Weak Ac Field Research Articles

Compound giant unilamellar vesicles as a bio-mimetic model for electrohydrodynamics of a nucleate cell.

The understanding obtained by studies on the electrohydrodynamics (EHD) of single giant unilamellar vesicles (sGUVs) has contributed significantly towards a better comprehension of the response of biological cells to electric fields. This has subsequently helped in developing technologies such as cell dielectrophoresis and cell electroporation. For nucleate eukaryotic cells though, a vesicle-in-vesicle compound giant unilamellar vesicle (cGUV) is a more appropriate bio-mimic than a sGUV. In this work, we present an improvised method for the formation of cGUVs, wherein the electrical conductivities of the inner, annular and outer regions of the cGUVs can be modified. A comprehensive experimental study is presented on the EHD of these cGUVs under weak AC fields over a wide range of frequencies, and an encouraging agreement is observed between the experiments and earlier published theoretical studies on concentric cGUVs. The spherical, prolate or oblate spheroidal deformations of a cGUV under AC electric fields depend upon the membrane electromechanical properties as well as the magnitude and direction of the electric traction at the membrane produced by the Maxwell stress that varies with the relative timescales associated with the frequency of the applied AC electric field and that of the membrane charging time and the Maxwell-Wagner relaxation time. This work establishes cGUVs as appropriate bio-mimics for conducting EHD studies relevant to eukaryotic cells.

High-order dynamic localization and tunable temporal cloaking in ac-electric-field driven synthetic lattices

Dynamic localization (DL) of photons, i.e., the light-motion cancellation effect arising from lattice's quasi-energy band collapse under a synthetic ac-electric-field, provides a powerful and alternative mechanism to Anderson localization for coherent light confinement. So far only low-order DLs, corresponding to weak ac-fields, have been demonstrated using curved-waveguide lattices where the waveguide's bending curvature plays the role of ac-field as required in original Dunlap-Kenkre model of DL. However, the inevitable bending losses pose a severe limitation for the observation of high-order DL. Here, we break the weak-field limitation by transferring lattice concepts from spatial to synthetic time dimensions using fiber-loop circuits and observe up to fifth-order DL. We find that high-order DLs possess superior localization and robustness against random noise over lower-order ones. As an exciting application, by judiciously combining low- and high-order DLs, we demonstrate a temporal cloaking scheme with flexible tunability both for cloak's window size and opening time. Our work pushes DL towards high-order regimes using synthetic-lattice schemes, which may find potential applications in robust signal transmission, protection, processing, and cloaking.

Repolarization processes in lead-free KNN ceramics

In this work, we studied the nature of repolarization processes in KNN-0.05Ta ceramics in the temperature range of structural phase transition. We have found the anomalous behavior of reverse dependences ε'(Ε =), while measuring the response in weak ac-fields in the sample exposed to the constant bias field. Behavior of ε'(Ε =) was compared with the polarization loops measured in sinusoidal fields of various amplitudes.

Resonant Precession Modes of a Dipole System under a Weak AC Field

Resonant Precession Modes of a Dipole System under a Weak AC Field

Dynamic magnetic susceptibility of a ferrofluid: The influence of interparticle interactions and ac field amplitude.

Based on numerical results of dynamic susceptibility, a simple theory of the dynamic response of a ferrofluid to an ac magnetic field is obtained that includes both the effects of interparticle dipole-dipole interactions and the dependence on field amplitude. Interparticle interactions are incorporated in the theory using the so-called modified mean-field approach. The new theory has the following important characteristics: in the noninteracting regime at a weak ac field, it gives the correct single-particle Debye theory results; it expands the applicability of known theories valid for high concentrations [Ivanov, Zverev, and Kantorovich, Soft Matter 12, 3507 (2016)10.1039/C5SM02679B] or large values of ac field amplitudes [Yoshida and Enpuku, Jpn. J. Appl. Phys. 48, 127002 (2009)10.1143/JJAP.48.127002], in accordance with their applicability. The susceptibility spectra are analyzed in detail. It is demonstrated that interparticle dipole-dipole interactions and an increase in field amplitude have an opposite effect on the dynamic response of ferrofluids, so that at certain field amplitudes, relaxation processes in the system of interacting particles are determined by the characteristic relaxation times for an ideal paramagnetic gas. The new theory correctly predicts the dynamic susceptibility and characteristic relaxation times of a ferrofluid at high ac field amplitudes as long as the Langevin susceptibility χ_{L}≲1, which is a complex characteristic of ferrofluid density and the intensity of interparticle dipole-dipole interactions.

