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

Abstract

In twist-bend nematic (N_TB) liquid crystals (LCs), the director (mean molecular orientation) exhibits heliconical structure with nanoscale periodicity. On the mesoscopic scale, N_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_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_eff of the N_TB phase from the measurements of the dynamic modulus G^*(w). It is found that B_eff of the N_TB phase is in the range of 10³–10⁶ Pa and it follows a temperature dependence, B_eff∼(T_TB−T) ², as predicted by the recent coarse-grained elastic theory. Our results show that the structural rheology of N_TB LCs is strikingly similar to that of the usual smectic LCs, although the temperature dependence of B_eff is much faster than that of smectic LCs as predicted by the coarse-grained models.