Adjacent Dipoles Research Articles

Supramolecular Polymerization of N,N',N″,N‴-tetra-(Tetradecyl)-1,3,6,8-pyrenetetracarboxamide: A Computational Study.

The role of molecular dipole orientations and intermolecular interactions in a derivative of pyrene on its supramolecular self-assembly in solution has been investigated using quantum chemical and force field based computational approaches. Five possible dipole configurations of the molecule have been examined, among which the one in which adjacent dipole vectors are antiparallel to each other is determined to be the ground state, on electrostatic grounds. Self-assembly of this molecule under realistic conditions has been studied using MD simulations. Dipolar relaxation in its liquid crystalline (LC) phase has been investigated and contrasted against that in the well-established benzene-1,3,5-tricarboxamide (BTA) family. The dihedral barrier related to the amide dipole flip is larger in the pyrene system than in BTA which explains the differences in their dipolar relaxation behaviors. The mechanism underlying polarization switching upon the application of an external electric field in the LC phase is investigated. Unlike in BTA, this switching is not associated with a reversal of the helical sense of the hydrogen bonded chains, due to differences in molecular symmetry. The observations enable general conclusions on the relationship between electric field induced chiral enhancement and symmetry to be drawn.

A compact broadband dual‐polarized omnidirectional antenna with high isolations for indoor DAS applications

ABSTRACTA compact broadband horizontally/vertically polarized omnidirectional antenna with high isolations is presented for indoor distributed antenna systems. The antenna is a combination of a horizontally polarized (HP) element and a vertically polarized (VP) element, which are connected by four broadside parallel strips (BPSs). Its HP and VP elements consist of four horizontal and vertical elliptical printed dipoles respectively. The vertical dipoles locate between adjacent horizontal dipoles, which makes the antenna compact in structure. The dual‐polarized antenna obtains a bandwidth of 45.5% from 1.7 to 2.7 GHz, and the port isolations are better than 30 dB without introducing large distance between vertical dipoles and horizontal dipoles. The HP and VP gains are higher than 1.7 and 2 dBi respectively. For both horizontal and vertical elements, the radiation patterns in the horizontal plane are omnidirectional with ripples <2.5 dB, and the cross‐polarization levels are also below −25 dB in the horizontal plane. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 59:176–180, 2017

Element decoupling of 7 T dipole body arrays by EBG metasurface structures: Experimental verification

Metasurfaces are artificial electromagnetic boundaries or interfaces usually implemented as two-dimensional periodic structures with subwavelength periodicity and engineered properties of constituent unit cells. The electromagnetic bandgap (EBG) effect in metasurfaces prevents all surface modes from propagating in a certain frequency band. While metasurfaces provide a number of important applications in microwave antennas and antenna arrays, their features are also highly suitable for MRI applications. In this work we perform a proof-of-principle experiment to study finite structures based on mushroom-type EBG metasurfaces and employ them for suppression of inter-element coupling in dipole transceive array coils for body imaging at 7T. We firstly show experimentally that employment of mushroom structures leads to reduction of coupling between adjacent closely-spaced dipole antenna elements of a 7T transceive body array, which reduces scattering losses in neighboring channels. The studied setup consists of two active fractionated dipole antennas previously designed by the authors for body imaging at 7T. These are placed on top of a body-mimicking phantom and equipped with the manufactured finite-size periodic structure tuned to have EBG properties at the Larmor frequency of 298MHz. To improve the detection range of the B1+ field distribution of the top elements, four additional elements were positioned along the bottom side of the phantom. Bench measurements of a scattering matrix showed that coupling between the two top elements can be considerably reduced depending on the distance to the EBG structure. On the other hand, the measurements performed on a 7T MRI machine indicated redistribution of the B1+ field due to interaction between the dipoles with the structure. When the structure is located just over two closely spaced dipoles, one can reach a very high isolation improvement of −14dB accompanied by a strong field redistribution. In contrast, when put at a certain height over the antennas the structure provides a moderate isolation improvement together with a slight increase of B1+ level. This study provides a tool for the decoupling of dipole antennas in ultrahigh field transceive arrays, possibly resulting in denser element placement and/or larger subject-element spacing.

