Influence of nano confinement on nematic liquid crystals

TL;DR

This study investigates how nanochannel confinement influences the nematic ordering of liquid crystals 5CB and 6CB, revealing a transition from continuous to discontinuous behavior depending on channel radius and wall interactions.

Contribution

It provides a phenomenological model linking channel radius to nematic ordering strength and identifies critical radii for phase behavior transitions in confined liquid crystals.

Findings

Nematic order increases with channel radius.

Critical radii R_c are 12.1 nm for 5CB and 14.0 nm for 6CB.

Wall roughness reduces effective nematic order.

Abstract

We explore the nematic ordering of the rod-like liquid crystals 5CB and 6CB, embedded into parallel-aligned nanochannels in mesoporous silicon and silica membranes as a function of mean channel radius (4.7<=R <=8.3 nm), and thus geometrical confinement strength, by optical birefringence measurements in the infrared region. The orientational order inside the nanochannels results in an excess birefringence, which is proportional to the nematic order parameter. It evolves continuously upon cooling with a precursor behavior, typical of a paranematic state at high temperatures. These observations are compared with the bulk behavior and analyzed within a phenomenological model. Such an approach indicates that the strength of the nematic ordering fields sigma is beyond a critical threshold sigma_c =1/2, that separates discontinuous from continuous paranematic-to-nematic behavior. In agreement with the predictions of the phenomenological approach a linear dependency of sigma on the inverse channel radius is found and we can infer therefrom the critical channel radii, R_c, separating continuous from discontinuous paranematic-to-isotropic behavior, for 5CB (12.1 nm) and 6CB (14.0 nm). Our analysis suggests that the tangential anchoring at the channel walls is of similar strength in mesoporous silicon and mesoporous silica membranes. A comparison with the bulk phase behavior reveals that the nematic order in nanoconfinement is significantly affected by channel wall roughness leading to a reduction of the effective nematic ordering.