Monitoring damage growth and topographical changes in plate structures using sideband peak count-index and topological acoustic sensing techniques.

Some topographies in plate structures can hide cracks and make it difficult to monitor damage growth. This is because topographical features convert homogeneous structures to heterogeneous one and complicate the wave propagation through such structures. At certain points destructive interference between incident, reflected and transmitted elastic waves can make those points insensitive to the damage growth when adopting acoustics based structural health monitoring (SHM) techniques. A newly developed nonlinear ultrasonic (NLU) technique called sideband peak count - index (or SPC-I) has shown its effectiveness and superiority compared to other techniques for nondestructive testing (NDT) and SHM applications and is adopted in this work for monitoring damage growth in plate structures with topographical features. The performance of SPC-I technique in heterogeneous specimens having different topographies is investigated using nonlocal peridynamics based peri-ultrasound modeling. Three types of topographies - "X" topography, "Y" topography and "XY" topography are investigated. It is observed that "X" and "XY" topographies can help to hide the crack growth, thus making cracks undetectable when the SPC-I based monitoring technique is adopted. In addition to the SPC-I technique, we also investigate the effectiveness of an emerging sensing technique based on topological acoustic sensing. This method monitors the changes in the geometric phase; a measure of the changes in the acoustic wave's spatial behavior. The computed results show that changes in the geometric phase can be exploited to monitor the damage growth in plate structures for all three topographies considered here. The significant changes in geometric phase can be related to the crack growth even when these cracks remain hidden for some topographies during the SPC-I based single point inspection. Sensitivities of both the SPC-I and the topological acoustic sensing techniques are also investigated for sensing the topographical changes in the plate structures.

Underwater acoustic sensing using the geometric phase.

We present a sensing modality using the geometric phase of acoustic waves propagating in an underwater environment. We experimentally investigate the effect of scattering by a small subwavelength perturbation on a flat submerged surface. We represent the state of an acoustic field in the unperturbed and perturbed cases as multidimensional vectors. The change in geometric phase is obtained by calculating the angle between those vectors. This angle represents a rotation of the state vector of the wave due to scattering by the perturbation. We perform statistical analysis to define a signal-to-noise ratio to quantify the sensitivity of the geometric phase measurement and compare it to magnitude based measurements. This geometric phase sensing modality is shown to have higher sensitivity than the magnitude based sensing approach.

Demonstration of a two-bit controlled-NOT quantum-like gate using classical acoustic qubit-analogues.

The Controlled-NOT (CNOT) gate is the key to unlock the power of quantum computing as it is a fundamental component of a universal set of gates. We demonstrate the operation of a two-bit C-NOT quantum-like gate using classical qubit acoustic analogues, called herein logical phi-bits. The logical phi-bits are supported by an externally driven nonlinear acoustic metamaterial composed of a parallel array of three elastically coupled waveguides. A logical phi-bit has a two-state degree of freedom associated with the two independent relative phases of the acoustic wave in the three waveguides. A simple physical manipulation involving the detuning of the frequency of one of the external drivers is shown to operate on the complex vectors in the Hilbert space of pairs of logical phi-bits. This operation achieves a systematic and predictable C-NOT gate with unambiguously measurable input and output. The possibility of scaling the approach to more phi-bits is promising.

Experimental classical entanglement in a 16 acoustic qubit-analogue.

The possibility of achieving and controlling scalable classically entangled, i.e., inseparable, multipartite states, would fundamentally challenge the advantages of quantum systems in harnessing the power of complexity in information science. Here, we investigate experimentally the extent of classical entanglement in a [Formula: see text] acoustic qubit-analogue platform. The acoustic qubit-analogue, a.k.a., logical phi-bit, results from the spectral partitioning of the nonlinear acoustic field of externally driven coupled waveguides. Each logical phi-bit is a two-level subsystem characterized by two independently measurable phases. The phi-bits are co-located within the same physical space enabling distance independent interactions. We chose a vector state representation of the [Formula: see text]-phi-bit system which lies in a [Formula: see text]-dimensional Hilbert space. The calculation of the entropy of entanglement demonstrates the possibility of achieving inseparability of the vector state and of navigating the corresponding Hilbert space. This work suggests a new direction in harnessing the complexity of classical inseparability in information science.

