Hierarchical elasticity of bimesogenic liquid crystals with twist-bend nematic phase

In 2001, Dozov predicted that twist-bend nematic phase can be spontaneously formed when K₃₃ < 0 and K₁₁/K₂₂ > 2, and this phase has recently been discovered in bimesogens. To verify Dozov's hypothesis, we have measured precisely the temperature dependence of the elastic constants of CB7CB in the entire temperature range of nematic phase and in twist-bend nematic phase close to the transition temperature by combing the Fréedericksz threshold methods for a twist nematic and an in-plane switching cells. Anomalous changes in K₂₂ and K₃₃ are observed across the phase transition. The elasticity estimated via extrapolation of the data in the high temperature region of the nematic phase seems to fully satisfy Dozov's hypothesis although the elasticity data in the vicinity of the phase transition exhibit opposite trends. This can be explained by the general nature of a hierarchical system where the macroscopic elasticity is governed mostly by the distortion of a higher level structure.