Direct flexoeffect in nematics close to clearing point

The peculiarities of direct flexoeffect in nematic liquid crystals (NLC) in the vicinity of the clearing point have been studied experimentally. N-butyl-aniline-methoxybenzylidene (MBBA) and dvuhkolchaty ether were the objects of the study. Flexoeffect, as a kind of piezoelectric effect, was observed as a result of the impact of transverse (shear) waves on the homeotropic NLC layer. Electrooptical NLC cell was of sandwich type, in which the excitation of the transverse shear wave was drived by the longitudinal oscillations of one glass substrate coated by conductive layer. The registration of a polarization voltage U1,2, induced in the NLC layer by a periodic deformation of the director field was carried out by two ways: the first - U2 was recorded between the movable plate and fixed substrate, the second - U1 was measured on the fixed substrate. The two possible mechanism of U1,2 voltage generation have been considered. The first one was associated with the polarization potential formation due to the flexoeffect, the second – because of orientational deformation of the surface polarization Ps layer. That is, both mechanisms to the detected signal in varying degrees contribute. Analyzing the temperature changes of the periodic shear induced polarization in the NLC layer in the neighborhood of a phase transition in the isotropic phase, the conclusion of significant contribution of the surface polarization was done. The recorded potential difference in the isotropic phase was related to the Maxwell effect.

Features of the direct flexoeffect in cholesterics and twist-structures of nematics

O.A. Denisova, O.A. Skaldin