Biaxial layering transition of hard rod-like particles in narrow slit-like pores

TL;DR

This study investigates the phase behavior of hard rectangular rods in narrow slit pores, revealing layering transitions and uniaxial-biaxial ordering changes driven by density and particle shape anisotropy.

Contribution

It introduces a detailed analysis of layering and ordering transitions of hard rods in confined geometries using density functional theory, highlighting new layering phenomena.

Findings

Layering transitions occur between fluids with n and n+1 layers.

Both uniaxial and biaxial phases are observed depending on particle aspect ratio.

Multiple layering transitions can occur with increasing density.

Abstract

The phase behavior of hard rectangular rods with length L and diameter D is studied in a narrow slit-like pore using the Parsons-Lee density functional theory. Using the restricted orientation approximation, we find strong adsorption at the walls with planar ordering, second order uniaxial-biaxial ordering transitions and first order layering transitions. The layering transition takes place between two fluids having n and n+1 layers, where the layer spacing is in the order of D. In the case of weak shape anisotropy (L/D=3), the coexisting fluids can be either uniaxial or biaxial, while both phases are found to be biaxial for L/D=6 and L/D=9. Interestingly, even two or more layering transitions can be observed with increasing density at a given shape anisotropy and pore width.