Smectic-like rheology and pseudo-layer compression elastic constant of a twist-bend nematic liquid crystal

TL;DR

This study investigates the rheological properties of twist-bend nematic liquid crystals, revealing their smectic-like behavior and measuring a pseudo-layer elastic constant that varies with temperature, aligning with recent elastic theories.

Contribution

It provides the first measurement of the pseudo-layer compression elastic constant in N extsubscript{TB} phases and compares their rheology to smectic liquid crystals, highlighting similarities and differences.

Findings

N extsubscript{TB} phase exhibits smectic-like rheology.

Pseudo-layer elastic constant $B_{eff}$ ranges from 10^3 to 10^6 Pa.

$B_{eff}$ follows a temperature dependence of $(T_{TB}-T)^2$.

Abstract

In twist-bend nematic (N\textsubscript{TB}) liquid crystals (LCs), the mean molecular orientation exhibits heliconical structure with nanoscale periodicity. On the mesoscopic scale, N\textsubscript{TB} resembles layered systems (like smectics), where the helical pitch is equivalent to "pseudo-layers" without a true mass density wave. We study rheological properties of a N\textsubscript{TB} phase and compare the results with those of an usual SmA phase. Analysing the shear response and adapting a simplified physical model for rheology of defect mediated lamellar systems we measure the pseudo-layer compression elastic constant $B_{eff}$ of N\textsubscript{TB} phase from the measurements of dynamic modulus $G^{*}(\omega)$. We find that $B_{eff}$ of the N\textsubscript{TB} phase is in the range of $10^{3}-10^{6}$ Pa and it follows a temperature dependence, $B_{eff}\sim (T_{TB}-T)^{2}$ as predicted by the recent coarse-grained elastic theory. Our results show that the structural rheology of N\textsubscript{TB} is strikingly similar to that of the usual smectic LCs although the temperature dependence of $B_{eff}$ is much faster than smectic LCs as predicted by the coarse-grained models.