Magnetic field effects on atomic and molecular collisions

The full quantal theory of heavy-particle collisions in the presence of a static magnetic field, B, has been formulated. A new mechanism of magnetic field effect was found which depends on the velocity of the molecular center of mass and plays an important role at high-speed collisions. A power series expansion of the cross section as a function of B was obtained, which can be applied to collisions at weak field. It was also found that the differential cross section can become zero due to the magnetic field when a rovibronic energy level of a molecule is in resonance with another energy level.     

Magnetic field effects on cytochrome c-mediated bioelectrocatalytic transformations

Constant magnetic fields affect many biological transformations, but we lack mechanistic understanding of the processes. The magnetohydrodynamic effect may account for the enhancement of bioelectrocatalytic transformations at interfaces. This is exemplified by the bioelectrocatalyzed cytochrome c-mediated reduction of oxygen and oxidation of lactate in the presence of cytochrome oxidase and lactate dehydrogenase, respectively. We observe significant magnetic field effects on the rates of bioelectrochemical transformations (ca. 3-fold increase) at the functionalized interfaces at field strengths, B, up to 1 T. We show that the limiting current is proportional to the B(1/3)C*(4/3), where C is the concentration of electroactive species. The results may have important implications on the understanding of the magnetic field effects on natural biocatalytic processes at membranes and on the enhancement of biotransformations in biotechnology.     

Magnetic field effects on the initiation of chain oxidation

1. Download : Download high-res image (79KB)
2. Download : Download full-size image

     

Magnetic field effects on recombination radiation in tetracene crystal

Recombination radiation in tetracene crystal is shown to be modified by the application of a magnetic field. Relative variations of radiation intensity with field and orientation can be explained in terms of molecular singlet fission and triplet exciton-charge carrier interaction.     

Magnetic field effects on microstructural variation of electrodeposited cobalt films

The electrodeposition process of Co films in a sulfuric acid solution was examined in a magnetic field (0–5 T). The surface morphology of Co films electrodeposited without a magnetic field was drastically modified with the variation of hydrogen gas evolution rate. Crystalline α-Co was formed in the range of pH = 1.5–6.0, while β-Co was not observed. When the magnetic field was superimposed perpendicular to the electric field in the acidic solution (pH = 1.5), the hydrogen evolution rate was promoted by MHD convection, which enhanced the ionic mass transfer (H⁺ and Co²⁺) near the electrode surface. Moreover, crystalline β-Co was formed simultaneously with the appearance of the elongated ridge-shape precipitates under a higher magnetic field (≥3 T). Contribution to special issue “Magnetic field effects in Electrochemistry”     

Magnetic field effects on the fluorescence of intramolecular electron-donor-acceptor systems

The external magnetic field effects on the exciplex fluorescence of α-(4-dimethylaminophenyl)-ω-(9-phenanthryl)alkanes have been studied by photostationary, time-resolved, and magnetic field modulation fluorescence spectroscopy. The singlet-triplet degeneracy of the radical ion pair is suggested to occur at a methylene chain length of about ten.     

Tactoids of plate-like particles: size, shape, and director field

We studied, by means of polarized light microscopy, the shape and director field of nematic tactoids as a function of their size in dispersions of colloidal gibbsite platelets in polar and apolar solvents. Because of the homeotropic anchoring of the platelets to the interface, we found large tactoids to be spherical with a radial director field, whereas small tactoids turn out to have an oblate shape and a homogeneous director field, in accordance with theoretical predictions. The transition from a radial to a homogeneous director field seems to proceed via two different routes depending in our case on the solvent. In one route, the what presumably is a hedgehog point defect in the center of the tactoid transforms into a ring defect with a radius that presumably goes to infinity with decreasing drop size. In the other route, the hedgehog defect is displaced from the center to the edge of the tactoid, where it becomes virtual again going to infinity with decreasing drop size. Furthermore, quantitative analysis of the tactoid properties provides us with useful information on the ratio of the splay elastic constant and the anchoring strength and the ratio of the anchoring strength and the surface tension.     

Magnetic field effects on heterogeneous triplet—triplet annihilation in anthracene-doped phenanthrene

The orientation of an 8 kG magnetic field with respect to the crystallographic axes is shown to affect the intensity of anthracene delayed fluorescence in an anthracene-doped phenanthrene crystal. These results are consistent with the calculated locations of level-crossing resonances for anthracene trapped-phenanthrene triplet exciton heterofusion. The line shapes of these level crossing resonances appear to be rather different from those reported for other cases of heterofusion or homofusion magnetic field resonances.     

Magnetic field effects on the optical spectrum of FeCl2

The intensity of the ligand-field zero-phonon line of FeCl₂ at 4272 Å has been measured in magnetic fields up to 40 kOe at temperatures of 8 and 11°K. The intensity first rises and then falls with increasing field, as the crystal passes through a phase transition from an antiferromagnetic (metamagnetic) to a ferromagnetic state.     

Magnetic self-assembly of gold nanoparticle chains using dipolar core-shell colloids

The preparation of gold nanoparticle (AuNP) assemblies was conducted by the synthesis and dipolar assembly of ferromagnetic core-shell nanoparticles composed of AuNP cores and cobalt NP shells. Dissolution of metallic Co phases with mineral acids afforded self-assembled AuNP chains and bracelets.     

