Measurement of Flexo-coefficients for Nematic LCs without the Influence of the Impurity Ions by Using the Chromatographic Isolation Method

The sum of the flexo-coefficients (e₁₁+e₃₃) was measured by the capacitance characteristic depending on the applied dc voltage in the HAN cell. The voltage for the minimum value of the capacitance was shifted by the influence of flexoelectric effect. One of the important problems for the evaluation for the flexo effect was the influence of impurity ions. Then, the chromatographic isolation phenomenon was used to separate impurity ions when the LC material was injected into the empty cell by the capillary action. The coefficient (e₁₁+e₃₃) of ZLI-4792 was measured, and a value of 26.0 pC/m was obtained.     

Enhancement of flexoelectric ratio of nematic liquid crystal by doping calamitic ferroelectric liquid crystal

We measured the flexoelectric ratio e* of a nematic liquid crystal (NLC) doped with a calamitic ferroelectric liquid crystal (FLC). We doped two kinds of commercial FLC into the pure NLC at a weight concentration of 5 wt%. The absolute value of the flexoelectric ratio was increased up to 49% compared to the pure NLC. The greater transverse dipole moment and the elastic constant of FLC are thought to be related to the increase of the flexoelectric ratio.     

Effect of electrode spacing on image flicker in fringe-field-switching liquid crystal display

Low-frequency driving of a liquid crystal display (LCD) panel to minimise power consumption has drawn much attention recently. In case an LCD panel is driven by a low-frequency fringe field, image flickering phenomenon is detected by the naked human eye when the sign of the applied electric field is reversed. We investigated the effect of electrode spacing on the image flickering phenomenon induced by the flexoelectric effect in a fringe-field-switching (FFS) liquid crystal cell. We found that the image flicker in an FFS cell under low-frequency driving can be eliminated in a simple manner by changing the electrode spacing.     

Complex zigzag flexoelectric walls in the Schlieren textures of MBBA: possibility for determination of two basic Schlieren textures with singular or non‐singular surface points

The flexo‐dielectric behaviour of a homeotropic MBBA nematic layer has been experimentally studied. Asymmetric strong‐weak anchoring of the homeotropic nematic layer was achieved by treating the glass plates – one of them with lecithin ensuring the strong anchoring and the other with usual soap ensuring the weak anchoring. The application of a dc voltage with a sufficient amplitude led to the appearance of a complex texture consisting of gradient flexo‐dielectric deformations including Schlieren texture with many singular points and zigzag flexoelectric walls. The application of additional orienting a.c. voltage brought clarification of the Schlieren texture resembling that of the smectic‐C liquid crystal. Inversely, the application of an ac voltage across the homeotropic nematic layer led to formation of a nice Schlieren texture. The additional application of a d.c. voltage created complex zigzag gradient flexoelectric walls which connected the singular points in the Schlieren texture. In this way, one can determine for the first time how many points in the initial Schlieren texture are singular and how many points are non‐singular. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

介电材料挠曲电系数测定平台的研制

挠曲电材料在传感、驱动方面呈现出的优良特性,使其在结构健康监测领域具有极大应用前景。挠曲电系数是衡量材料挠曲电效应的重要指标,对其测定受限于微小位移、微小电量的精确测量。本文分析了测定挠曲电系数的关键环节,提出了测定挠曲电系数的实验方案,利用锁定放大器和压电微驱动器以及超景深三维显微镜构建测试平台,精确测量微电量及微位移,进而准确测定挠曲电系数。在室温下对钛酸锶钡(Ba0.75Sr0.25TiO3)试样进行了研究,初步测量了纵向挠曲电系数以及横向挠曲电系数,验证了用该平台测定挠曲电系数的可行性,为进一步开展挠曲电材料的研究和应用提供了基础实验平台。     

Flexoelectric effect modelling

A statistical theory of dipole flexoelectric (FE) polarization in liquid crystals is used to calculate temperature dependence of order parameters, elastic constants and FE coefficients. Two systems with polar wedge-shaped and banana-shaped molecules are investigated. In both cases, the FE coefficients are proportional to the dipole moment component parallel to the molecule symmetry axis. It results from the symmetries of interactions and of the Mayer function. The origin of the FE effect and microscopic pictures of the distorted phases are discussed.     

Nematic twist-bend phase under external constraints

ABSTRACT

The recently discovered twist-bend nematic phase, N_TB, has short-pitched heliconical structure with doubly degenerate handedness. In contrast to the classic nematic, in the N_TB phase the director is spontaneously distorted, resulting in unusual elastic properties. The response of the N_TB phase to external stimuli, like chiral doping or applied fields might provide further information about its structure and can find utilisation in practical applications. Here, the N_TB behaviour is theoretically investigated under chiral doping and strong electric fields. We show that the chiral doping removes the N_TB degeneration and modifies the conical tilt angle, leaving the pitch unchanged. Thus, the N_TB helical twisting power is very high and strongly non-linear. Under electric field, we consider separately the ferroelectric, flexoelectric and dielectric couplings. We show that the experiments reported so far disagree with the ferroelectric behaviour, indicating that the N_TB phase is not spontaneously polarised. On the contrary, the observed polar effects fit well with the flexoelectric coupling, confirming the degenerated heliconical structure of the phase. Under very strong fields, we predict a second-order twist-bend nematic – nematic phase transition due to the dielectric torque on the director.     

PHOTOISOMERISING EFFECTS IN NITROAZO FLEXOELECTRIC DIMERIC NEMATIC SYSTEMS

Abstract

The photoisomerisation of a flexoelectric chiral nematic bimesogen system dyed with an azo dye has been investigated. The host material has a pitch and field dependent tilt angle that are temperature independent. Upon illumination by ultra violet, the azo dye molecules undergo a shape change from their trans to cis isomer. The effect of the shape change of the dye on the mixture is to decrease the I-N* transition temperatures, to increase the response times and to decrease the transmitted optical intensity. For the same reduced temperatures, the tilt angles, pitch and threshold voltages for the transition from focal conic to homeotropic textures are unchanged. The macroscopic parameters observed suggest that the orientational order parameter of the system is reduced by UV illumination. The cis isomers do not appear to separate from the host material or significantly change the flexoelectric coefficient.     

Flexoelectric-Induced Voltage Shift in Hybrid Aligned Nematic Liquid Crystal Cell

Flexoelectric-induced voltage shift in a weak anchoring hybrid aligned nematic liquid crystal cell is investigated theoretically. Based on the elastic theory of liquid crystal and the variation method, the equations for the bulk and the boundary of the cell are derived. By computer simulation, the dependence of the
shift voltage on the sum of the flexoelectric coefficients and the anchoring energy strength is obtained. As a result, a novel method to determine the sum of the flexoelectric coefficients by measuring the shift voltage is put forward.