transmission loss that outperforms the mass density law. A meta-panel sample is fabricated via 3D printing technology and we evaluate its sound insulation performance by measuring transmission loss and insertion loss in an impedance tube and an anechoic chamber. Experimental measurements show that the meta-panel can exhibit excellent insulation performance while being significantly lighter in weight (about 5%) compared to a steel plate, and furthermore, it effectively suppresses low-frequency road noise and high-frequency motor noise from an actual electric car.

Improved voice acquisition using acoustic metamaterials

Voice acquisition and recognition are important, because sound is the most convenient and efficient media to realize seamless and hands-free human–machine interaction. Many applications, such as smart phones, humanoid robots, smart home, autonomous vehicles, and Internet of Things, demand for high-quality voice acquisition, especially in a noisy environment. In this report, we show that the evanescent portion of human voice can be amplified by setting an acoustic metamaterial in front of a microphone; thus, the signal to noise ratio can be enhanced remarkably. Such an effect is demonstrated through a double negative acoustic metamaterial slab with simultaneous negative density and bulk modulus induced by coexisting and coupled Helmholtz and Fabry–Pérot resonances. This work provides a passive and physical means, rather than the traditional active and algorithmic method, to improve the quality of voice sound acquisition.

Estimating effective acoustic properties of various configuration of perforated panels

Acoustic metamaterial attains uncommon material properties over natural material such as negative effective mass density, negative effective bulk modulus, or both. To start with, the present research demonstrates and establishes robust method to estimate and measure acoustic metamaterial properties of a Helmholtz resonator analytically using transfer matrix (TM) method and experimentally in detail. The proposed method extracts the reflection and transmission coefficients from corresponding TM to evaluate the effective acoustic metamaterial properties. For a single Helmholtz resonator, the effective bulk modulus has been observed negative; however, the effective density remains positive. In order to attain double negative material properties, the perforated panel (PP) and microperforated panel (MPP) have been introduced in parallel to resonator. The 1D electro-acoustic modeling has been carried out for all configurations to estimate effective properties analytically. Successively, the experiments have been conducted to measure the effective properties from acoustic metamaterial prospective. The analytical and experimental investigations on five different cases reveal that one can attain negative effective compressibility and effective density by cascading two PPs or MPPs with a finite duct. Moreover, by backing a Helmholtz resonator from both sides with two PPs or MPPs in parallel, one can attain double negative properties comparatively higher than earlier configuration in low-frequency zone in broadband. In total, eight different configurations have been investigated analytically and experimentally, where it has been demonstrated that the proposed compact model is more effective than a finite array of same Helmholtz resonators.

Evidence for abundant organic matter in a Neoarchean banded iron formation

Abstract Microbial Fe(II) oxidation has been proposed as a major source of Fe minerals during deposition of banded iron formations (BIFs) in the Archean and Proterozoic Eons. The conspicuous absence of organic matter or graphitic carbon from BIFs, however, has given rise to divergent views on the importance of such a biologically mediated iron cycle. Here, we present mineral associations, major element concentrations, total carbon contents and carbon isotope compositions for a set of lower amphibolite-facies BIF samples from the Neoarchean Zhalanzhangzi BIF in the Qinglonghe supracrustal sequence, Eastern Hebei, China. Graphite grains with crystallization temperatures (~470 °C) that are comparable to that predicted for the regional metamorphic grade are widely distributed, despite highly variable iron (12.9 to 54.0 wt%) and total organic carbon (0.19 to 1.10 wt%) contents. The crystalline graphite is interpreted to represent the metamorphosed product of syngenetic bio-mass, based on its co-occurrence with apatite rosettes and negative bulk rock δ13Corganic values (–23.8 to –15.4‰). Moreover, the crystalline graphite is unevenly distributed between iron- and silica-rich bands. In the iron-rich bands, abundant graphite relicts are closely associated with magnetite and/or are preserved within carbonate minerals (i.e., siderite, ankerite, and calcite) with highly negative bulk rock δ13Ccarb values (–16.73 to –6.33‰), indicating incomplete reduction of primary ferric (oxyhydr) oxides by organic matter. By comparison, only minor graphite grains are observed in the silica-rich bands. Normally, these grains are preserved within quartz or silicate minerals and thus did not undergo oxidation by Fe(III). In addition, the close association of graphite with iron-bearing phases indicates that ferric (oxyhydr)oxides may have exerted a first order control on the abundance of organic matter. Combined, the biological oxidation of Fe(II) in the oceanic photic zone and subsequent burial of ferric (oxyhydr)oxides and biomass in sediments to form BIFs, suggests that a BIF-dependent carbon cycle was important in the Archean Eon. Although significant re-adsorption of phosphorus to ferric (oxyhydr)oxides and the formation of authigenic phosphate minerals at the sediment-water interface would be expected, oxidation of biomass in BIFs may have recycled at least a portion of the P (and other nutrients) released from reactions between organic matter and ferric (oxyhydr)oxides to the overlying water column, potentially promoting further primary productivity.

