Surface and smectic layering transitions in binary mixtures of parallel hard rods

TL;DR

This study investigates the surface phase behavior of binary mixtures of aligned hard rods near a wall, revealing complex layering, wetting, and adsorption phenomena influenced by pressure and phase transitions.

Contribution

It provides a detailed theoretical analysis of surface layering and wetting transitions in binary hard-rod mixtures, highlighting the effects of composition and pressure.

Findings

Layering transitions occur at high pressures.

Complete wetting involves infinitely thick smectic films.

Critical adsorption is observed at low pressures.

Abstract

The surface phase behavior of binary mixtures of colloidal hard rods in contact with a solid substrate (hard wall) is studied, with special emphasis on the region of the phase diagram that includes the smectic A phase. The colloidal rods are modelled as hard cylinders of the same diameter and different lengths, in the approximation of perfect alignment. A fundamental--measure density functional is used to obtain equilibrium density profiles and thermodynamic properties such as surface tensions and adsorption coefficients. The bulk phase diagram exhibits nematic-smectic and smectic-smectic demixing, with smectic phases having different compositions; in some cases they are microfractionated. The calculated surface phase diagram of the wall-nematic interface shows a very rich phase behavior, including layering transitions and complete wetting at high pressures, whereby an infinitely thick smectic film grows at the wall via an infinite sequence of stepwise first--order layering transitions. For lower pressures complete wetting also obtains, but here the smectic film grows in a continuous fashion. Finally, at very low pressures, the wall-nematic interface exhibits critical adsorption by the smectic phase, due to the second-order character of the bulk nematic-smectic transition.