Investigations of twist elastic constant K22 of new nematic liquid crystal materials using threshold IPS method

Parka, Janusz; Dąbrowski, M.; Kowerdziej, Rafał

doi:10.2478/s11772-010-0074-z

Cited by 10 publications

(5 citation statements)

References 18 publications

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“…Similar to the π-cell technique, the estimated error bar of our approach is ±8%, which is still pretty good as compared to other methods. [14] To minimise experimental errors, more accurate measurements for the input parameters are required, especially for the bend elastic constant K 33 . Besides, using a thicker cell gap or preparing a larger amount of sample would also reduce the total error.…”

Section: Discussionmentioning

confidence: 99%

“…Another approach is to use IPS cell, in which twist deformation can be induced by an electric field. [13,14] However, specific designs, like wall-shaped electrode or aluminium electrodes, are required to ensure pure twist rotation, otherwise the non-uniform LC reorientation would cause a large uncertainty in K 22 measurement. Raynes et al proposed another technique using a wedge LC cell filled with a chiral dopant.…”

Section: Introductionmentioning

confidence: 99%

“…For K 11 and K 33 , a common measurement technique has been established. [10,11] But for K 22 , even though several methods have been proposed, [12][13][14][15][16][17][18][19][20][21] each approach has its own merits and demerits.…”

Section: Introductionmentioning

confidence: 99%

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A simple method to measure the twist elastic constant of a nematic liquid crystal

Chen

Zhu

2015

Liquid Crystals

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We demonstrate a simple method for measuring the twist elastic constant (K 22 ) of a nematic liquid crystal (LC). By adding some chiral dopant to an LC host, the LC directors rotate 180°in a homogeneous cell, which is known as 180°super-twisted nematic (STN) cell. By preparing two such STN cells with different chiral concentrations and measuring their Fréedericksz threshold voltages, we can obtain the K 22 and helical twisting power simultaneously. In the whole process, there is no need to measure the pitch length. Our obtained K 22 values agree well with those reported by using other methods.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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A simple method to measure the twist elastic constant of a nematic liquid crystal

Chen

Zhu

2015

Liquid Crystals

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“…The director reorientation was probed by measuring the transmitted light intensity passing normally through the LC cell between crossed polarizers. 11 K was determined from the relationship There are several approaches to measure 22 K , such as the Frederiks transition in a twisted nematic (TN) cell [33], in-plane realignment in a planar cell [34], a dynamic light scattering method [35], etc. The measured value of 22 K in these methods is usually less accurate than those of 11 K and 33 K , since the twist deformations are accompanied by bend and splay.…”

Section: Elastic Constantsmentioning

confidence: 99%

Comparative analysis of anisotropic material properties of uniaxial nematics formed by flexible dimers and rod-like monomers

et al. 2016

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We report temperature dependencies of material properties such as dielectric anisotropy, birefringence, splay ( 11 K ), twist ( 22 K ), and bend ( 33 K ) elastic constants of the uniaxial nematic (N) phase formed by flexible dimers of DTC5C9 and compare their behavior to that of a corresponding monomer MCT5. DTC5C9 forms a twist-bend nematic (Ntb) at temperatures below the N phase. Anisotropic properties of MCT5 are typical of the rodlike mesogens. In particular, birefringence increases as the temperature is reduced, following the classic behavior, described by Haller. The elastic constants also follow the standard behavior, with their ratios being practically temperature-independent. In contrast, DTC5C9 shows a dramatic departure from the standard case. Birefringence changes non-monotonously with temperature, decreasing on approaching the N-Ntb phase transition. 33 K decreases strongly to 0.4 pN near the N -Ntb transition, although remains finite. The ratios of the elastic constants in DTC5C9 show a strong temperature dependence that can be associated with the bend-induced changes in the orientational distribution function. The measured elastic properties are consistent with the tendency of the dimeric molecules to adopt bent configurations that give rise to the Ntb phase. Keywords: dimeric nematic; elastic constants; birefringence; negative dielectric anisotropy; twist-bend nematic. N phase. Uniform bend, however, cannot be realized in space without other types of deformations, either splay or twist. Thus two different variations of the nematic with spontaneous bend have been proposed by Meyer [8] and Dozov [9]: a twist-bend nematic and a splay-bend nematic (yet to be discovered experimentally). The relative stability of the two is controlled by the ratio of the splay 11 K to twist 22 K constants. Namely, in the twist-bend phase, 11 22 /2 KK  , while in the splay-bend case, 11 22 /2 KK  [9, 10]. As for the ChOH state, Meyer [11] predicted that it can be induced by an external electric or magnetic field in a cholesteric liquid crystal only when 33 22 KK  . In calamitic nematics formed by rod-like molecules, the latter condition is not satisfied; the expected and universally observed trend follows the inequalities 33 22 11 K K K  [13]. Prior research indicates that the dimeric materials feature elastic properties very different from those of rod-like nematogens. Atkinson et al. [14] demonstrated a strongodd-even effect in the behavior of 33 K . Two dimers, one with an even number of alkyl groups in the spacer chain and another one with an odd number, were characterized by the splay Frederiks transition technique. In this approach, 11 K is determined from the threshold field of reorientation, and 33 K is obtained by extrapolating the voltage dependence of capacitance far above the threshold. 33 K of the odd homolog was about (3-4) pN, much smaller (by a factor 10 ) than the corresponding value for the even homolog; the results were in good agreement with the theoretical consideration of the odd-even effect by...

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“…Another optical methods were also developed including conoscopic techniques [9][10][11], half leaky guided wave measurements [12] and methods based on liqht scattering [13][14][15][16][17][18][19]. Other techniques used cells with special in-plane electrodes [20][21][22][23][24]. Further experiments were carried out on wedge- [25,26] and π-cells [27].…”

mentioning

confidence: 99%

Dielectric technique to measure the twist elastic constant of liquid crystals: The case of a bent-core material

et al. 2012

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The effect of director pretilt on the twist magnetic Fréedericksz transition of nematics was investigated in a planar cell. The director configuration was calculated as a function of magnetic inductance. The dielectric and optical response of the nematic liquid crystal was numerically modeled. A dielectric measurement method for determining the elastic constant K_{22} is presented. The influence of the conditions for the Mauguin effect is discussed. The theoretical predictions were confirmed by our experiments. Experimental data for all elastic constants of a bent-core nematic material are presented and discussed.

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Investigations of twist elastic constant K22 of new nematic liquid crystal materials using threshold IPS method

Cited by 10 publications

References 18 publications

A simple method to measure the twist elastic constant of a nematic liquid crystal

A simple method to measure the twist elastic constant of a nematic liquid crystal

Comparative analysis of anisotropic material properties of uniaxial nematics formed by flexible dimers and rod-like monomers

Dielectric technique to measure the twist elastic constant of liquid crystals: The case of a bent-core material

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