Density functional theory for colloidal mixtures of hard platelets, rods, and spheres

TL;DR

This paper develops a geometry-based density functional theory for mixtures of hard spheres, needles, and platelets, accurately capturing their interactions and phase behavior, including the isotropic-nematic transition, with good agreement to simulations.

Contribution

It introduces a new density functional approach for ternary mixtures of anisotropic particles, ensuring correct second virial behavior and extending previous models to include platelets and needles.

Findings

Accurately describes the isotropic-nematic transition of hard platelets.

Provides explicit analytic expressions for systems with planar symmetry.

Shows good agreement with simulation results for coexistence densities.

Abstract

A geometry-based density functional theory is presented for mixtures of hard spheres, hard needles and hard platelets; both the needles and the platelets are taken to be of vanishing thickness. Geometrical weight functions that are characteristic for each species are given and it is shown how convolutions of pairs of weight functions recover each Mayer bond of the ternary mixture and hence ensure the correct second virial expansion of the excess free energy functional. The case of sphere-platelet overlap relies on the same approximation as does Rosenfeld's functional for strictly two-dimensional hard disks. We explicitly control contributions to the excess free energy that are of third order in density. Analytic expressions relevant for the application of the theory to states with planar translational and cylindrical rotational symmetry, e.g. to describe behavior at planar smooth walls, are given. For binary sphere-platelet mixtures, in the appropriate limit of small platelet densities, the theory differs from that used in a recent treatment [L. Harnau and S. Dietrich, Phys. Rev. E 71, 011504 (2004)]. As a test case of our approach we consider the isotropic-nematic bulk transition of pure hard platelets, which we find to be weakly first order, with values for the coexistence densities and the nematic order parameter that compare well with simulation results.