Trimeric metasurfaces for independent control of bright and dark modes of Fano resonances

In this paper, we present a simple trimeric metasurface consisting of three dipolar resonators in each unit cell, to achieve the independent controlling over both the broad bright mode and the sharp dark mode of Fano resonances. Through both the finite difference time domain simulation and microwave experiment, we find that spectral positions of the bright and dark modes are linearly dependent on, respectively, the global spacing between adjacent unit cells and the local spacing between adjacent dipoles within each unit cell. The dependence of the spectral position of bright (dark) mode on the global (local) spacing is independent without mutual influence, which provides a facile pathway to control the Fano resonance with large flexibility. Our proposed scheme to control Fano resonance is highly desired in various fields including lasing spaser and biosensing with improved performance.

A COMPACT DUAL-BAND DUAL-POLARIZED ANTENNA FOR BASE STATION APPLICATION

A compact dual-band dual-polarized antenna is proposed in this paper. The two pair dipoles with strong end coupling are used for the lower frequency band, and cross-placed patch dipoles are used for the upper frequency band. The ends of the dipoles for lower frequency band are bent to increase the coupling between adjacent dipoles, which can benefit the compactness and bandwidth of the antenna. Breaches are introduced at the ends of the dipoles of the upper band, which also benefit the compactness and matching of the antenna. An antenna prototype was fabricated and measured. The measured results show that the antenna can cover from 790MHz to 960MHz (19.4%) for lower band and from 1710MHz to 2170MHz (23.7%) for upper band with VSWR < 1.5. It is expected to be a good candidate design for base station antennas.

Smooth Teeth: Why Multipoles Are Perfect Gears

What if gears could mesh perfectly smoothly and without friction or wear? Allowing adjacent magnetic dipoles to rotate about axes orthogonal to their dipole moments produces special orientations, for which the ground state is a degenerate continuum. In these cases, when one dipole spins around its axis the next dipole follows, without any counterforce, creating a gear train. Different multipoles allow various gear ratios. This is of immediate practical relevance, from micromachines to lab-on-a-chip actuators driven by an external magnetic dipole.

Induced Dipole–Dipole Interactions Influence the Unfolding Pathways of Wild-Type and Mutant Amyloid β-Peptides

Amyloid-forming proteins undergo a structural transition from α-helical to disordered conformations and, ultimately, cross-β fibrils. The unfolding and aggregation of the amyloid β-peptide (Aβ) have been implicated in the development and progression of Alzheimer’s disease (AD) and cerebral amyloid angiopathy (CAA). However, the events underlying the initial structural transition leading to the disease state remain unclear. Although most cases are sporadic, several genetic variants exist that alter the electrostatic properties of Aβ and lead to more rapid unfolding and more severe phenotypes. In the present study, the enhanced unfolding is shown to be due to the mutated side chains altering the local peptide-bond dipole moments leading to local destabilization of the α-helix, as determined from polarizable molecular dynamics (MD) simulations of wild-type (WT) Aβ fragments and several common mutations. The local perturbation of the helix then leads to progressive unwinding of the α-helix in a cooperative fashion due to decreases in adjacent (i ± 1) and hydrogen-bonded (i + 4) peptide-bond dipole moments. Side-chain dynamics, subsequent variations in dipole moments, and ultimately the response in the peptide-bond dipole moments are all modulated by solvent dielectric properties based on simulations in water versus ethanol. The polarizable simulation results, along with simulations using the additive CHARMM36 force field, further indicate that cooperativity due to the alignment of peptide bonds leading to enhanced dipole moments is a fundamental force in stabilizing α-helices.

A DPIV study on the effects of separation distance upon the vortical behaviour of jet–cylinder impingements

An experimental investigation based on laser-induced fluorescence (LIF) and digital particle image velocimetry (DPIV) was conducted to provide a better understanding into the effects of jet–cylinder separation distance on the vortical structures and dynamic behaviour resulting from jet impingements upon convex cylinders. Separation distances of 1, 2 and 4 jet diameters were investigated, while cylinder–jet diameter ratio was maintained at 2 throughout. LIF and DPIV results show that jet ring-vortex initiation, wall-separated vortex initiation, vortex dipole formation and vortex separation occur further away from the impingement point as the separation distance decreases. Varying the separation distance also influences the recirculating wake region size at the cylinder lee sides, as well as producing two distinct flow modes between adjacent vortex dipoles along the cylinder straight edges. Mean and instantaneous skin friction coefficient distributions determined from DPIV results not only showcase the different effects of separation distance on the wall shear stress, but also illustrate how the flow dynamics influence the local wall shear stress levels. On the other hand, wall-separated vortex initiation, vortex dipole formation and separation events incur little distinctive changes upon the local wall shear stress distributions that may ease their unique identification.

