The aggregation of ferrocolloids in a magnetic field

The structure of chain aggregates forming in a low-concentration monodisperse ferrocolloid in a uniform permanent magnetic field was studied. The configuration integral of a flexible fluctuating chain of arbitrary length was calculated using the matrix method of coordinate system rotation with allowance made for the dipole-dipole interaction of the magnetic moments of nearest ferroparticles. The equilibrium distribution over aggregate chain lengths was determined from the condition of minimum of the free energy functional. It was shown that, even for infinitely diluted ferrofluids, an increase in the strength of external magnetic field led to a noticeable lengthening of chain aggregates at strong interparticle magnetodipole interaction. This results in faster (compared to Langevins law) growth of the magnetization of a system in weak and intermediate fields.Translated from Kolloidnyi Zhurnal, Vol. 66, No. 6, 2004, pp. 766–774.Original Russian Text Copyright © 2004 by Ivanov.     

Superfluorescent transitions in an external magnetic field

Polarisation properties of the superfluorescence in the near-infrared regime have been investigated between high-lying levels of Sr and Ba under the influence of a static homogeneous external magnetic field. In some transitions the time-resolved measurements show a change of the polarisation of the superfluorescence depending on the magnetic field strength. In suitable experimental conditions intensity modulations were observed. These were assigned as Zeeman quantum beats or indirectly observed Zeeman superfluorescent beats. The experimental findings of superfluorescence in two-level, three-level, or multi-level configurations in dependence on the magnetic field strength can be explained well in a semiclassical model of multi-level superfluorescence.     

The isotropic-nematic interface of colloidal goethite in an external magnetic field

Polarization microscopy was used to study the behavior around the isotropic-nematic interface of colloidal goethite dispersions in a magnetic field. It has been found before that the nematic phase is favored in an external field. In the case of goethite this was also observed; nematic droplets formed inside the isotropic phase and coalesced with the nematic phase. However, the behavior was found to be much richer because of the particle rotation around a certain critical field strength. The simultaneous occurrence of (parallel)nematic-(perpendicular)nematic phase separation under the influence of a magnetic field also plays a role here.     

Radiative properties of an electrically-vaporized thin film plasma in an external magnetic field

An external magnetic field of a few kilogauss is used to alter significantly the radiative properties of the plasmas generated by the capacitive discharge vaporization ofthin Ag films. The magnetic field is generated by the plasma current in a large inductor surrounding the plasma. The magnetic field is oriented normal to the electric field in the plasma. This generates a drift motion of the plasma normal to the plane established by the electric and magnetic field vectors. The drift motion can direct the plasma toward the plastic substrate on which the Ag film and a powder or solution residue sample were originally located. This increases the plasma-sample interaction. Time integrated, spatially resolved photographic spectra and spatially resolved photoelectric radiation profiles show that with the magnetic field, the continuum background radiation is confined to a relatively small region near the substrate surface. This results in significantly larger analyte line-to-background intensity ratios in the region 1–2 cm above the substrate surface.     

Control of particle assisted wetting by an external magnetic field

It is shown that repulsive particles can assist wetting of a water surface by an organic liquid even at a particle density substantially less than a close packed monolayer. By applying external fields, one can change the interparticle interactions from net attractive to net repulsive and thus induce a transition from nonwetting to wetting conditions. This was achieved by applying superparamagnetic polystyrene particles together with a polymerizable organic liquid (trimethylolpropane trimethacrylate) to a water surface in the middle of a solenoid. Passing a current through the solenoid created a magnetic field perpendicular to the interface that polarized the particles and induced repulsive dipole-dipole forces. Without the field, lenses of the organic liquid that included aggregates of particles floating on the water surface were observed. In the presence of the field, the organic liquid and the particles were evenly distributed across the surface. The interparticle distance increases proportional to the square root of the area per particle and is close to the value expected for hexagonal order.     

Exact density functionals for two-electron systems in an external magnetic field

In principle, the extension of density functional theory (DFT) to Coulombic systems in a nonvanishing magnetic field is via current DFT (CDFT). Though CDFT is long established formally, relatively little is known with respect to any generally applicable, reliable approximate E(XC) and A(XC) functionals analogous with the workhorse approximate functionals (local density approximation and generalized gradient approximation) of ordinary DFT. Progress can be aided by having benchmark studies on a solvable correlated system. At zero field, the best-known finite system for such purposes is Hooke's atom. Recently we extended the exact ground state solutions for this two-electron system to certain combinations of nonzero external magnetic fields and confinement strengths. From those exact solutions, as well as high-accuracy numerical results for other field and confinement combinations, we construct the correlated electron density and paramagnetic current density, the exact Kohn-Sham orbitals, and the exact DFT and CDFT exchange-correlation energies and potentials. We compare with results from several widely used approximate functionals, all of which exhibit major qualitative failures, whether in CDFT or in naive application of ordinary DFT. We also illustrate how the CDFT vorticity variable nu is a computationally difficult quantity which may not be appropriate in practice to describe the external B field effects on E(XC) and A(XC).     