Sharp demagnetization of closed-loop HTS coil in first cycle of external AC fields induced by unexpected dynamic resistance

Dynamic resistance leads to the demagnetization of high-T c superconducting (HTS) permanent magnets (such as closed-loop coils and tape stacks) when exposed to AC fields. This letter reports on the systematic study of an unexpected dynamic resistance occurring in first cycle AC field, which can be induced by any weak AC field—even below the threshold value ‘conventionally’ defined for generating dynamic resistance. This resistance is predicted by a critical-state analytical calculation, and its physical origin is shown to be from the asymmetrical flux penetration. Numerically, this resistance is verified, and simulated for a closed-loop HTS coil, showing a non-negligible demagnetization effect. Experimentally, the current decay (i.e. demagnetization) characteristics of a closed-loop HTS coil in a sufficient range of AC fields are measured. This resistance manifests itself in the closed-loop coil as a relatively sharp current decay in the first cycle of the applied field; this observation is in qualitative agreement with the simulation. A comparative experiment is performed to eliminate the contribution from the index loss or ‘conventional’ dynamic resistance that also possibly results in a greater demagnetization in the first cycle owing to the larger coil current. As the result, this unexpected dynamic resistance is verified. The unexpected, sharp demagnetization caused by this resistance might need to be considered when designing HTS closed-loop coil applications.

Numerical modeling of the magnetic response of interacting superparamagnetic particles to an ac field with arbitrary amplitude

The dynamic magnetic response of immobilized superparamagnetic nanoparticles to an ac field with arbitrary amplitude is studied using numerical simulations. The nanoparticles are considered to be distributed randomly within an implicit solid matrix, but the easy axes of the particles are aligned parallel to the ac magnetic field. Modeling of dynamic response is based on the Fokker–Planck–Brown equation in which the interparticle dipole–dipole interactions are included within the framework of the modified mean-field theory. The effects of the magnetic crystallographic anisotropy barrier, the ac field amplitude, and the interparticle interactions on the dynamic magnetization, susceptibility, and relaxation time are analyzed. It is shown that an increase in the amplitude of the ac field significantly accelerates relaxation processes in the system under consideration whereas an increase in magnetic anisotropy of a particle and interparticle dipole–dipole interactions slows them down. The numerical results for dynamic susceptibility and relaxation time are compared against theories reliable in the weak ac field, and an excellent agreement is obtained.

High-Speed Piezoresponse Force Microscopy and Machine Learning Approaches for Dynamic Domain Growth in Ferroelectric Materials.

Domain dynamics has been one of the hottest research topics for ferroelectric materials in order to understand the ferroelectric mechanisms and to develop the related applications. By using high-speed piezoresponse force microscopy (HSPFM), it is possible to observe the dynamic domain evolution in an ultrashort time increment. This paper combines the HSPFM experiments and machine learning to study the domain growth under a weak AC field in ferroelectric materials. Here, the Bayesian optimized support vector machine is employed to classify the switching domain and stable domain. The results indicate that the machine learning classifier is capable of discerning the switching area. In addition, the domain associated characteristics, such as domain pinning and domain wall pinning, can also be observed and analyzed by combining experiments and machine learning. The machine learning approach can fast and deeply extract the complicated features related to free energy from the multidimensional signals obtained by HSPFM.

