Phase transitions and ordering of confined dipolar fluids

We apply a modified mean-field density functional theory to determine the phase behavior of Stockmayer fluids in slitlike pores formed by two walls with identical substrate potentials. Based on the Carnahan-Starling equation of state, a fundamental-measure theory is employed to incorporate the effects of short-ranged hard sphere - like correlations while the long-ranged contributions to the fluid interaction potential are treated perturbatively. The liquid-vapor, ferromagnetic liquid - vapor, and ferromagnetic liquid - isotropic liquid first-order phase separations are investigated. The local orientational structure of the anisotropic and inhomogeneous ferromagnetic liquid phase is also studied. We discuss how the phase diagrams are shifted and distorted upon varying the pore width.Comment: 15 pages including 8 figure

Towards efficient methods for the study of pattern formation in ferrofluid films

Hexagonal and labyrinthine patterns appear in thin ferrofluid films after application of a magnetic field perpendicular to the film. The pattern size and the stability of the hexagonal and labyrinthine structures can be predicted by free energy approaches. Several approximations are used in the literature to accelerate the calculation of the magnetic energy. They are usually based on the use of a uniform, average or constant magnetization. In the uniform approximation the magnetization at all points in the pattern is assumed to be equal to its value at the center of the stripes or cylinders in the labyrinthine or hexagonal patterns. Recent papers indicate that this approximation gives qualitatively wrong results. This is corroborated here by a comparison with accurate results. When a volume-averaged magnetization is used during the calculation of the demagnetization field, from which the magnetic energy is evaluated, the theoretical results are only slightly modified with respect to the accurate results. Thus, we can propose a new method which gives results in good agreement with the accurate values and accelerates the calculations by a factor of 1000. The influence of the approximations is explained by a study of the evolution of the demagnetization field in the patterns. This study indicates that the volume-averaged approximation might only be reliable for patterns with a homogeneously distributed magnetic fluid. Another approximation of a constant magnetization, which is widely used in the literature, assumes that the magnetization does not change during the pattern formation in contrast to the uniform and average approximations. A different way of computing the constant magnetization than that usually employed markedly improves the agreement with the accurate results. This is explained by the derivation of a direct relationship between the approximations of a constant and an average magnetization

The self-organization of magnetic nanoparticles : a Monte Carlo study

International audienc

Theories of structural and dynamic properties of ions in discrete solvents. Application to magnetic resonance imaging

The molecular Ornstein-Zernike (MOZ) formalism used to compute the structure of a liquid solution is briefly presented. Its ability to describe the equilibrium properties of aprotic solvents and of their electrolyte solutions is demonstrated from selected examples. The potential of mean force and the relative motion of ions in water are studied by the powerful method of intermolecular nuclear magnetic relaxation dispersion (NMRD) in paramagnetic solutions. The interest of the ion-ion dynamics in medical magnetic resonance imaging (MRI) is shown by a typical NMRD study involving paramagnetic gadolinium Gd3+ complexes

Ion association and solvation of perchlorate salts in N,N-dimethylformamide and N,N-dimethylacetamide

The results of dielec. relaxation expts. on solns. of LiClO4, NaClO4 and Bu4NClO4 in N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMA), performed at 25 °C in the frequency range 0.2 ≤ ν/GHz ≤ 89 are presented. For all electrolytes, a solute relaxation process due to ion pairs is obsd. in addn. to the solvent relaxation. The alkali salts form predominantly solvent-shared ion pairs in DMF and DMA, whereas contact ion pairs are obsd. for Bu4NClO4. In all cases, the rate of ion-pair formation is nearly diffusion-controlled. Effective solvation nos. were deduced from the solvent dispersion amplitude. Data suggest that the anion-solvent interactions are weak, whereas the alkali ions form a well-defined primary solvation shell. Interestingly, irrotational bonding of solvent mols. by Bu4N+ is obsd., possibly due to steric interactions