Phase effects on the optical near-filed interaction of a nanohole dimer milled in a gold film

The near-field optical interaction of a nanohole dimer milled in a gold film and illuminated by an optical plane-wave is investigated. Depending on the polarization direction of the incident light, two resonant peaks are generated, in the normalized transmission spectrum due to the strong interaction between the nanoholes. In order to elucidate the nature of the interaction between the nanohole dimer elements at the resonant peaks, the symmetry breaking idea is applied. It is found that, in contrast to the complementary metallic nanoparticle dimers, the short and long wavelength peaks of the transmission spectrum can be assigned to in-phase and out-of-phase interaction of two adjacent magnetic dipoles, respectively. Furthermore, it is shown that breaking the symmetry in line with the polarization of the incident magnetic field results in severely antiphase interaction, while breaking the symmetry in line with the polarization of the incident electric field results in slightly out-of-phase interaction, at the long wavelength peak. The numerical simulation results are verified with Magnetic Coupled Dipole Approximation (MCDA) method.

Antiferroelectric Materials, Applications and Recent Progress on Multiferroic Heterostructures

Antiferroelectric (AFE) materials with adjacent dipoles oriented in antiparallel directions have a double polarization hysteresis loops. An electric field (E-field)-induced AFE–ferroelectric (FE) phase transition takes place in such materials, leading to a large lattice strain and energy change. The high dielectric constant and the distinct phase transition in AFE materials provide great opportunities for the realization of energy storage devices like super-capacitors and energy conversion devices such as AFE MEMS applications. Lots of work has been done in this field since 60–70 s. Recently, the strain tuning of the spin, charge and orbital orderings and their interactions in complex oxides and multiferroic heterostructures have received great attention. In these systems, a single control parameter of lattice strain is used to control lattice–spin, lattice–phonon, and lattice–charge interactions and tailor properties or create a transition between distinct magnetic/electronic phases. Due to the large strain/stress arising from the phase transition, AFE materials are great candidates for integrating with ferromagnetic (FM) materials to realize in situ manipulation of magnetism and lattice-ordered parameters by voltage. In this paper, we introduce the AFE material and it's applications shortly and then review the recent progress in AFEs based on multiferroic heterostructures. These new multiferroic materials could pave a new way towards next generation light, compact, fast and energy efficient voltage tunable RF/microwave, spintronic and memory devices promising approaches to in situ manipulation of lattice-coupled order parameters is to grow epitaxial oxide films on FE/ferroelastic substrates.

Grid-free 2D plasma simulations of the complex interaction between the solar wind and small, near-Earth asteroids

We present results from a new grid-free 2D plasma simulation code applied to a small, unmagnetized body immersed in the streaming solar wind plasma. The body was purposely modeled as an irregular shape in order to examine photoemission and solar wind plasma flow in high detail on the dayside, nightside, terminator and surface-depressed ‘pocket’ regions. Our objective is to examine the overall morphology of the various plasma interaction regions that form around a small body like a small near-Earth asteroid (NEA). We find that the object obstructs the solar wind flow and creates a trailing wake region downstream, which involves the interplay between surface charging and ambipolar plasma expansion. Photoemission is modeled as a steady outflow of electrons from illuminated portions of the surface, and under direct illumination the surface forms a non-monotonic or “double-sheath” electric potential upstream of the body, which is important for understanding trajectories and equilibria of lofted dust grains in the presence of a complex asteroid geometry. The largest electric fields are found at the terminators, where ambipolar plasma expansion in the body-sized nightside wake merges seamlessly with the thin photoelectric sheath on the dayside. The pocket regions are found to be especially complex, with nearby sunlit regions of positive potential electrically connected to unlit negative potentials and forming adjacent natural electric dipoles. For objects near the surface, we find electrical dissipation times (through collection of local environmental solar wind currents) that vary over at least 5 orders of magnitude: from 39μs inside the near-surface photoelectron cloud under direct sunlight to ≫1s inside the particle-depleted nightside wake and shadowed pocket regions.