Acoustic propagation through a duct metamaterial attached with periodic multi-resonant cavities

Presently, noise pollution in ducts is considered seriously, and numerous studies concerning sound absorption in pipes are ongoing globally. Aiming at the difficulty to eliminate the low-frequency noise in pipeline, a multi-resonant cavity duct metamaterial is proposed. The acoustic properties of the metamaterial are calculated using the energy method based on the Hamilton principle and the transfer matrix methodology. The result demonstrates that the duct metamaterial, due to its band gaps and negative bulk modulus, obtains several sound absorption peaks and is advantageous to sound elimination in low frequency. The good acoustic characteristics of the duct metamaterial for sound insulation can be attributed to the plate-cavity interaction. Parametric analysis of the proposed structure is also carried out. This study demonstrates the application prospects of duct metamaterials for multi-frequency sound absorption in engineering practice.

Prognostic Impact of Unsupervised Early Assessment of Bulk and Leukemic Stem Cell Measurable Residual Disease in Acute Myeloid Leukemia.

Acute myeloid leukemias (AML) are clonal diseases that develop from leukemic stem cells (LSC) that carry an independent prognostic impact on the initial response to induction chemotherapy, demonstrating the clinical relevance of LSC abundance in AML. In 2018, the European LeukemiaNet published recommendations for the detection of measurable residual disease (Bulk MRD) and suggested the exploration of LSC MRD and the use of multiparametric displays. We evaluated the performance of unsupervised clustering for the post-induction assessment of bulk and LSC MRD in 155 patients with AML who received intensive conventional chemotherapy treatment. The median overall survival (OS) for Bulk+ MRD patients was 16.7 months and was not reached for negative patients (HR, 3.82; P < 0.0001). The median OS of LSC+ MRD patients was 25.0 months and not reached for negative patients (HR, 2.84; P = 0.001). Interestingly, 1-year (y) and 3-y OS were 60% and 39% in Bulk+, 91% and 52% in Bulk-LSC+ and 92% and 88% in Bulk-LSC-. In this study, we confirm the prognostic impact of post-induction multiparametric flow cytometry Bulk MRD in patients with AML. Focusing on LSCs, we identified a group of patients with negative Bulk MRD but positive LSC MRD (25.8% of our cohort) with an intermediate prognosis, demonstrating the interest of MRD analysis focusing on leukemic chemoresistant subpopulations.

Experimental demonstration of scalable active metamaterials with interacting cells of tunable bulk modulus

Active metamaterials can be designed to exhibit acoustic properties that are challenging to obtain using passive structures without relying on narrowband resonant behavior. Arrangements of unit cells that independently sense the local acoustic conditions and produce a coherent response, which is determined by the gain of electronics between the sensor and driver, act as a material with programmable properties. Such an active metamaterial has been previously demonstrated, but only for a single or a few non-interacting cells due to a limited understanding of how the programmed gains relate to the effective properties. Recently we developed a polarized source model to address this challenge, and based on that work we now demonstrate experimentally an active metamaterial composed of interacting cells with programmable bulk modulus. Each unit cell consists of a speaker and microphone mounted in the bottom plate of a 2D waveguide, with electronics of adjustable gain stored underneath. The field generated by a 2 × 3 arrangement of cells in response to an incident pulse was compared to the simulated scattered field of the equivalent continuous material. By tuning the gain, we accurately reproduced the behavior of a medium with negative, fractional, and large relative bulk modulus.

