Dipolar Effects Research Articles

Computed inverse MRI (CIMRI) for intrinsic brain magnetic susceptibility mapping

In magnetic resonance imaging (MRI), tissue magnetization in the main field B0 is a necessary preparation for magnetic resonance signal formation that imposes an inherent dipole effect on MRI signals, which predisposes an artifact on tissue MRI. In the MRI principle, T2*-weighted MRI can be described by a cascade of data transformations: from the source of tissue magnetic susceptibility (denoted by χ) to the output of complex-valued T2* image (in a magnitude and phase pair). Under the linear approximation of the T2* phase MRI, we can computationally reconstruct the source χ by quantitative susceptibility mapping (QSM), which is an inverse solution that is modeled by computed inverse MRI (CIMRI). For a brain function study using MRI (fMRI), we can reconstruct a timeseries of brain χ images to represent the intrinsic brain function activity called functional QSM (fQSM). This intrinsic depiction is defined as the removal of the artifactual dipole effect and other MRI-introduced distortions from phase data through inverse mapping. With one high-resolution QSM experiment and one group (20 subjects) low-resolution fQSM experiment, we show that the dipole effect manifests as ripples around vessels and a spatial split at a local activation blob and that the dipole effect could be removed by CIMRI. In the context of inverse imaging or undoing MRI transformations (including dipole convolution), we computationally achieve brain intrinsic structural depiction by QSM and intrinsic functional depiction by fQSM.

Effects of Dipole–Dipole Interaction on Vortex Motion in Bose–Einstein Condensates

In this paper, we study the dynamics of a single vortex and a corotating vortex pair in a quasi-two-dimensional Bose–Einstein condensate. The numerical results are obtained by solving the Gross–Pitaevskii equation. It is shown that the vortex motion is strongly dependent on the dipole–dipole interaction (DDI). For a single vortex, the periodic motion occurs with increasing dipole strength. The x-coordinate and y-coordinate as a function of time close to the shape of cosine wave and triangular wave when DDI is an isotropic repulsive interaction, respectively. In addition, it is evident that strong and anisotropic DDI makes the movement of vortex become fast and the amplitude enlarges. In the case of a corotating vortex pair, the dynamics is qualitatively consistent with a single vortex for weak dipole strength. Moreover, the periodic dynamics only appears for strong and anisotropic DDI and more rapid vortex motion is indicated.

Magnetic dipole and thermal radiation effects on hybrid base micropolar CNTs flow over a stretching sheet: Finite element method approach

The finite element method (FEM) is applied to study the impacts of prominent parameters on microrotation, velocity, and temperature to know the characteristics of the flow of incompressible water-ethylene glycol base fluids (60% water + 40% ethylene glycol) with single-wall and multiwall carbon nanotube nanoparticles micropolar ferromagnetic fluid due to porous stretching surface. A magnetic dipole of significant strength together the applied magnetic field contributes to better saturation of magnetic nanoparticles. Appropriate similarity transforms are applied to acquire the ordinary differential form of the governing non-linear partial differential equations and resulting equations are discretized in the prospectus of FEM. The detailed parametric study has been carried out, the results are presented in graphical and tabular form. The increment in the ferromagnetic interaction parameter slows down the fluid velocity but it upsurges the microrotation and thermal distribution. The multiwall carbon nanotube (MWCNT) in comparison to the single-wall carbon nanotube (SWCNT) has a greater impact on velocity and microrotation profiles also single-wall carbon nanotube (SWCNT) is compared to the multiwall carbon nanotube (MWCNT) has a greater impact on the temperature profile. The validation of the MATLAB code and the numerical scheme has been verified with an excellent comparison of present results with previous ones in the existing literature.