Effect of Ligand Adsorption on the Electronic Properties of the PbS(100) Surface.

A first-principles density functional theory calculation was carried out to study the adsorption of acetic acid, methyl amine, methanethiol, and hydrogen iodide on the (100) surface of PbS. All four ligands are common capping agents used in colloidal PbS quantum dot-based photovoltaics. Interestingly, among the considered adsorbates, dissociative adsorption was energetically preferred for hydrogen iodide, while associative adsorption was favorable for the rest. Associative adsorption was driven by strong interactions between the electronegative elements (Y) in the respective ligands and the Pb surface atoms via Pb 6p-Y np bond hybridization (n represents the valence quantum number of the respective electronegative elements). Importantly, the adsorption of ligands altered the work function of PbS, with contrasting trends for associative (decrease in the work function) versus dissociative (increase in the work function) adsorption. The changes in the work function correlates well with a corresponding shift in the 5d level of surface Pb atoms. Other important observations include variations in the work function that linearly change with increasing the surface coverage of adsorbed ligands as well as with the strength of the adsorption of ligands.

Experimental demonstration of coherent superpositions in an ultrasonic pseudospin.

We experimentally demonstrate the existence and control of coherent superpositions of elastic states in the direction of propagation of an ultrasonic pseudospin i.e., a φ-bit. The experimental realization of this mechanical pseudospin consists of an elastic aluminum rod serving as a waveguide sandwiched between two heavy steel plates. The Hertzian contact between the rod and the plates leads to restoring forces which couple the directions of propagation (forward and backward). This coupling generates the coherence of the superposition of elastic states. We also demonstrate φ-bit gate operations on the coherent superposition analogous to those used in quantum computing. In the case of a φ-bit, the coherent superposition of states in the direction of propagation are immune to wave function collapse upon measurement as they result from classical waves.

Spectral analysis of amplitudes and phases of elastic waves: Application to topological elasticity.

The topological characteristics of waves in elastic structures are determined by the geometric phase of waves and, more specifically, by the Berry phase, as a characterization of the global vibrational behavior of the system. A computational procedure for the numerical determination of the geometrical phase characteristics of a general elastic structure is introduced: the spectral analysis of amplitudes and phases method. Molecular dynamics simulation is employed to computationally generate the band structure, traveling modes' amplitudes and phases, and subsequently the Berry phase associated with each band of periodic superlattices. In an innovative procedure, the phase information is used to selectively excite a particular mode in the band structure. It is shown analytically and numerically, in the case of one-dimensional elastic superlattices composed of various numbers of masses and spring stiffness, how the Berry phase varies as a function of the spatial arrangement of the springs. A symmetry condition on the arrangement of springs is established, which leads to bands with Berry phase taking the values of 0 or π. Finally, it is shown how the Berry phase may vary upon application of unitary operations that mathematically describe transformations of the structural arrangement of masses and springs within the unit cells.

Sono-electrochemical recovery of metal ions from their aqueous solutions.

Metal recovery from aqueous waste streams is an important goal for recycling, agriculture and mining industries. The development of more effective methods of recovery have been of increasing interest. The most common methods for metal recovery include precipitation, electrochemical, ion exchange, flocculation/coagulation and filtration. In the current work, a sono-electrochemical technique employing sound field at megasonic frequency (500kHz or 1MHz) in conjunction with electrochemistry is evaluated for enhanced recovery of selected metal ions (palladium, lead and gallium) with different redox potentials from their aqueous solutions. The surface morphology and elemental composition of the metal deposits were characterized using scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. The percent recovery was found to depend on the type of metal ion and the megasonic frequency used. Palladium was recovered in its metal form, while lead and gallium were oxidized during or after the recovery process.

Structure of ZnCl2 Melt. Part I: Raman Spectroscopy Analysis Driven by Ab Initio Methods.