VELAS: An open-source toolbox for visualization and analysis of elastic anisotropy

Although elastic properties and their anisotropy play a crucial role in materials science and technology, it is not straightforward to visualize how properties such as shear modulus vary with the direction of stress/strain. Here, written in GNU Octave, we have developed an easy-to-use, flexible, and user-friendly open-source toolbox with a graphical user interface, VELAS, which automatically visualizes and analyses the elastic anisotropy of arbitrary crystal systems using second-order elastic constants. Using Voigt-Reuss-Hill averaging scheme, VELAS allows the calculation of more than ten significant mechanical properties such as modulus, hardness, anisotropy index, ductility, and bond type. It also supports the determination of the mechanical stability of crystals at atmospheric and high pressures using the Born mechanical stability criterion. VELAS provides a new easy-to-use tool for detecting and identifying unusual mechanical properties such as negative Poisson's ratio, negative linear compressibility, and negative bulk modulus. VELAS offers several alternative visualization solutions for elastic properties, such as unit spherical projection, and map projection, and supports direct output of high-quality images. Furthermore, VELAS provides users an interface to read data from the Materials Project API in both online and offline modes. After introducing the basic theory, we detail the software framework, technical route, and installation specifications of VELAS. Moreover, the reliability and versatility of VELAS are confirmed by the analysis of the cases of negative linear compressibility, negative Poisson's ratio, hardness, and fracture toughness. Program summaryProgram Title: VELASCPC Library link to program files:https://doi.org/10.17632/bj27474v6f.1Developer's repository link:https://github.com/ranzhengcode/VELASLicensing provisions: GNU General Public License 3Programming language: GNU OctaveNature of problem: To determine the mechanical stability of any crystal at atmospheric and high pressures. To automatically visualize and analyze the elastic anisotropy of arbitrary crystals and identify unusual elastic properties.Solution method: Firstly, the determination of the mechanical stability of crystals at atmospheric and high pressures using the Born mechanical stability criterion. Secondly, more than ten significant mechanical properties such as elastic moduli, ratios, hardness, and anisotropy index are analyzed using Voigt-Reuss-Hill averaging scheme. Then, the mechanical behavior of crystals in the elastic regime is derived from a comprehensive tensor analysis of the second-order elastic constants. Finally, Pugh Ratio, Hardness, and Fracture Toughness in 3D space are calculated based on our approximate treatment of elastic properties.

Molecular dynamics study of crystal lattice effect on mechanical properties of SiO2: A density functional tight binding investigation

In this study, molecular dynamics simulations (MD) based on quantum mechanical method in which the interactions were expressed by self-consistent charge density functional tight binding (SCC-DFTB) to investigate the mechanical properties of four different SiO2 structures. The Young's, shear and bulk modulus of systems are calculated and the results compared to experimental and other computational findings. We have observed that the hexagonal SiO2 exhibits more different mechanical behavior than other SiO2 structures because of its negative bulk modulus value. While the length of a-lattice parameters for four SiO2 crystal structures increase with the change of MD time, the length of the b and c-lattice vectors shortens. Moreover, it is analyzed the stress-strain behavior and yield points are determined from this curve of SiO2 structures. It is revealed that the plastic deformation occurs at higher stress for hexagonal SiO2 than other SiO2 structures because of its rigid solid property. These results can provide a reference for understanding the mechanical properties of various SiO2 crystal structures.

Compact acoustic metamaterial based on the 3D Mie resonance of a maze ball with an octahedral structure

Acoustic metamaterials (AMs) offer ever-expanding possibilities for manipulating sound waves. Potential applications include diagnostic medical imaging, super-absorption, acoustic sensing, and acoustic stealth. In spite of recent progress, the investigation of AMs with a three-dimensional (3D) response is lagging behind, in particular for those that exhibit an isotropic response. Here, we demonstrate a highly compact subwavelength maze-like multi-shell plastic sphere, which generates Mie resonances with isotropic monopolar and anisotropic dipole, quadrupole, and octupole modes at low frequencies for airborne sound, based on an octahedral structure. Eigenmode analysis shows that the proposed maze ball exhibits a negative bulk modulus at the monopole Mie resonance frequency in the absence of viscous losses, which is a signature of strong transmission blocking. With a diameter of 0.17λ and a volume filling factor of 13.5%, a constructed single 3D maze ball reduces the experimentally-measured transmitted acoustic energy by 67%, limited mainly by viscous losses. With optimized fabrication, the proposed 3D Mie resonator should provide a versatile approach for the manipulation of sound waves on a subwavelength scale, and lead to the realization of practical 3D metamaterial devices.