Synthesis of low surface energy thin films of nonhalogenated polyepichlorohydrin-triazoles with side alkyl chain

The dispersive and polar components of surface energy of a polymer thin film are hypothesized to be reduced by introduction of intramolecular interactions to pull side chains to the backbone and by establishment of hydrophobic interactions between extended alkyl chains to compact the molecular volume. A series of new polymer, polyepichlorohydrin (PECH)-triazole polymer with side chain with different number of methylene groups, were synthesized to prove this proposal. The newly synthesized polymer has high film-formation capacity and no halogens. The interactions between ether oxygen and imine nitrogen atoms depress the dipole effects and the hydrophobic interactions between extended methylene groups of the side chain on triazole compact the molecular volume, so the yielded surface energy has been effectively reduced to a level lower than PTFE. The synthesized polymer is a nonhalogenated low surface-energy material with potential applications.

Exploring magnetic dipole contribution on ferromagnetic nanofluid flow over a stretching sheet: An application of Stefan blowing

The study is achieved to explore the novel of a two-dimensional ferromagnetic fluid flow over a flat elastic sheet with the magnetic dipole effect. The flow design was chosen because it is often used in engineering and bioengineering applications including magnetic drug targeting systems. The Buongiorno model has been used to incorporate Brownian movement and thermophoresis effects. Further, Stefan blowing effect and convective boundary constraint is taken in to the account. The governing expressions for assumed flow are reduced to ordinary differential equations by opting suitable similarity variables. The reduced equations are solved using Runge–Kutta–Fehlberg 45 method by adopting Shooting technique. The attained numerical outcomes are then illustrated graphically . Result reveals that, upsurge in values of thermophoresis and Brownian motion parameters improves the thermal gradient and the fluid shows high heat transfer in existence of Stefan blowing condition. Convective boundary condition has tendency to improve the heat transfer feature of liquid in presence of Stefan blowing condition. Further, the justification of current outcomes is accomplished by developing comparison with available published work.

Broadband and extremely low frequency sound isolation by a programmable shunted electromechanical diaphragm with force dipole effect

Broadband sound isolation in low frequencies by a partition with a small dynamic mass is a challenge to the mass law. We review the conventional sound isolation panels and membrane-type acoustic metamaterials and find that the former is equivalent to a monopole identical to a single path control system and the latter is similar to a dipole equivalent to a feed-forward system with zero time delay. Neither is likely to enhance sound isolation in a broad bandwidth and at low frequencies. Here, we introduce a “force dipole” effect, which is a passive feedback force countering the incident sound. The device is based on a passively shunted electromechanical diaphragm (SEMD), consisting of a moving-coil attached diaphragm, a permanent magnet generating a DC magnetic field, and a programmable analog circuit shunting the coil. It isolates sound in a super broad bandwidth down to the infrasound region. In reaction to an incident sound, a Lorentz force is exerted on the moving-coil opposing the incident pressure force, forming a near-perfect dipole. The net residual force is greatly reduced and so is the sound wave transmission. The force dipole effect is determined by the shunt circuit and our experiments in an impedance tube show that the spectrum of transmission loss (TL) of the SEMD can be programed by a smart circuit; it is maintained at 20 dB or above from 15 Hz to 772 Hz. The mass and stiffness laws are broken over 5.7 octaves. The lumped-parameter theoretical model is verified by experiment, and further analysis predicts that a superconducting circuit will achieve a super broadband band gap.

Electronic structure of aqueous two-dimensional photocatalyst

The electronic structure, in particular the band edge position, of photocatalyst in presence of water is critical for photocatalytic water splitting. We propose a direct and systematic density functional theory (DFT) scheme to quantitatively predict band edge shifts and their microscopic origins for aqueous 2D photocatalyst, where thousands of atoms or more are able to be involved. This scheme is indispensable to correctly calculate the electronic structure of 2D photocatalyst in the presence of water, which is demonstrated in aqueous MoS2, GaS, InSe, GaSe and InS. It is found that the band edge of 2D photocatalysts are not rigidly shifted due to water as reported in previous studies of aqueous systems. Specifically, the CBM shift is quantitatively explained by geometric deformation, water dipole and charge redistribution effect while the fourth effect, i.e., interfacial chemical contact, is revealed in the VBM shift. Moreover, the revealed upshift of CBM in aqueous MoS2 should thermodynamically help carriers to participate in hydrogen evolution reaction (HER), which underpin the reported experimental findings that MoS2 is an efficient HER photocatalyst. Our work paves the way to design 2D materials in general as low-cost and high-efficiency photocatalysts.

