Orientational order in dipolar fluids consisting of nonspherical hard particles

We investigate fluids of dipolar hard particles by a certain variant of density-functional theory. The proper treatment of the long range of the dipolar interactions yields a contribution to the free energy which favors ferromagnetic order. This corrects previous theoretical analyses. We determine phase diagrams for dipolar ellipsoids and spherocylinders as a function of the aspect ratio of the particles and their dipole moment. In the nonpolar limit the results for the phase boundary between the isotropic and nematic phase agree well with simulation data. Adding a longitudinal dipole moment favors the nematic phase. For oblate or slightly elongated particles we find a ferromagnetic liquid phase, which has also been detected in computer simulations of fluids consisting of spherical dipolar particles. The detailed structure of the phase diagram and its evolution upon changing the aspect ratio are discussed in detail.Comment: 35 pages LaTeX with epsf style, 11 figures in eps format, submitted to Phys. Rev.

Crystal structures and freezing of dipolar fluids

We investigate the crystal structure of classical systems of spherical particles with an embedded point dipole at T=0. The ferroelectric ground state energy is calculated using generalizations of the Ewald summation technique. Due to the reduced symmetry compared to the nonpolar case the crystals are never strictly cubic. For the Stockmayer (i.e., Lennard-Jones plus dipolar) interaction three phases are found upon increasing the dipole moment: hexagonal, body-centered orthorhombic, and body-centered tetragonal. An even richer phase diagram arises for dipolar soft spheres with a purely repulsive inverse power law potential $\sim r^{-n}$. A crossover between qualitatively different sequences of phases occurs near the exponent $n=12$. The results are applicable to electro- and magnetorheological fluids. In addition to the exact ground state analysis we study freezing of the Stockmayer fluid by density-functional theory.Comment: submitted to Phys. Rev.

Inhomogeneous magnetization in dipolar ferromagnetic liquids

At high densities fluids of strongly dipolar spherical particles exhibit spontaneous long-ranged orientational order. Typically, due to demagnetization effects induced by the long range of the dipolar interactions, the magnetization structure is spatially inhomogeneous and depends on the shape of the sample. We determine this structure for a cubic sample by the free minimization of an appropriate microscopic density functional using simulated annealing. We find a vortex structure resembling four domains separated by four domain walls whose thickness increases proportional to the system size L. There are indications that for large L the whole configuration scales with the system size. Near the axis of the mainly planar vortex structure the direction of the magnetization escapes into the third dimension or, at higher temperatures, the absolute value of the magnetization is strongly reduced. Thus the orientational order is characterized by two point defects at the top and the bottom of the sample, respectively. The equilibrium structure in an external field and the transition to a homogeneous magnetization for strong fields are analyzed, too.Comment: 17 postscript figures included, submitted to Phys. Rev.

Particle dynamics of a cartoon dune

The spatio-temporal evolution of a downsized model for a desert dune is observed experimentally in a narrow water flow channel. A particle tracking method reveals that the migration speed of the model dune is one order of magnitude smaller than that of individual grains. In particular, the erosion rate consists of comparable contributions from creeping (low energy) and saltating (high energy) particles. The saltation flow rate is slightly larger, whereas the number of saltating particles is one order of magnitude lower than that of the creeping ones. The velocity field of the saltating particles is comparable to the velocity field of the driving fluid. It can be observed that the spatial profile of the shear stress reaches its maximum value upstream of the crest, while its minimum lies at the downstream foot of the dune. The particle tracking method reveals that the deposition of entrained particles occurs primarily in the region between these two extrema of the shear stress. Moreover, it is demonstrated that the initial triangular heap evolves to a steady state with constant mass, shape, velocity, and packing fraction after one turnover time has elapsed. Within that time the mean distance between particles initially in contact reaches a value of approximately one quarter of the dune basis length

Ferromagnetic Liquid Thin Films Under Applied Field

Theoretical calculations, computer simulations and experiments indicate the possible existence of a ferromagnetic liquid state, although definitive experimental evidence is lacking. Should such a state exist, demagnetization effects would force a nontrivial magnetization texture. Since liquid droplets are deformable, the droplet shape is coupled with the magnetization texture. In a thin-film geometry in zero applied field, the droplet has a circular shape and a rotating magnetization texture with a point vortex at the center. We calculate the elongation and magnetization texture of such ferromagnetic thin film liquid droplet confined between two parallel plates under a weak applied magnetic field. The vortex stretches into a domain wall and exchange forces break the reflection symmetry. This behavior contrasts qualitatively and quantitatively with the elongation of paramagnetic thin films.Comment: 10 pages, 4 figures, Submitted to Phys. Rev.