Magnetic field effects on the open circuit potential of ferromagnetic electrodes in corroding solutions

Magnetic fields shift the open circuit potential (OCP) of ferromagnetic electrodes (Fe, Co, and Ni) in corroding solutions. The OCP changes we observe (a) follow the series Fe>Co>Ni; (b) increase with the magnetic flux density; (c) reach a maximum with disk electrodes approximately 1 mm in diameter; and (d) depend on the orientation of the electrode. We report that when the surface of the electrode is oriented parallel (theta = 90 degrees) or perpendicular (theta = 0 degrees) to the magnetic field, the open circuit potential moves in opposite directions (positive and negative, respectively) with the largest changes occurring when the electrode surface is parallel to the magnetic field. Nonconvective sleeve electrodes produce the same behavior. The overall experimental evidence suggests that the magnetic field changes the OCP by modifying the surface concentrations of the paramagnetic participants in the corrosion process of the ferromagnetic electrode by species in solution; this in turn is accomplished by imposing a field-gradient driven mode of mass transfer upon paramagnetic species in solution (magnetophoresis). Simulations of the magnetic field around the ferromagnetic electrode at the two extreme orientations considered here show that in one case (theta = 90 degrees) field gradients actually repel, while in the other case (theta = 0 degrees) they attract paramagnetic species in the vicinity of the electrode.     

Magnetic field effects on the copolymerization of water‐soluble and ionic monomers

The effect of magnetic field (MF) on the radical copolymerization of a series of water‐soluble and ionic monomers is presented including acrylamide (AM), acrylic acid (AA), its ionized form acrylate (A^?), and diallyldimethylammonium chloride (DADMAC). The following combinations have been studied: AM/AA, AM/A^?, AM/DADMAC, and AA/DADMAC. In addition to the MF, strong electrostatic interactions are present for the majority of monomer combinations and conditions. Although the monomer consumption rate (R_p) increased up to 65% applying a MF of 0.1 Tesla, the composition of the resulting copolymers was not affected under such conditions. Despite this increase of R_p by MF, the electrostatic repulsion between ionic monomers and charged growing radicals dominates R_p and governs the copolymer composition with and without MF. The order of the experimentally obtained reactivity ratios reflects the extent of electrostatic interaction: r_AM/AA (1.41) < r (3.10) < r_AA/DADMAC (4.25) < r_AM/DADMAC (6.95) and r_AA/AM (2.20) > r_DADMAC/AA (0.25) > r (0.17) > r_DADMAC/AM (0.03). Overall, weak MF offers to reduce the production time without modifying the product composition. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 373–383, 2009     

Magnetic field effects on the crystal orientation and surface morphology of electrodeposited iron films

The effects of a magnetic field (5 T) on crystal orientation and surface morphology were investigated for iron films electrodeposited in ferrous aqueous solution. XRD measurements for the iron films showed that the preferred orientation parallel to the substrate was determined by the current density and not influenced by the magnetic field. By X-ray pole figure measurements, however, the crystal texture of the iron films electrodeposited at 10 mA cm^–2 and 30 mA cm^–2 was found to be controlled by the magnetic field. That is, the (110) planes were orientated in same direction of the magnetic field vector at angles of 30° and 35° to the direction normal to the substrate plane at 10 mA cm^–2 and 30 mA cm^–2, respectively. When the morphology was observed by SEM, iron grains at 30 mA cm^–2 changed from a triangular pyramid shape at 0 T to a complex star-like shape at 5 T.Presented at the 3rd International Symposium on Electrochemical Processing of Tailored Materials held at the 53rd Annual Meeting of the International Society of Electrochemistry, 15–20 September 2002, Düsseldorf, Germany     

Screening effects on structure and diffusion in confined charged colloids

Using molecular dynamics computer simulations we investigate structural and dynamic (diffusion) properties of charged colloidal suspension confined to narrow slit pores with structureless, uncharged walls. The system is modeled on an effective level involving only the macroions, which interact via a combination of a soft-sphere and a screened Coulomb potential. The aim of our study is to identify the role of the range of the macroion-macroion interaction controlled by the inverse Debye screening length, kappa. We also compare to bulk properties at the same chemical potential as determined in parallel grand canonical Monte Carlo simulations. Our results reveal a significant influence of the interaction range which competes, however, with the influence of density. At liquidlike densities a decrease of range yields a decreasing mobility (and a corresponding enhancement of local structure) in the bulk system, whereas the reverse effect occurs in narrow slits with thickness of a few particle diameter. These differences can be traced back to the confinement-induced, and kappa-dependent, reduction of overall density compared to the bulk reservoir. We also show that an increase of kappa softens the oscillations in the normal pressure as function of the wall separation, which is consistent with experimental observations concerning the influence of addition of salt.     

Magnetic field effects on the recombination of radical ions in reaction centers of photosynthetic bacteria

The amount of triplet products formed upon recombination of the bacteriochlorophyll dimer cation and the bacteriopheophytin anion in modified bacterial reaction centres can be manipulated by external magnetic fields. Making use of this effect, two methods provide information about structural and dynamic properties of the reaction centre. The two methods differ in that one (MARY) uses low static magnetic fields (0 ? B₀ < 1 kG) whereas the other (RYDMR) uses microwaves and high static fields (B₀ > 1 kG). As is shown here the two methods are equivalent from a theoretical point of view, the interpretation of experimental spectra being equally involved in both cases.     

Magnetic field effects on natural convection flow of a non-Newtonian fluid in an L-shaped enclosure

The effect of magnetic field on natural convection heat transfer in an L-shaped enclosure filled with a non-Newtonian fluid is investigated numerically. The governing equations are solved by finite-volume method using the SIMPLE algorithm. The power-law rheological model is used to characterize the non-Newtonian fluid behavior. It is revealed that heat transfer rate decreases for shear-thinning fluids (of power-law index, n?<?1) and increases for shear-thickening fluids (n?>?1) in comparison with the Newtonian ones. Thermal behavior of shear-thinning and shear-thickening fluids is similar to that of Newtonian fluids for the angle of enclosure α?<?60° and α?>?60°, respectively.