A Novel Design of CPW Feed Planar Omnidirectional Circularly Polarized Antenna

A novel planar omnidirectional circularly polarized (CP) antenna is presented. The omnidirectional circular polarization characteristics of the planar antenna are the result of the combined effect of a planar quasi-magnetic dipole (PQMD) and a printed electric dipole (PED) in this paper. The CP radiation pattern of the proposed antenna can be achieved by distributing appropriate current amplitude and phase to both elements, respectively. A power divider is used to adjust the amplitude and phase relationship between two basic components. In order to achieve a planar structure, coplanar waveguide (CPW) feed is adopted to feed the quasi-magnetic dipole for the first time. The overall electrical size of antenna is 1.61λ × 0.38λ. Finally, the correctness of our theoretical analysis is verified by processing and measuring this antenna. Through the analysis of the measurement results, we have obtained the following conclusions: the operating bandwidth of the proposed antenna obtained by measurement is from 5.67 to 5.86 GHz, in which the reflection coefficient is less than −10 dB and the axial ratio is less than 3 dB. Within the available bandwidth, the proposed antenna achieves omnidirectional radiation characteristics with a gain between 0.89 and 2.48 dBic.

Spatial dispersion contribution to second harmonic generation in inversion-symmetric materials

It is well known that second harmonic generation (SHG) from dipoles within the bulk vanishes in inversion-symmetric semiconductor materials as a consequence of parity symmetry. Hence SHG is then ascribed in the form of either surface dipole effects or bulk-related electric quadrupole or magnetic dipole effects. By incorporating the redefined spatial dispersion into the simplified bond-hyperpolarizability model, we show that the SHG spatial dispersion contribution for Si(001) and Si(111) facet orientations can be reformulated by a third-rank tensor containing one independent parameter, namely, the complex SHG spatial dispersion hyperpolarizability. Our results show that certain unexplained rotational anisotropy SHG intensity features for different incoming light wavelengths or optical penetration depths can be well reproduced only if the contribution from spatial dispersion is incorporated.

Beam performance of the SHINE dechirper

A corrugated structure is built and tested on many FEL facilities, providing a ‘dechirper’ mechanism for eliminating energy spread upstream of the undulator section of X-ray FELs. The wakefield effects are here studied for the beam dechirper at the Shanghai high repetition rate XFEL and extreme light facility (SHINE), and compared with analytical calculations. When properly optimized, the energy spread is well compensated. The transverse wakefield effects are also studied, including the dipole and quadrupole effects. By using two orthogonal dechirpers, we confirm the feasibility of restraining the emittance growth caused by the quadrupole wakefield. A more efficient method is thus proposed involving another pair of orthogonal dechirpers.

Finite-Element Analysis of Noise Preceding the Arrival of S-Wave in Ultrasonic Measurements of Rock Velocities.