The structure of molten ZnCl2 is investigated using a combination of computer simulation and experimental methods. Ab initio molecular dynamics (AIMD) is used to model the structure of ZnCl2 at 600 K. The structure factors and pair distribution functions derived from AIMD show a good match with those previously measured by neutron diffraction (ND). In addition, Raman spectroscopy is used to investigate the structure of liquid ZnCl2 and identify the relative fractions of constituent structural units. To ascertain the assignment of each Raman mode, a series of ZnCl2 crystalline prototypes are modeled and the corresponding Raman modes are derived by first-principles calculations. Curve fitting of experimental Raman spectra using these mode assignments shows excellent agreement with both AIMD and ND. These results confirm the presence of significant fractions of edge-sharing tetrahedra in liquid ZnCl2. The presence of these structural motifs has significant impact on the fragility of this tetrahedral glass-forming liquid. The assignment of Raman bands present in molten ZnCl2 is revised and discussed in view of these results.

Graphene mediated self-assembly of fullerene nanorods.

A simple procedure for solution-based self-assembly of C60 fullerene nanorods on graphene substrates is presented. Using a combination of electron microscopy, X-ray diffraction and Raman spectroscopy, it is shown that the size, shape and morphology of the nanorods can be suitably modified by controlling the kinetics of self-assembly.

Size-dependent permittivity and intrinsic optical anisotropy of nanometric gold thin films: a density functional theory study.

Physical properties of materials are known to be different from the bulk at the nanometer scale. In this context, the dependence of optical properties of nanometric gold thin films with respect to film thickness is studied using density functional theory (DFT). We find that the in-plane plasma frequency of the gold thin film decreases with decreasing thickness and that the optical permittivity tensor is highly anisotropic as well as thickness dependent. Quantitative knowledge of planar metal film permittivity's thickness dependence can improve the accuracy and reliability of the designs of plasmonic devices and electromagnetic metamaterials. The strong anisotropy observed may become an alternative method of realizing indefinite media.

A first-principles characterization of water adsorption on forsterite grains.

Numerical simulations examining chemical interactions of water molecules with forsterite grains have demonstrated the efficacy of nebular gas adsorption as a viable mechanism for water delivery to the terrestrial planets. Nevertheless, a comprehensive picture detailing the water-adsorption mechanisms on forsterite is not yet available. Towards this end, using accurate first-principles density functional theory, we examine the adsorption mechanisms of water on the (001), (100), (010) and (110) surfaces of forsterite. While dissociative adsorption is found to be the most energetically favourable process, two stable associative adsorption configurations are also identified. In dual-site adsorption, the water molecule interacts strongly with surface magnesium and oxygen atoms, whereas single-site adsorption occurs only through the interaction with a surface Mg atom. This results in dual-site adsorption being more stable than single-site adsorption.

Theoretical methodology for prediction of tropospheric oxidation of dimethyl phosphonate and dimethyl methylphosphonate.

Rate constants for the reactions of OH radicals with dimethyl phosphonate [DMHP, (CH(3)O)(2)P(O)H] and dimethyl methylphosphonate [DMMP, (CH(3)O)(2)P(O)CH(3)] have been calculated by ab initio structural methods and semiclassical dynamics modeling and compared with experimental measurements over the temperature range 250-350 K. The structure and energetics of reactants and transition structures are determined for all hydrogen atom abstraction pathways that initiate the atmospheric oxidation mechanism. Structures are obtained at the CCSD/6-31++G** level of chemical theory, and the height of the activation barrier is determined by a variant of the G2MP2 method. A Transfer Hamiltonian is used to compute the minimum energy path in the neighborhood of the transition state (TS). This calculation provides information about the curvature of the potential energy surface in the neighborhood of the TS, as well as the internal forces that are needed by the semiclassical flux-flux autocorrelation function (SCFFAF) dynamics model used to compute the temperature-dependent reaction rate constants for the various possible abstraction pathways. The computed temperature-dependent rate curves frequently lie within the experimental error bars.

Architecture-dependent signal conduction in model networks of endothelial cells.