A robust three-parameter reference curve for condensed phase materials

Many isothermal equations of state lead to numerical problems under tension conditions due to negative bulk modulus. We summarize the properties of four existing models and devise a continuous non-piecewise reference curve to address this issue. The final functional form contains three fitting parameters that are directly related to the bulk modulus and its pressure derivative at the reference state and at infinite pressures. We also include bounds that guarantee the monotonicity of its pressure derivative in all volume space. To demonstrate the qualitative features of each isotherm, we conduct calibrations against experimental data for cyclotetramethylene tetranitramine, magnesium oxide, gold, and platinum. Pressure residuals and root mean square deviations show that the proposed functional form is capable of adequately fitting the data compared to other three-parameter models.

Weak-form homogenization of one-dimensional systems with hidden degrees of freedom

Acoustic metamaterials are synthetic structures that exhibit specific, often exotic, effective material properties for long-wavelength stimuli. One of the principal methods that exotic material properties, such as zero or negative mass density or bulk modulus, may be obtained is through hidden degrees of freedom. An example of a hidden degree of freedom is a resonator attached in shunt to a duct and modeling the system as just a duct with effective material properties. In this example, the resonator is “hidden” from the model by incorporating its (rather profound) effect on the system into the material properties. This paper demonstrates how hidden degrees of freedom may be incorporated into a recently developed homogenization scheme that utilizes the weak form of the dynamic system [Muhlestein, J. Acoust. Soc. Am. 147 (2020)]. Two specific examples will be considered: the shunt resonator, as described above, and a membrane inside a duct. The results are compared with other standard homogenization methods.

Polarization Self-Screened Multiple Quantum Wells for Deep Ultraviolet Light-Emitting Diodes to Enhance the Optical Power

We experimentally and numerically propose a deep ultraviolet light-emitting diode (DUV LED) possessing the quantum barriers (QBs) with the gradually reduced Al composition along the [0001] orientation. The induced negative polarization bulk charges in the graded QBs can screen the positive polarization sheet charges at the QB/quantum well (QW) interface, i.e., generating the polarization self-screening effect, which weakens the electric field in the QWs. The polarization self-screened feature is important in enhancing the overlap level of the electron and hole wave functions within the active region and increasing the hole injection, thus improving the electroluminescence intensity and the optical power.

Multi-material topology optimisation of micro-composites with reduced stress concentration for optimal functional performance

This study develops a new multi-material topology optimisation framework for design of periodic micro-composites with optimal functional performance and reduced stress concentration. First, multi-material topology optimisation is developed based on the alternating active phase algorithm and inverse homogenisation method with the sensitivity analysis derived for specific property objective i.e., negative Poisson's ratio (NPR) or maximum effective bulk modulus (EBM) and (p-norm macroscopic) stress objective. Then, the effects of initial material distribution and weight ratio (w1, w2 assigned to the property and stress objectives, respectively) are investigated, and the evaluation indices are also developed to obtain the optimal solution. Further, two cases related to the design of micro-composites for maximised either NPR or EBM with reduced maximum stress are performed. The results show that when designing the multi-material NPR micro-composites, the decrease of w1/w2 contributes to a general decease of both NPR and maximum stress. While in designing the maximum EBM, decreasing w1/w2 leads to the reduced maximum stress and simultaneously reduced EBM; hereby, a decision-making method as well as the proposed evaluation index are both applied and compared for acquiring the optimal result. This study provides new methods and solutions to multi-material micro-composites design for future industrial applications.

Experimental and numerical investigations of ventilated acoustic metamaterial based in-parallel arrangement of Helmholtz resonator for façade screen

Understanding urban noise as a serious environmental problem in urban centers, the development and application of noise control strategies have demanded a recent effort by several researches. In this case, the development of acoustic metamaterial artificially designed to manipulate the wave phenomena has become a recent topic, aiming at optimized responses, and enables the development of subwavelength devices with potential application in passive ventilation and noise mitigation, providing better environmental conditions in buildings. The present paper intends to contribute to the knowledge in this field by investigating the concept of an acoustic metamaterial with negative bulk modulus based in a parallel arrangement of Helmholtz Resonators. Experimental and numerical investigations are carried out to determine the acoustic potential of the proposed meta structure in terms of sound absorption and sound transmission loss. The developed concept exhibits significant benefits in the properties of sound transmission loss, and seems a potential application for noise control at specific frequency bands (mainly at low to middle frequency) in building façades.