For oil and gas seismic exploration, rock velocities are essential parameters to tease out reservoir properties from seismic data. The ultrasonic pulse transmission (UPT) method has been a gold standard to estimate reservoir rock velocities in the laboratory. Regarding the UPT method, accurate determination of the travel time of waves plays a significant role in robustly measuring rock velocities. One of the most conventional ways to obtain the travel time is through the arrival picking. However, unclear noise virtually exists preceding the arrival of S-wave interfering with this arrival picking, which, sometimes, can cause enormous errors to measured S-wave velocity. Herein, we develop a 2-D, three-component (2D-3C) finite-element modeling (FEM) algorithm aiming to interpret the noise by combining with UPT measurements. The proposed 2D-3C FEM not only can efficiently compute ultrasonic wavefield radiated by circular P- or S-wave transducers but also able to obtain synthetic waveforms in the testing of S-wave velocity where polarization directions of S-wave transducers are arranged as nonparallel. To analyze the simulated ultrasonic waveforms, we introduce frequently-used concepts of edge and direct plane waves to build elastodynamic models of the ultrasonic wavefield. Then, we compare numerical results with experimental measurements. Our 2D-3C FEM results show good agreement with experimental waveforms both in P- and S-wave velocity testings. Whereafter, we pinpoint constitutions of the noise preceding the arrival of S-wave. Comparison of numerical and experimental waveforms suggests that the edge P-wave with its reflected and converted modes partially contributes to this noise, while the rest part of the noise may stem from the effects of the compressional dipole, the couplant smeared between a transducer and a sample, and inherently parasitic longitudinal vibrations of S-wave transducers. The interpretations on this noise have the potential to benefit future design of more effective S-wave transducers.

Unshielded bent folded-end dipole 9.4 T human head transceiver array decoupled using modified passive dipoles.

To develop an unshielded dipole transceiver array for human head imaging at 9.4 Tesla and to improve decoupling of adjacent dipole elements, a novel array design with modified passive dipole antennas was developed, evaluated, and tested. The new array consisted of 8 bent folded-end dipole elements placed in a single row and surrounding the head. Adjacent elements of RF transceiver arrays are usually decoupled by introducing circuits electrically connected to elements. These methods are difficult to use for dipole arrays because of the distant location of the adjacent antennas. A recently developed decoupling technique using passive dipoles is simple and does not require any electrical connection. However, common parallel passive dipoles can produce destructive interference with the RF field of the array itself. To minimize this interference, we placed the passive dipoles perpendicularly to the active dipoles and positioned them at the ends of the array. We also evaluated the effect of different passive dipoles on the array transmit performance. Finally, we optimized the array transmit performance by varying the length of the dipole folded portion. By rotating the passive dipoles 90º and moving them toward the ends of the array, we minimized the destructive interference to an acceptable level without compromising decoupling and the transmit efficiency. While keeping the benefits of the passive dipole decoupling method, the new modified dipoles produce substantially less destructive interference with the RF field of the array than the common design. The constructed transceiver array demonstrated good decoupling and whole-brain coverage.

Comparison of Huggins Coefficients and Osmotic Second Virial Coefficients of Buffered Solutions of Monoclonal Antibodies.

The Huggins coefficient kH is a well-known metric for quantifying the increase in solution viscosity arising from intermolecular interactions in relatively dilute macromolecular solutions, and there has been much interest in this solution property in connection with developing improved antibody therapeutics. While numerous kH measurements have been reported for select monoclonal antibodies (mAbs) solutions, there has been limited study of kH in terms of the fundamental molecular interactions that determine this property. In this paper, we compare measurements of the osmotic second virial coefficient B22, a common metric of intermolecular and interparticle interaction strength, to measurements of kH for model antibody solutions. This comparison is motivated by the seminal work of Russel for hard sphere particles having a short-range “sticky” interparticle interaction, and we also compare our data with known results for uncharged flexible polymers having variable excluded volume interactions because proteins are polypeptide chains. Our observations indicate that neither the adhesive hard sphere model, a common colloidal model of globular proteins, nor the familiar uncharged flexible polymer model, an excellent model of intrinsically disordered proteins, describes the dependence of kH of these antibodies on B22. Clearly, an improved understanding of protein and ion solvation by water as well as dipole–dipole and charge–dipole effects is required to understand the significance of kH from the standpoint of fundamental protein–protein interactions. Despite shortcomings in our theoretical understanding of kH for antibody solutions, this quantity provides a useful practical measure of the strength of interprotein interactions at elevated protein concentrations that is of direct significance for the development of antibody formulations that minimize the solution viscosity.