Detection of Magnetosome-Like Structures in Eukaryotic Cells Using Nonlinear Longitudinal Response to ac Field

Although magnetosomes have been discovered in bacteria since several decades, until today the question remains open whether such biomineralized structures do exist in eukaryotic cells. Herein, evidence was provided for the existence of magnetosome-like Fe-based structures in different viable eukaryotic cells by the registration of second harmonic of magnetization M2(H) of longitudinal nonlinear response to weak ac field. The behavior of the field hysteresis of the M2 response from cells in suspension and/or in pellet indicated a multi-domain state of magnetosome-like structures in certain type of cells, and a single-domain state in other cell lines. The amounts of magnetosomes in cells range from ≤ 1÷2 to 5÷8 per cell. The presence of magnetosome-like structures was analyzed in normal tissue samples obtained from Wistar rats and C57/Bl6 mice. Additionally, the tumor tissue (orthotopic rat C6 glioma and mouse GL261 glioma) were assessed for magnetosomes. Detected magnetosomes in certain tissues (i.e., brain, heart, lungs) matched to a single-domain magnetite nanoparticle, whereas in other organs they exhibited characteristics attributable to a multi-domain state, better corresponding to Fe(0) composition of their magnetic cores. Subsequent studies are necessary to elucidate the role of the Fe-based magnetosome-like structures in the biology and physiology of eukaryotic cells.

Linear complex susceptibility of long-range interacting dipoles with thermal agitation and weak external ac fields

An analytical formula for the linear complex susceptibility of dipolar assemblies subjected to thermal agitation, long-range interactions, and an externally applied uniform sinusoidal field of weak amplitude is derived using the forced rotational diffusion equation of Cugliandolo et al. [Phys. Rev. E 91, 032139 (2015)] in the virial approximation. If the Kirkwood correlation factor of the dipolar assembly ${g}_{K}$ exceeds unity, a thermally activated process arising from the interaction-specific component arises, while for ${g}_{K}<1$ the susceptibility spectrum normalized by its static value is practically unaltered with respect to that of the ideal gas phase.

Differential polarization of cortical pyramidal neuron dendrites through weak extracellular fields

The rise of transcranial current stimulation (tCS) techniques have sparked an increasing interest in the effects of weak extracellular electric fields on neural activity. These fields modulate ongoing neural activity through polarization of the neuronal membrane. While the somatic polarization has been investigated experimentally, the frequency-dependent polarization of the dendritic trees in the presence of alternating (AC) fields has received little attention yet. Using a biophysically detailed model with experimentally constrained active conductances, we analyze the subthreshold response of cortical pyramidal cells to weak AC fields, as induced during tCS. We observe a strong frequency resonance around 10-20 Hz in the apical dendrites sensitivity to polarize in response to electric fields but not in the basal dendrites nor the soma. To disentangle the relative roles of the cell morphology and active and passive membrane properties in this resonance, we perform a thorough analysis using simplified models, e.g. a passive pyramidal neuron model, simple passive cables and reconstructed cell model with simplified ion channels. We attribute the origin of the resonance in the apical dendrites to (i) a locally increased sensitivity due to the morphology and to (ii) the high density of h-type channels. Our systematic study provides an improved understanding of the subthreshold response of cortical cells to weak electric fields and, importantly, allows for an improved design of tCS stimuli.

Concentration-dependent zero-field magnetic dynamic response of polydisperse ferrofluids

In this contribution, we discuss the experimental results obtained for seven ferrofluid samples with the same particle size distribution that differ only in concentration of magnetic material. The dynamic response to a weak linearly-polarised probing AC field is measured for each sample at five different temperatures. We investigate Cole-Cole diagrams and phase shifts in order to describe the impact of ferroparticle concentration on the initial magnetic susceptibility. Our findings show that the main contribution comes from the increasing effective viscosity of the ferrofluid that results in the growth of the Brownian relaxation time. This mechanism is very important for the systems containing even a small portion of large magnetic colloids. The influence of dipolar correlations was separately analysed using computer simulations.

Narrow-bandwidth sensing of high-frequency fields with continuous dynamical decoupling

State-of-the-art methods for sensing weak AC fields are only efficient in the low frequency domain (<10 MHz). The inefficiency of sensing high-frequency signals is due to the lack of ability to use dynamical decoupling. In this paper we show that dynamical decoupling can be incorporated into high-frequency sensing schemes and by this we demonstrate that the high sensitivity achieved for low frequency can be extended to the whole spectrum. While our scheme is general and suitable to a variety of atomic and solid-state systems, we experimentally demonstrate it with the nitrogen-vacancy center in diamond. For a diamond with natural abundance of 13C, we achieve coherence times up to 1.43 ms resulting in a smallest detectable magnetic field strength of 4 nT at 1.6 GHz. Attributed to the inherent nature of our scheme, we observe an additional increase in coherence time due to the signal itself.

