Orientational order in dipolar fluids consisting of nonspherical hard particles

TL;DR

This study uses density-functional theory to analyze dipolar fluids of nonspherical particles, revealing phase behaviors including ferromagnetic and nematic phases, and correcting previous theoretical models.

Contribution

It introduces a proper treatment of long-range dipolar interactions in density-functional theory, improving phase diagram predictions for dipolar ellipsoids and spherocylinders.

Findings

Agreement with simulation data for nonpolar phase boundaries

Longitudinal dipoles promote nematic ordering

Existence of ferromagnetic liquid phase in certain particle shapes

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.