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

• Received 18 August 2020
• Revised 7 October 2020
• Accepted 13 October 2020

DOI:https://doi.org/10.1103/PhysRevMaterials.4.115601