Nuclear magnetic relaxation of ionic nuclei moving in an external electric field

We have experimentally investigated nuclear magnetic relaxation of some quadrupolar ionic nuclei (²³Na⁺, ⁸⁷Rb⁺ and ³⁵Cl⁻ ) in electrolyte solutions in non-equilibrium states. The measurements of relaxation rates 1/T₁ in the presence of a direct electric field, and thus also an electric current, revealed that differences can occur in comparison with relaxation in the absence of the field. In some systems no change in the relaxation rate was observed, but in certain other (non-aqueous) systems there was a change in the quadrupolar relaxation rate in the presence of the field.     

Reciprocity relation in scattering by media subjected to an external magnetic field

Summary Krishnan's reciprocity theorem in colloid optics,ϱ _u=1+l/ϱ_h/1+1/ϱ _v is generalised for the case when the scattering medium is subjected to an external orienting field. It is shown theoretically that a general relation of the typeI _B ^A =I′ _A ^B results in this case, whereI _B ^A is the intensity of the component of the scattered light having its electric vector inclined at an angleB to the vertical with the incident light polarised at an angleA to the vertical, the external field direction being parallel to the incident beam.I′ _A ^B is the corresponding intensity with the magnetic field parallel of the scattered ray. Experimental verification of the above generalisation is also given.

---

Zusammenfassung Krishnans Reziprozit?tstheoremϱ _u=1+1/ϱ _h/1+1/ϱ _v wird für den Fall verallgemeinert, da\ das streuende Medium einem ?u\eren orientierenden Feld unterliegt. Es l?\t sich theoretisch zeigen, da\ in diesem Fall eine allgemeine Beziehung des TypsI _B ^A =I′ _A ^B resultiert, in derI _B ^A die Intensit?t der Komponente des Streulichts mit dem elektrischen Vektor um den WinkelB zur Vertikalen geneigt bei einfallendem Licht um einen WinkelA zur Vertikalen polarisiert angibt, und das ?u\ere Feld parallel zum einfallenden Strahl liegt.I′ _A ^B ist die entsprechende Intensit?t mit dem Magnetfeld parallel zum gestreuten Strahl. Eine experimentelle Best?tigung der genannten Verallgemeinerung wird gegeben.

     

Reactivity dynamics of a confined molecule in presence of an external magnetic field

Reactivity dynamics and stability of a confined hydrogen molecule in presence of an external magnetic field has been studied using quantum fluid density functional theory. Dynamic profiles of various reactivity parameters such as hardness, electrophilicity, magnetizability, phase volume, entropy, etc. have been studied within a confined environment. Responses in the reactivity parameters as well as the associated electronic structure principles validate the stability of the confined H₂ molecule in ground and excited states in presence of an external magnetic field. Confinement to the system has been imposed by the Dirichlet type boundary condition. Confinement and excitation act in opposite directions. Ground state type dynamics is obtained on simultaneous electronic excitation and confinement. © 2014 Wiley Periodicals, Inc.     

Phase separation of a model binary polymer solution in an external field

The phase separation of a simple binary mixture of incompatible linear polymers in solution is investigated using an extension of the sedimentation equilibrium method, whereby the osmotic pressure of the mixture is extracted from the density profiles of the inhomogeneous mixture in a gravitational field. In Monte Carlo simulations the field can be tuned to induce significant inhomogeneity, while keeping the density profiles sufficiently smooth for the macroscopic condition of hydrostatic equilibrium to remain applicable. The method is applied here for a simplified model of ideal but mutually avoiding polymers, which readily phase separate at relatively low densities. The Monte Carlo data are interpreted with the help of an approximate bulk phase diagram calculated from a simple, second-order virial coefficient theory. By derivation of effective potentials between polymer centers of mass, the binary mixture of polymers is coarse-grained to a "soft colloid" picture reminiscent of the Widom-Rowlinson model for incompatible atomic mixtures. This approach significantly speeds up the simulations and accurately reproduces the behavior of the full monomer resolved model.     

Effect of an external magnetic field on photolysis of aqueous solutions of cyclohexanone

The effect of an external magnetic field on the ratio of the yields of the basic products of photolysis of aqueous solutions of cyclohexanone was investigated by high-resolution NMR spectroscopy. It was shown that the magnetic field does not significantly affect the yield of the products of the reaction with a 0.2 M concentration of the starting ketone, the yield of 5-hexenal decreases, and the yield of caproic acid and the rate of photolysis increase below 0.02 M. The observed effects are explained by association of cyclohexan-one with involvement of the HFI mechanism from the theory of radical pairs.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1196–1197, May, 1991.     

