On the measurement of the bend elastic constant in nematic liquid crystals close to the nematic-to-SmA and the nematic-to-NTB phase transitions

We propose to measure the ratio κ31 of the bend elastic constant K3 over the splay elastic constant K1 of a nematic phase by using an optical method based on the measurement of the birefringence of homeotropic/planar hybrid samples. This method is compared to the classical measurements using the Freedericksz transition under AC electric field and its advantages and disadvantages are discussed. Both methods are applied to the measurement of K1 and K3 close to the nematic-to-SmA phase transition of the liquid crystal 8CB and the nematic-to-NTB phase transition of the mixture 8CB + 50 wt% CB7CB. The role of flexoelectricity in these measurements is analysed, as well as the role of the pretilt angle and the anchoring energy of the molecules on the plates limiting the samples.