Thin-thick coexistence behavior of 8CB liquid crystalline films on silicon

TL;DR

This study investigates the wetting and coexistence behavior of 8CB liquid crystalline films on silicon, revealing complex reentrant phenomena near phase transition temperatures through optical and x-ray reflectivity measurements.

Contribution

It provides the first experimental phase diagram showing broad thick-thin coexistence regions and reentrant wetting behavior near phase transitions of 8CB films on silicon.

Findings

Reentrant wetting behavior observed twice near phase transitions.

Coexistence regions depend on film coverage, merging at lower thickness.

Behavior near the second-order nematic-to-smectic transition is novel and unexpected.

Abstract

The wetting behavior of thin films of 4'-n-octyl-4-cyanobiphenyl (8CB) on Si is investigated via optical and x-ray reflectivity measurement. An experimental phase diagram is obtained showing a broad thick-thin coexistence region spanning the bulk isotropic-to-nematic ($T_{IN}$) and the nematic-to-smectic-A ($T_{NA}$) temperatures. For Si surfaces with coverages between 47 and $72\pm3$ nm, reentrant wetting behavior is observed twice as we increase the temperature, with separate coexistence behaviors near $T_{IN}$ and $T_{NA}$. For coverages less than 47 nm, however, the two coexistence behaviors merge into a single coexistence region. The observed thin-thick coexistence near the second-order NA transition is not anticipated by any previous theory or experiment. Nevertheless, the behavior of the thin and thick phases within the coexistence regions is consistent with this being an equilibrium phenomenon.