Abstract
The specific features of the direct flexoelectric effect in cholesteric liquid crystals (LC) (MBBA + cholesteryl chloride ~ 2%) and twist structures of nematic liquid crystals under the periodic shear oscillations are studied experimentally. The cell represented the assembly of their three plates: the lower and upper plates were separated by spacers of a given thickness, whereas the central one floated freely in the LC. Two methods of oscillation excitation in an LC cell were used in this work [3-5]. To observe the flexoeffect, 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 dependence of the director deflection angle, as well as the magnitude of the flexoelectric effect on the plate oscillation rate, was described by a linear relationship, which is explained by the Eriksen-Leslie theory. The frequency dependences of the flexoelectric polarization induced by the periodic shift, which has a resonance character, are experimentally obtained. It is shown that the resonance character of the detected potential difference U1,2 is related to the scattering of the propagating elastic-viscous wave by the helicoidal twist structure formed in the nematic and weak cholesteric.
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