A theoretical study of spin level crossing induced by an external magnetic field of ring molecule magnet models

The dependence of the magnetization of model systems on an external magnetic field has been investigated. An ab initio path integral Monte Carlo method is used to study the spin level crossing phenomena of molecules with ring structures such as those in the ferric wheel [Fe(OMe)₂(O₂CCH₂Cl)]₁₀. The ab initio treatment is essential to calculate the magnetization in a system with a large contribution from next-neighbor interactions. A possible use as a molecular device for switching or molecular recognition is suggested.     

The aza-Pudovik reaction accelerated in external constant magnetic field

The aza-Pudovik reaction of N-benzyl aromatic and aliphatic aldimines with dimethyl H-phosphonate was found to be accelerated significantly when carried out in an external magnetic field. Its conversion rates were analyzed and compared with those obtained during the aza-Pudovik reaction carried out under standard conditions. Magnetic acceleration was found to have a similar degree as in that observed in the Kabachnik–Fields reaction. However, the formation of the starting imines was found not to be susceptible to constant magnetic field–no acceleration was observed.     

Magnetogranulometric analysis of ferrocolloids: Second-order modified mean field theory

A statistical model of highly concentrated ferrocolloids is proposed. The model is based on the use of the mean field theory for the single-particle orientational distribution function of the magnetic moments of ferroparticles. The expression for the effective mean field is derived with the method of perturbation theory of the second order with respect to the concentration and intensity of magnetodipole interparticle interaction. It is proposed to apply the magnetogranulometric analysis for describing magnetization curves for highly concentrated ferrocolloids. The high efficiency of the modified mean field theory is demonstrated during the determination of the parameters of disperse composition of ferrocolloids by the magnetogranulometric analysis.     

The influence of chain aggregates on the magnetic properties of ferrocolloids

Static magnetic properties of monodisperse low-concentrated aggregated ferrocolloid are studied within the framework of the model of flexible chain aggregates. For the account of long-range magnetic dipoledipole interaction between all ferroparticles in a system, it was proposed to use the modified mean field theory. It is shown that the model of short flexible chains, in combination with the modified mean field theory, quantitatively explains anomalously high magnetic characteristics of aggregated ferrocolloids obtained in computer experiments.     

Initial magnetic susceptibility of ferrocolloids: The influence of chain aggregates

The behavior of a system of noninteracting chain aggregates of ferroparticles suspended in a liquid matrix and subjected to a weak uniform permanent magnetic field was studied based on the free energy functional of a monodisperse low-concentration magnetic colloid. The orientational response of a flexible chain to a weak external field showed that the model of rigid rods is applicable only for short chains (doublets and triplets) of large ferroparticles characterized by a strong magnetodipole interaction. The calculated values of the initial magnetic susceptibility of aggregated ferrofluid agree well with the computer simulation data.Translated from Kolloidnyi Zhurnal, Vol. 66, No. 6, 2004, pp. 756–765.Original Russian Text Copyright © 2004 by Ivanov.     

Density-functional theory study of electric and magnetic properties of hexafluorobenzene in the vapor phase

A series of electric and magnetic properties of hexafluorobenzene have been calculated, including the electric dipole polarizability, magnetizability, electric quadrupole moment, and nonlinear mixed electric dipole-magnetic dipole-electric quadrupole hyperpolarizabilities needed to obtain estimates of the Kerr, Cotton-Mouton, Buckingham, Jones, and magnetoelectric birefringences in the vapor phase. Time-dependent density-functional theory was employed for the calculation of linear-, quadratic, and cubic response functions. A number of density functionals have been considered, along with Sadlej's triple-zeta basis set and the augmented correlation-consistent polarized valence double zeta and augmented correlation-consistent polarized valence triple zeta basis sets. Comparisons have been made with experiment where possible. The analysis of results allows for an assessment of the capability of time-dependent density-functional theory for high-order electromagnetic properties of an electron-rich system such as hexafluorobenzene.     

Desorption of hydrogen from the electrode surface under influence of an external magnetic field

The effect of a uniform magnetic field in perpendicular-to-electrode configuration, on the hydrogen evolution reaction was investigated during the deposition of Fe and CoFe alloys. It was found that the desorption of hydrogen is supported in a magnetic field. This effect is discussed in details with respect to the micro-magnetohydrodynamic convection, which arises due to the fluctuation of the current distribution close to the H₂ bubble.     

Influence of an external magnetic field on the dynamics of magnetite reduction by hydrogen

Russian Chemical Bulletin -