Magnetic phase separation and unusual scenario of its temperature evolution in porous carbon-based nanomaterials doped with Au and Co

Two porous glassy carbon-based samples doped with Au and Co were investigated. The magnetization study as well as measurements of the nonlinear longitudinal response to a weak ac field (NLR) and electron magnetic resonance give evidences for a presence of magnetic nanoparticles (MNPs) embedded in paramagnetic/ferromagnetic matrix respectively, both samples being in magnetically phase-separated state at temperatures above 300K. Matrix, forming by paramagnetic centers located in matrix outside the MNPs, reveals exchange interactions providing its ferromagnetic (FM) ordering below TC≈ 210K in Au-doped sample and well above 350K in Co-doped one. For the former, NLR data suggest a percolation character of the matrix long-range FM order, which is mainly caused by a porous amorphous sample structure. Temperature dependence of the magnetization in the Au-doped sample evidences presence of antiferromagnetic (AF) interactions of MNPs with surrounding matrix centers. At magnetic ordering below TC these interactions promote origination of “domains” involving matrix fragment and surrounding MNPs with near opposite orientation of their moments that decreases the magnetostatic energy. On further cooling, the domains exhibit AF ordering below Tcr∼140K<TC, resulting in formation of a peculiar “ferrimagnet”. The porous amorphous structure leads to absence of translational and other symmetry features through the samples that allows canted ordering of magnetic moments in domains and in whole sample providing “canted ferrimagnetism”. At low temperatures Ttr∼3K, “order-oder” transition, evidencing the non-Heisenberg character of this magnetic material, occurs from ordering like “canted ferrimagnet” to FM alignment, which is stimulated by external magnetic field. The data for Co-doped sample imply the similar evolution of magnetic state but at higher temperatures above 350K. This state exhibits more homogeneous arrangement of the FM nanoparticles and the FM matrix. Order-order transition occurs in it at higher Ttr∼10–15K as well and followed by formation of long-range FM ordering found earlier by neutron diffraction. Doping of carbon-based nanomaterials by magnetic metals provides advantages for their possible practical applications as Co-doped sample with higher TC (>350K) and larger remanent magnetization evidences.

Spatially dispersive dynamical response of hot carriers in doped graphene

We study theoretically wave-vector and frequency dispersion of the complex dynamic conductivity tensor (DCT), σlm(k,ω), of doped monolayer graphene under a strong dc electric field. For a general analysis, we consider the weak ac field of arbitrary configuration given by two independent vectors, the ac field polarization and the wave vector k. The high-field transport and linear response to the ac field are described on the base of the Boltzmann kinetic equation. We show that the real part of DCT, calculated in the collisionless regime, is not zero due to dissipation of the ac wave, whose energy is absorbed by the resonant Dirac quasiparticles effectively interacting with the wave. The role of the kinematic resonance at ω=vF|k| (vF is the Fermi velocity) is studied in detail taking into account deviation from the linear energy spectrum and screening by the charge carriers. The isopower-density curves and distributions of angle between the ac current density and field vectors are presented as a map which provides clear graphic representation of the DCT anisotropy. Also, the map shows certain ac field configurations corresponding to a negative power density, thereby it indicates regions of terahertz frequency for possible electrical (drift) instability in the graphene system.

Magneto-Electronic Phase Separation in La&lt;sub&gt;0.7&lt;/sub&gt;Sr&lt;sub&gt;0.3&lt;/sub&gt;MnO&lt;sub&gt;3&lt;/sub&gt; with Metallic Behavior in Paramagnetic Region