Structural Determinants of Tau Aggregation Inhibitor Potency

Small-molecule Tau aggregation inhibitors are under investigation as potential therapeutic agents against Alzheimer disease. Many such inhibitors have been identified in vitro, but their potency-driving features, and their molecular targets in the Tau aggregation pathway, have resisted identification. Previously we proposed ligand polarizability, a measure of electron delocalization, as a candidate descriptor of inhibitor potency. Here we tested this hypothesis by correlating the ground state polarizabilities of cyanine, phenothiazine, and arylmethine derivatives calculated using ab initio quantum methods with inhibitory potency values determined in the presence of octadecyl sulfate inducer under reducing conditions. A series of rhodanine analogs was analyzed as well using potency values disclosed in the literature. Results showed that polarizability and inhibitory potency directly correlated within all four series. To identify putative binding targets, representative members of the four chemotypes were added to aggregation reactions, where they were found to stabilize soluble, but SDS-resistant Tau species at the expense of filamentous aggregates. Using SDS resistance as a secondary assay, and a library of Tau deletion and missense mutants as targets, interaction with cyanine was localized to the microtubule binding repeat region. Moreover, the SDS-resistant phenotype was completely dependent on the presence of octadecyl sulfate inducer, but not intact PHF6/PH6* hexapeptide motifs, indicating that cyanine interacted with a species in the aggregation pathway prior to nucleus formation. Together the data suggest that flat, highly polarizable ligands inhibit Tau aggregation by interacting with folded species in the aggregation pathway and driving their assembly into soluble but highly stable Tau oligomers.

Electromagnetic dipole coupling mechanism in layered terahertz metamaterials

Interplay between adjacent dipoles is an experimental priori for designing artificially-engineered structure because the dipole coupling is one critical factor for determining the electromagnetic response in metamaterials. Although numerous investigations have been performed to study the coupling effect of the split-ring resonator (SRR), the interlayer dipole coupling of its complementary SRR, called C-SRR, has been largely unexplored. Here, we present experimental and theoretical investigations on the electromagnetic coupling effect in the two stacks of layered C-SRR structures. By adjusting the relative lateral distance between the two-dimensionally stacked meta-structures, we observe that the confined magnetic dipole plays an important role in determining the resonance frequency and the bandwidth broadening of the C-SRR, exhibiting an exactly opposite behavior to the SRR structure. Our investigation provides experimental basis for developing frequency tunable three-dimensional metamaterial devices.

Printed, coplanar, magnetic-dipole multiantenna system for 5-GHz wlan applications

A printed, three-magnetic-dipole antenna system symmetrically placed on a single-layered FR4 substrate suitable for 5-GHz WLAN applications is presented. The dipoles are set in a sequential, rotating arrangement on a small substrate with an equal inclination angle to form a symmetrical multiantenna structure. Each antenna occupies an area of 7 mm × 10 mm and is spaced 5.4 mm apart from the adjacent dipole. The magnetic dipole has a U-shaped radiating arm and is capacitively excited by a driven coupling strip. With a compact size (radius less than 15 mm) of the antenna system, the design yet shows a wide operating band with good port isolation attained. Details of a design prototype are described and discussed in the article. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:1726–1731, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26886

Theory to exciton dissociation at the interface between a conjugated polymer and an electron acceptor

Experimental studies evidence that excitons in organic materials can efficiently dissociate into free electrons and holes at an intrinsic donor/acceptor interface despite the strong Coulomb attraction between the charges provided by the low dielectric constant in organics. Using a semi-analytic theory for the dissociation probability of electron-hole pairs we show that although delocalization of charge carriers along the polymer chains can help the carriers to overcome the Coulomb potential barrier, this effect is not enough to explain the efficient and field-independent dissociation of pairs at the interfaces.Furthermore we study the influence of the dipole layer built in the dark at the donor/acceptor interface considering two distinct forms of this dipole layer: a continuous layer and a system of discrete dipoles. In the latter case very high dissociation probabilities can be reached. The main effect here is provided by the quantization energy of holes restrained by the adjacent dipoles.

Electric Field-Driven Water Dipoles: Nanoscale Architecture of Electroporation

Electroporation is the formation of permeabilizing structures in the cell membrane under the influence of an externally imposed electric field. The resulting increased permeability of the membrane enables a wide range of biological applications, particularly as a way to deliver normally excluded substances into cells. While electroporation is used extensively in biology, biotechnology, and medicine, its molecular mechanism is not well understood, which limits the ability to control and fine-tune the process. Using molecular dynamics simulations we demonstrate that the initial trigger of the pore formation is the reorganization of the water dipoles at the water-lipid interface into narrow protrusions of water molecules whose dipole moments are aligned with the external electric field. Such alignment is not possible in the bulk phase due to the unfavorable energetics of adjacent parallel dipole moments. Contrary to some previous studies which focused on lipid behavior, our model indicates that interfacial water molecules are the main players in the process, its initiators and drivers. The role of the lipid bilayer is then reduced to that of a relatively passive “sticky” barrier. We present simulations that show that a process similar to electroporation happens even in the absence of the lipid bilayer, with water at a vacuum interface. This new model of electroporation indicates that this process can be modeled as a simpler solvent-only system and opens up new opportunities in both theoretical studies of the process as well as development of new technologies to better control it or to use it in new innovative ways.