Dipole-Induced Raman Enhancement Using Noncovalent Azobenzene-Functionalized Self-Assembled Monolayers on Graphene Terraces.

Graphene is a promising material in the field of interface science, especially for noncovalent functionalization, sensing, and for applications in catalysis and nanoelectronics. The noncovalent self-assembly of aromatic molecules on graphene promotes electronic coupling through π-π interactions that allows for quenching of the fluorescence of adsorbent molecules and the enhancement of their Raman spectra via graphene-enhanced Raman spectroscopy (GERS). Although recent work has explored the Raman enhancement on mono- and bilayer graphene, the layer dependence of both electronic phenomena (i.e., fluorescence quenching and Raman enhancement) has largely remained underexplored. Similarly, the effect of near-surface molecular dipoles on GERS has sparsely been examined. In this work, we employ self-assembled monolayers of azobenzene-decorated triazatriangulene molecules (AzoTATA) on graphene terraces to examine the effect of switchable molecular dipoles on the GERS effect, which occurs as a function of azobenzene photoisomerization. Furthermore, using empirical and computational methods, we present a systematic study for deriving the mechanism of GERS enhancement and fluorescence quenching on graphene terraces.

Formation Mechanism of PFN Dipole Interlayer in Organic Solar Cells

Water‐/alcohol‐soluble polyelectrolyte poly[(9, 9‐bis (3′‐(N,N‐dimethylamino) propyl)‐2, 7‐fluorene)‐alt‐2, 7‐(9, 9‐dioctylfluorene)] (PFN) used in organic solar cells (OSCs) reduces the work function of the electrode due to the effect of an interfacial dipole, which is beneficial for the energy‐level alignment between the electrode and the active layer. To date, the studies on the working mechanism of PFN are mainly conducted through topographical and electronic research. Herein, a dynamic insight into the formation mechanisms of the PFN interlayer at the molecular structural level is established. The charge transfer between PFN and the substrates is verified for the first time by X‐ray photoelectron spectroscopy (XPS) and density functional theory (DFT) studies, which results in chemisorption dipoles with their direction aligned with the intrinsic dipole of the PFN molecule, thereby reducing the work function of the substrate. The larger adsorption energy in the substrates of the nitrogen‐containing side chains of PFN is also identified, which induces the preferential orientation of PFN molecule to reduce the work function of the substrate. By incorporating this interlayer, high efficiency in single‐junction OSCs is achieved using commercial materials. The findings are of great significance for understanding and optimizing the polymer dipole interlayers for OSCs.

Squaraine Organic Crystals with Strong Dipole Effect Toward Stable Lithium-Organic Batteries

Although organic redox molecules as electrode materials can achieve a high specific capacity through molecular design engineering, effective strategies to overcome their low cycling stability have not yet been developed. Herein, we establish a novel strategy that utilizes a strong dipole effect to suppress the high solubility of organic molecules and, thus, improve cycling stability. Two organic single crystals of squaraine derivative molecules with different substituents, 2,4-di(piperidin-1-yl)cyclobutane-1,3-dione (SAPD) and 2,4-di(pyrrolidin-1-yl)cyclobutane-1,3-dione (SAPL), were synthesized by facile routes. In particular, the SAPL cathode exhibited a discharge capacity of 371 mAh g-1 at 40 mA g-1 (0.2 C) and a nearly 78% capacity retention after 1000 cycles at 0.2 A g-1 (1 C). It also exhibited a capacity of 135 mAh g-1 even at 4 A g-1v (20 C) without hybridizing with graphene or carbon nanotubes. We studied the molecular interactions of SAPD and SAPL via detailed density functional theory calculations and revealed their lithium-ion storage mechanisms by using ex-situ technologies. Through comparing their intermolecular interactions, we found the stronger dipole effect can elevate cycling performance better. Hence, our design strategy can serve as a promising path for constructing high-performance lithium-organic batteries with long-term cycling stability.

