Orientational order in dipolar fluids consisting of nonspherical hard particles

Abstract

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.

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

“…This phase boundary still depends on the way the extrapolation to higher densities is done (here linear or quadratic) but is in good agreement with other works. For example density functional theory of Groh and Dietrich [4] are in a reasonable good agreement with our results. However due to their approximation the X 0 to 1/X 0 symmetry is exact which is however clearly broken in the PY result.…”

“…As a function of the aspect ratio or density a fluid of hard ellipsoids can now also undergo an isotropic to nematic (I-N) transition. This is expected from the Onsager solution of hard spherocylinders [2], has been found by computer simulations [3] and also by density functional theory [4].…”

“…This was done with a linear (circles) and a quadratic (squares) interpolation. For comparison we plotted the I-N-transition which arises from density functional theory [4]. Also the results from ref.…”

Understanding and Describing the Liquid-Crystalline States of Polypeptide Solutions: A Coarse-Grained Model of PBLG in DMF

“…A number of studies have been carried out after the early attempts by Kimura 71 to describe the properties of a system which combines Onsager's hard-rod model with anisotropic dispersion forces. [72][73][74][75][76][77][78][79][80][81][82][83] Some progress has also been made in introducing dipolar [84][85][86][87][88][89] or chiral [90][91][92][93] interactions between LC molecules. Of particular relevance to our current work is the closed-form algebraic equation of state developed within a van der Waals-Onsager treatment for systems of attractive hard-core particles, 94 in which the attractive potential is expanded in spherical harmonics 95 representing different multipolar contributions to the anisotropic attractions.…”

Magnetic Properties of Colloidal Suspensions of Interacting Magnetic Particles

“…Groh and Dietrich [206] study the phase transition properties of nonspherical dipolar hard particles using a density functional ansatz. In the special case of DHS they find no dilute/dense transition between isotropic phases but an isotropic/ferromagnetic transition that becomes noncontinuous at λ > 1.33 and φ ≈ 0.2.…”