Orientational ordering of confined hard rods: the effect of shape anisotropy on surface ordering and capillary nematization

TL;DR

This study uses a density functional theory approach to analyze how shape anisotropy affects surface ordering and capillary nematization of confined hard rods, revealing divergence of nematic film thickness and the nature of phase transitions.

Contribution

It introduces a detailed theoretical analysis of shape anisotropy effects on surface and confinement-induced ordering in hard rod systems using the Zwanzig approximation.

Findings

Nematic wetting is complete for any shape anisotropy.

Surface ordering transition depends on wall separation H, especially for H<L.

The bulk isotropic-nematic transition becomes a surface and capillary phase transition in confinement.

Abstract

We examine the ordering properties of rectangular hard rods with length L and diameter D at a single planar wall and between two parallel hard walls using the second-virial density functional theory. The theory is implemented in the three-state Zwanzig approximation, where only three mutually perpendicular directions are allowed for the orientations of hard rods. The effect of varying shape anisotropy is examined at L/D=10, 15 and 20. In contact with a single hard wall, the density profiles show planar ordering, damped oscillatory behavior, and wall induced surface ordering transition below the coexisting isotropic density of bulk isotropic-nematic (I-N) phase transition. Upon approaching the coexisting isotropic density, the thickness of nematic film diverges logarithmically, i.e. the nematic wetting is complete for any shape anisotropy. In the case of confinement between two parallel hard walls, it is found that the continuous surface ordering transition depends strongly on the distance between confining walls (H) for H<L, while it depends weakly on H for H>L. The minimal density at which a surface ordering transition can be realized is located at around H~2D for all studied shape anisotropies due to the strong interference effect between the two hard walls. The first order I-N phase transition of the bulk system becomes surface ordered isotropic (IB)-capillary nematic (NB) phase transition in the slit pore. This first order IB-NB transition weakens with decreasing pore width and terminates in a critical point for all studied shape anisotropies.