Dielectric Responses of Polyurethane/Zinc Oxide Blends for Dry-Type Cast Cold-Curing Resin Transformers.

The influence of different concentrations (0.5, 1.0, and 2.0 wt.%) of Zinc Oxide (ZnO) filler on the dielectric properties of the cold-curing polyurethane (PU) resin is presented in this study. For this purpose, the direct DC conductivity and the broadband dielectric spectroscopy measurements were used to describe the changes in dielectric responses of PU/ZnO nanocomposites over the frequency and temperature range, respectively. It can be stated that, the 1.0 wt.% nanoparticles and lower caused a decrease in the real relative permittivity compared to the pure PU resin, while the higher concentration of nanoparticles for frequencies above 1 Hz had the opposite effect. The presence of nanoparticles in the polyurethane resin affected the segmental dynamics of the polymer chain and changed a charge distribution in the given system. These changes caused a shift of local relaxation peaks in the spectra of imaginary permittivity and dissipation factor of nanocomposites. It is suggested that the temperature-dependent transition of the electric properties in the nano-composite is closely associated with the -relaxation and intermediate dipolar effects (IDE).

Gourmet powder functionalization of SnO2 for high-performance perovskite solar cells made in air

In spite of great advances in low-cost fabrication of air-processed perovskite solar cells (PSCs), there are still many opportunities to further improve device performance. Herein, we show that, by introducing monosodium glutamate (MSG) with carboxyl group, amino group, and Na+ cationic, known as gourmet powder, it is likely to simultaneously enhance the quality of perovskite film and electron transport layer, and interfacial charge extraction. As a multifunctional modify agent, the electric dipole effect and strong crosslinking of MSG can effectively reduce defects, accelerate interfacial charge transfer, alleviate hysteresis, and greatly improve performance of air-processed PSCs. Due to these unique characteristics of MSG, the champion power conversion efficiency (PCE) of 17.71% with negligible hysteresis has been achieved based on the structure of FTO/MSG-SnO2/FA0.5MA0.5PbI3/Spiro-OMeTAD/Au. Moreover, the unencapsulated device exhibits remarkable long-term stability, preserving over 91% of its initial PCE after storage in air for 1320 h. Our work provides a green and promising functionalization method for high-performance air-processed PSCs.

Electric field induced giant valley polarization in two dimensional ferromagnetic WSe2/CrSnSe3 heterostructure

Valleytronics is receiving extensive research efforts. Thus, we investigated the electric field-induced valley polarization in the WSe2/CrSnSe3 heterostructures by varying the stacking order. The heterostructure shows indirect band gaps of 270 and 330 meV in the two most stable structures. The WSe2/CrSnSe3 heterostructure displays a ferromagnetic ground state with out-of-plane anisotropy (0.02 meV) in one stable stacking (S-1) while a small in-plane anisotropy (−0.01 meV) is found in other stacking (S-2). The Curie temperature is slightly enhanced to 73 K compared to the monolayer CrSnSe3. We have found the valley splitting of 4 meV in S-1 whereas it became 9 meV in the S-2 system. The valley splitting is further enhanced if an electric field is applied from CrSnSe3 to the WSe2 layer whereas it is suppressed in the reversed electric field. Particularly, the S-2 structure shows a giant valley splitting of 67 meV at an electric field of 0.6 V Å−1. We attribute this electric field-dependency to the dipolar effect. Overall, we propose that the WSe2/CrSnSe3 heterostructure can be a potential structure for obtaining a giant valley splitting.

Dipolar and catalytic effects of an Fe3O4 based nitrogen-doped hollow carbon sphere framework for high performance lithium sulfur batteries

Well-designed hollow carbon sphere with embedded Fe3O4 nanoparticles is fabricated as sulfur host for Li–S batteries. The high catalytic activity of Fe3O4 can accelerate the redox conversion of polysulfide, facilitating the reaction kinetics.