Ordering transitions of weakly anisotropic hard rods in narrow slit-like pores

TL;DR

This study investigates how confinement affects the positional and orientational ordering of weakly anisotropic hard rods in narrow slit-like pores, revealing structural transitions and phase behavior changes with density and pore width.

Contribution

It provides a detailed analysis of ordering transitions of rectangular rods under confinement, highlighting new phase behaviors and structural transitions not previously characterized.

Findings

Transition from planar to homeotropic layers with increasing density

Existence of bilayer structures with mixed orientations at high densities

Observation of first-order phase transitions between different ordered structures

Abstract

The effect of strong confinement on the positional and orientational ordering is examined in a system of hard rectangular rods with length L and diameter D (L>D) using the Parsons-Lee modification of the second virial density functional theory. The rods are nonmesogenic (L/D<3)and confined between two parallel hard walls, where the width of the pore (H) is chosen in such a way that both planar (particle's long axis parallel to the walls) and homeotropic (particle's long axis perpendicular to the walls) orderings are possible and a maximum of two layers are allowed to form in the pore. In the extreme confinement limit of ,where only one layer structures appear, we observe a structural transition from a planar to a homeotropic fluid layer with increasing density, which becomes sharper as L->H. In wider pores (2D<H<3D) planar order with two layers, homeotropic order, and even combined bilayer structures (one layer is homeotropic, while the other is planar) can be stabilized at high densities. Moreover, first order phase transitions can be seen between different structures. One of them emerges between a monolayer and a bilayer with planar orders at relatively low packing fractions.