An integrative pharmacological approach to radio telemetry and blood sampling in pharmaceutical drug discovery and safety assessment

BackgroundA successful integration of the automated blood sampling (ABS) and telemetry (ABST) system is described. The new ABST system facilitates concomitant collection of physiological variables with blood and urine samples for determination of drug concentrations and other biochemical measures in the same rat without handling artifact.MethodIntegration was achieved by designing a 13 inch circular receiving antenna that operates as a plug-in replacement for the existing pair of DSI's orthogonal antennas which is compatible with the rotating cage and open floor design of the BASi Culex® ABS system. The circular receiving antenna's electrical configuration consists of a pair of electrically orthogonal half-toroids that reinforce reception of a dipole transmitter operating within the coil's interior while reducing both external noise pickup and interference from other adjacent dipole transmitters.ResultsFor validation, measured baclofen concentration (ABST vs. satellite (μM): 69.6 ± 23.8 vs. 76.6 ± 19.5, p = NS) and mean arterial pressure (ABST vs. traditional DSI telemetry (mm Hg): 150 ± 5 vs.147 ± 4, p = NS) variables were quantitatively and qualitatively similar between rats housed in the ABST system and traditional home cage approaches.ConclusionThe ABST system offers unique advantages over traditional between-group study paradigms that include improved data quality and significantly reduced animal use. The superior within-group model facilitates assessment of multiple physiological and biochemical responses to test compounds in the same animal. The ABST also provides opportunities to evaluate temporal relations between parameters and to investigate anomalous outlier events because drug concentrations, physiological and biochemical measures for each animal are available for comparisons.

Comment on “New statistical mechanical model for calculating Kirkwood factors in self-associating liquid systems and its application to alkanol+cyclohexane mixtures” [J. Chem. Phys. 127, 114501 (2007)

A brief overview of the current state of the research on calculation of Kirkwood factor of liquid alcohols and their mixtures is presented. On the basis of the accurate solution for the correlation factor, obtained in the author's published papers, it was stated that the formulas for Kirkwood factor of alcohols g(K) and their mixtures with inert solvents (hydrocarbons) obtained in the paper by Vasiltsova and Heintz [J. Chem. Phys. 127, 114501 (2007)], in the opinion of the present author, are erroneous and contradict all preceding results obtained in this area of research. On the basis of the rigorous solution for Kirkwood factor of liquids with predominant chainlike aggregation it has been shown that the generally accepted opinion based on the famous Kirkwood formula g(K)=1+z<cos gamma>, where z is coordination number, <cos gamma> is average cosine of angle between adjacent dipoles in liquids ([J. G. Kirkwood, J. Chem. Phys. 7, 911 (1939)], Eq. (19)) that Kirkwood factor as determined by the mean cosine of the angle between adjacent dipoles should be modified for the liquids consisting of nonlinear molecules with the complex type of the supramolecular organization. Alcohols are examples of such liquids, in which long-range intermolecular correlations beyond the nearest coordination shells are essential for their structural organization.

Are Adenine Strands Helical H-Aggregates?

On the basis of our quantum mechanical calculation, we propose that homogeneous single-stranded adenine bases (Ade-DNA) form helical H aggregates, and the photoexcited states can be described as Frenkel excitons. The calculated excitonic coupling between adjacent transition dipoles is in good agreement with the measured absorption spectrum of 20-base homogeneous adenine stacks that exhibits a blue shift of 2.6 nm relative to that of the monomeric species.

Internal dipole radiation as a tool for particle identification

A numerical approach for the calculation of the internal dipole radiation associated with particles of arbitrary morphology is investigated by using the discrete-dipole approximation (DDA) method. The DDA and analytical solutions for the total radiated power and radiation pattern are compared in the case of spherical host particles. It is shown that the DDA can be quite accurate under the condition that m <or approximately 2, and mkd<0.5, where m is the refractive index of the host particle, k=2pi/lambda is the wavenumber in vacuum, and d is the distance between two adjacent dipoles in the DDA cubic dipole array. Furthermore, the DDA solutions for the dipole radiation patterns associated with nonspherical host particles are compared with their corresponding counterparts obtained from the finite-difference time-domain method. Excellent agreement between the two results is noted. The DDA method is also applied to the computation of the internal dipole radiation associated with simulated nonspherical sporelike particles. The results suggest that the internal dipole radiation patterns contain a great deal of information about the morphology and composition of the host particle.