Multi-Wavelength Implications of the Companion Star in Eta Carinae

Eta Carinae is considered to be a massive colliding wind binary system with a highly eccentric (e \sim 0.9), 5.54-yr orbit. However, the companion star continues to evade direct detection as the primary dwarfs its emission at most wavelengths. Using three-dimensional (3-D) SPH simulations of Eta Car's colliding winds and radiative transfer codes, we are able to compute synthetic observables across multiple wavebands for comparison to the observations. The models show that the presence of a companion star has a profound influence on the observed HST/STIS UV spectrum and H-alpha line profiles, as well as the ground-based photometric monitoring. Here, we focus on the Bore Hole effect, wherein the fast wind from the hot secondary star carves a cavity in the dense primary wind, allowing increased escape of radiation from the hotter/deeper layers of the primary's extended wind photosphere. The results have important implications for interpretations of Eta Car's observables at multiple wavelengths.Comment: 5 pages, 4 figures, To be published in the proceedings of the meeting 'Four Decades of Research on Massive Stars' in honor of Tony Moffat, 11-15 July 2011, Saint-Michel-des-Saints, Quebe

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

Fluids of hard ellipsoids: Phase diagram including a nematic instability from Percus-Yevick theory

An important aspect of molecular fluids is the relation between orientation and translation parts of the two-particle correlations. Especially the detailed knowledge of the influence of orientation correlations is needed to explain and calculate in detail the occurrence of a nematic phase. The simplest model system which shows both orientation and translation correlations is a system of hard ellipsoids. We investigate an isotropic fluid formed of hard ellipsoids with Percus-Yevick theory. Solving the Percus-Yevick equations self-consistently in the high density regime gives a clear criterion for a nematic instability. We calculate in detail the equilibrium phase diagram for a fluid of hard ellipsoids of revolution. Our results compare well with Monte Carlo Simulations and density functional theory.Comment: 7 pages including 4 figure

Fluids of platelike particles near a hard wall

Fluids consisting of hard platelike particles near a hard wall are investigated using density functional theory. The density and orientational profiles as well as the surface tension and the excess coverage are determined and compared with those of a fluid of rodlike particles. Even for low densities slight orientational packing effects are found for the platelet fluid due to larger intermolecular interactions between platelets as compared with those between rods. A net depletion of platelets near the wall is exhibited by the excess coverage, whereas a change of sign of the excess coverage of hard-rod fluids is found upon increasing the bulk density.Comment: 6 pages, 9 figure

Phase Coexistence of a Stockmayer Fluid in an Applied Field

We examine two aspects of Stockmayer fluids which consists of point dipoles that additionally interact via an attractive Lennard-Jones potential. We perform Monte Carlo simulations to examine the effect of an applied field on the liquid-gas phase coexistence and show that a magnetic fluid phase does exist in the absence of an applied field. As part of the search for the magnetic fluid phase, we perform Gibbs ensemble simulations to determine phase coexistence curves at large dipole moments, $\mu$. The critical temperature is found to depend linearly on $\mu^2$ for intermediate values of $\mu$ beyond the initial nonlinear behavior near $\mu=0$ and less than the $\mu$ where no liquid-gas phase coexistence has been found. For phase coexistence in an applied field, the critical temperatures as a function of the applied field for two different $\mu$ are mapped onto a single curve. The critical densities hardly change as a function of applied field. We also verify that in an applied field the liquid droplets within the two phase coexistence region become elongated in the direction of the field.Comment: 23 pages, ReVTeX, 7 figure