The data on transport and magnetic properties (ac linear and nonlinear (second and third orders) susceptibilities) are presented for La0.7Sr0.3MnO3 single crystal with metallic behavior in paramagnetic (PM) region and ferromagnetic (FM) metallic ground state. The FM clusters originate in the PM matrix of the compound below some temperature T* &gt; 425 K, their nonlinear response being weakly T-dependent down to 367 K. This was attributed to clusters arising in the preferable sites formed by chemical inhomogeneities introduced by doping. On cooling below T# ≈ 366 K &gt; TC ≈ 363.3 K, a fast growth of cluster response without the change of its parameters is observed that was attributed to the development of homogeneous nucleation of the FM clusters. The latter stage continues below TC and is accompanied by a crossover to the steeper decreasing of resistivity with cooling that suggests metallic properties of the clusters. The cluster nonlinear response masks completely that of matrix at T = 360.3 K &lt; TC, where it is well described by the model of ensemble of magnetic single-domain nanoparticles in superparamagnetic regime based on the formalism involving Gilbert-Landau-Lifshits equation. Below TD = 359.6 K at the stage of domain formation, a weak interaction of matrix and cluster subsystems leads to their mutual ordering, which is accompanied by a sharp decrease of the nonlinear response to a weak ac field in small steady field H. The latter suggests an “antiferromagnetic” type of arrangement of these subsystems that provides decreasing the magnetostatic energy of the sample.

Weak ac field-induced patterns in vertical deposition of colloids at various evaporation rates

Pattern formation in colloids by weak ac fields in vertical deposition-like configuration at different temperatures has been studied experimentally. At low evaporation (room temperature), the effect of the field leads to the evolution of a one-dimensional array of clusters along the contact line and columnar colloidal dried deposits are obtained at higher evaporation. We investigate the flow dynamics involved in this pattern formation. Homogeneous variation of the contact angle by electrowetting effect becomes unstable and breaks the translational symmetry at the meniscus. Electrokinetic forces together with capillary forces result in the accumulation of particles for pattern formation. The movement of electrically charged colloidal particles is controlled by weak ac electric field even at higher temperatures. We observe the effect of increasing initial particle concentration on the behavior of the clusters for various field frequencies. The average distance between clusters increase monotonically with an increase in the initial particle concentration. We also observe that the average width of columns increases according to the applied field strength.

Nonlinear normal and anomalous response of non-interacting electric and magnetic dipoles subjected to strong AC and DC bias fields

The perturbation theory approach via the Smoluchowski equation to the nonlinear dielectric relaxation of noninteracting permanent electric dipoles (Coffey and Paranjape, Proc R Ir Acad A 78:17, 1978) and the analogous Brownian magnetic relaxation of ferrofluids where Neel relaxation is ignored is revisited for the particular case of a strong dc bias field superimposed on a strong ac field. Unlike weak ac and strong bias dc fields, a frequency-dependent dc term now appears in the response as well as additional nonlinear terms at the fundamental and second harmonic frequencies. These may be experimentally observable particularly in the ferrofluid application. The corresponding results for the dc term for anomalous relaxation based on the fractional Smoluchowski equation are also given.

Robust unidirectional rotation in three-tooth Brownian rotary ratchet systems

We apply a simple Brownian ratchet model to an artificial molecular rotary system mounted in a biological membrane, in which the rotor always maintains unidirectional rotation in response to a linearly polarized weak ac field. Because the rotor and stator compose a ratchet system, we describe the motion of the rotor tip with the Langevin equation for a particle in a two-dimensional three-tooth ratchet potential of threefold symmetry. Unidirectional rotation can be induced under the field and optimized by stochastic resonance, wherein the mean angular momentum (MAM) of the rotor exhibits a bell-shaped curve for the noise strength. We obtain analytical expressions for the MAM and power loss from the corresponding Fokker-Planck equation, via a Markov transition model for coarse-grained states (six-state model). The MAM expression reveals a significant effect depending on the chirality of the ratchet potential: in achiral cases, the MAM approximately vanishes with respect to the polarization angle φ of the field; in chiral cases, the MAM does not crucially depend on φ, but depends on the direction of the ratchet; i.e., the parity of the unidirectional rotation is inherent in the ratchet structure. This feature is useful for artificial rotary systems to maintain robust unidirectional rotation independent of the mounting condition.