Signal conduction between endothelial cells along the walls of vessels appears to play an important role in circulatory function. A recently developed approach to calculate analytically the spectrum of propagating compositional waves in models of multicellular architectures is extended to study putative signal conduction dynamics across networks of endothelial cells. Here, compositional waves originate from negative feedback loops, such as between Ca2+ and inositol triphosphate (IP3) in endothelial cells, and are shaped by their connection topologies. We consider models of networks constituted of a main chain of endothelial cells and multiple side chains. The resulting transmission spectra encode information concerning the position and size of the side branches in the form of gaps. This observation suggests that endothelial cell networks may be able to "communicate" information regarding long-range order in their architecture.

Calcium wave propagation in chains of endothelial cells with nonlinear reaction dynamics: Green's function approach.

We present a Green's function-based perturbative approach to solving nonlinear reaction-diffusion problems in networks of endothelial cells. We focus on a single component (Ca2+), piecewise nonlinear model of endoplasmic calcium dynamics and trans-membrane diffusion. The decoupling between nonlinear reaction dynamics and the linear diffusion enables the calculation of the diffusion part of the Green's function for network of cells with nontrivial topologies. We verify analytically and then numerically that our approach leads to the known transition from propagation of calcium front to failure of propagation when the diffusion rate is varied relative to the reaction rates. We then derive the Green's function for a semi-infinite chain of cells with various boundary conditions. We show that the calcium dynamics of cells in the vicinity of the end of the semi-infinite chain is strongly dependent on the boundary conditions. The behavior of the semi-infinite chain with absorbing boundary conditions, a simple model of a multicellular structure with an end in contact with the extracellular matrix, suggests behavioral differentiation between cells at the end and cells embedded within the chain.

Multiscale modeling of materials based on force and charge density fidelity.

The approximate representation of a quantum solid as an equivalent composite semiclassical solid is considered for insulating materials. The composite is comprised of point ions moving on a potential energy surface. In the classical bulk domain this potential energy is represented by potentials constructed to give the same structure and elastic properties as the underlying quantum solid. In a small local quantum domain the potential is determined from a detailed quantum calculation of the electronic structure. The new features of this well-studied problem are (1) a clearly stated theoretical context in which approximations leading to the model are introduced, (2) the representation of the classical domain by potentials focused on reproducing the specific quantum response being studied, (3) development of "pseudoatoms" for a realistic treatment of charge densities where bonds have been broken to define the environment of the quantum domain, and (4) inclusion of polarization effects on the quantum domain due to its distant bulk environment. This formal structure is illustrated in detail for a SiO(2) nanorod. More importantly, each component of the proposed modeling is tested quantitatively for this case, verifying its accuracy as a faithful multiscale model of the original quantum solid. To do so, the charge density of the entire nanorod is calculated quantum mechanically to provide the reference by which to judge the accuracy of the modeling. The construction of the classical potentials, the rod, the pseudoatoms, and the multipoles is discussed and tested in detail. It is then shown that the quantum rod, the rod constructed from the classical potentials, and the composite classical/quantum rod all have the same equilibrium structure and response to elastic strain. In more detail, the charge density and forces in the quantum subdomain are accurately reproduced by the proposed modeling of the environmental effects even for strains beyond the linear domain. The accuracy of the modeling is shown to apply for two quite different choices for the underlying quantum chemical method: transfer Hamiltonian and density functional methods.

A pseudoatom approach to molecular truncation: application in ab initio MBPT methods.

In this paper, we test the performance of the molecular truncation method of Mallik et al., which was originally applied at the semiempirical NDDO level, in ab initio MBPT methods. Pseudoatoms developed for the replacement of -OCH(3) and -OCH(2)CH(3) functional groups are used in optimizations of selected clusters, and the resulting geometries are compared to reference values taken from the full molecules. It is shown that the pseudoatoms, which consist of parametrized effective core potentials for the nearest neighbor interactions and an external charge field for long-range Coulomb effects, perform well at the MP2 and CCSD levels of theory for the suite of molecules to which they were applied. Representative timings for some of the pseudoatom-terminated clusters are presented, and it is seen that there is a significant reduction in computational time, yet the geometric configurations and deprotonation energies of the pseudoatom-terminated clusters are comparable to the more computationally expensive all-atom molecules.