Smecticlike rheology and pseudolayer compression elastic constant of a twist-bend nematic liquid crystal

Abstract

In twist-bend nematic (NTB) liquid crystals (LCs), the director (mean molecular orientation) exhibits heliconical structure with nanoscale periodicity. On the mesoscopic scale, NTB 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 NTB 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 Beff of the NTB phase from the measurements of the dynamic modulus GGêù(-ë). It is found that Beff of the NTB phase is in the range of 103-106 Pa and it follows a temperature dependence, BeffGê+(TTB-T)2, as predicted by the recent coarse-grained elastic theory. Our results show that the structural rheology of NTB LCs is strikingly similar to that of the usual smectic LCs, although the temperature dependence of Beff is much faster than that of smectic LCs as predicted by the coarse-grained models.