2100 OET ASSAY OF NON-MOTILE SPERM TO PREDICT TWO-CELL EMBRYO AND LIVE-BIRTH RATES USING A MOUSE ICSI MODEL

You have accessJournal of UrologyInfertility: Physiology, Pathophysiology, Basic Research1 Apr 20122100 OET ASSAY OF NON-MOTILE SPERM TO PREDICT TWO-CELL EMBRYO AND LIVE-BIRTH RATES USING A MOUSE ICSI MODEL Maurice Garcia, Aaron Ohta, Ryuzo Yanagimachi, W. Steve Ward, Stefan Moisyadi, and Joel Marh Maurice GarciaMaurice Garcia San Francisco, CA More articles by this author , Aaron OhtaAaron Ohta Honolulu, HI More articles by this author , Ryuzo YanagimachiRyuzo Yanagimachi Honolulu, HI More articles by this author , W. Steve WardW. Steve Ward Honolulu, HI More articles by this author , Stefan MoisyadiStefan Moisyadi Honolulu, HI More articles by this author , and Joel MarhJoel Marh Honolulu, HI More articles by this author View All Author Informationhttps://doi.org/10.1016/j.juro.2012.02.2267AboutPDF ToolsAdd to favoritesDownload CitationsTrack CitationsPermissionsReprints ShareFacebookTwitterLinked InEmail INTRODUCTION AND OBJECTIVES Optoelectronic Tweezers (OET) is a non-invasive motility-independent assay that employs weak AC fields and light to assess a cell's dielectric potential. We have shown that living cells display a strong attraction (+) response to an OET field, whereas dead cells are weakly repulsed (−) or are neutral (do not respond) to OET. We have previously reported how sperm can be assayed with OET to predict sperm viability in vitro. Using a mouse ICSI model, we sought to assess whether OET assay lowers pup rates, and, whether it predicts ICSI outcomes among non-motile sperm, comparing (presumably healthy) and sperm damaged by freeze-thaw in a dose-dependent fashion. METHODS Sperm from healthy B6D2F1 mice was harvested and suspended in CZB (highly electrically conductive). Sperm were randomized to 3 experimental groups suspended in Mannitol Electrofusion Medium (MEM) (OET compatible because minimally conductive): Group 1 (no treatment), Group 2 (Freeze-Thaw in MEM x 1). After treatment, random samples of only non-motile sperm underwent OET assay, followed by ICSI and embryo transfer. Positive and negative assay and experimental control groups were also included. (See Table). ICSI survival, %-Fertilization, %-2PN rates, and pup-rates following embryo transfer were assessed. Mixed regression modeling was applied to assess differences among groups. RESULTS Non-motile sperm had the following mean OET assay responses (Maximum OET induced velocity; += attraction, −= repulsion): +2.5μm/sec (SD 2.6) (untreated group), − 0.15μm/sec (SD .08) (freeze-thaw x1 group), and 0μm/sec (0.21). ICSI outcomes: (see Table). All pups were live-born and healthy appearing. CONCLUSIONS We demonstrate that it is feasible to use OET to assay non-motile sperm for ICSI using an in vivo mouse model. Our results also show that non-motile (vs. motile) sperm have equal 2-cell rates, but significantly lower live birth rate, even in optimized medium. OET assay discriminated among all 3 sperm groups, and predicted a significantly greater 2-cell embryo rate among sperm not exposed to the harmful effects of freeze-thaw. While a trend suggests that OET response predicts live birth rates, the study was underpowered due to the clustered nature (i.e. recipients) of embryo transfer data. To definitively assess potential harm and ability to predict live birth-rates, future studies should include an equal number of embryos transferred and a greater number of recipients, and, should seek to optimize OET media. © 2012 by American Urological Association Education and Research, Inc.FiguresReferencesRelatedDetails Volume 187Issue 4SApril 2012Page: e846-e847 Advertisement Copyright & Permissions© 2012 by American Urological Association Education and Research, Inc.MetricsAuthor Information Maurice Garcia San Francisco, CA More articles by this author Aaron Ohta Honolulu, HI More articles by this author Ryuzo Yanagimachi Honolulu, HI More articles by this author W. Steve Ward Honolulu, HI More articles by this author Stefan Moisyadi Honolulu, HI More articles by this author Joel Marh Honolulu, HI More articles by this author Expand All Advertisement Advertisement PDF downloadLoading ...