An Electric Bilayer Model of the Transient Current in a Nematic Liquid Crystal Cell
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The transient currents were observed in nematic liquid crystal (LC) cells when the polarity of the applied square wave voltage was reversed. The magnitude of these currents were shown to increase when the used LCs have a tendency to form dimers and the orientation films have a high electrical polarization. The origin of the transient currents is explained by assuming the switching of LC layers associated with the bilayers made of decomposed monomers adsorbed on the orientation films. The origin of the crosstalk appearing in the STN dot matrix LCDs is discussed in conjunction with the decomposition of dimers in the surface alignment region.Keywords:
Transient (computer programming)
Abstract The general rule that addition of a non-mesogenic solute causes a sharp decrease in the nematic-isotropic transition temperature (TN→ I) of a nematic solvent is not obeyed when the solute and solvent can enter into a donor-acceptor interaction. Addition of 4-aminobiphenyl to the nematic liquid crystal 4-cyano-4′-pentylbiphenyl (nematic range ∼ 25–35°) leads to an increase in TN→ I and a decrease in the crystal-nematic transition temperature. The maximum nematic range (21–38°) is achieved at ∼ 7 mole % solute.
Mesogen
Atmospheric temperature range
Biaxial nematic
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Biaxial nematic
Thermal fluctuations
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The results of volumetric measurements under high pressure near three different phase transitions in heptyloxyazoxybenzene (70AB), a nonreentrant nematic liquid crystal, and octyloxycyanobiphenol (80CB), a reentrant nematic liquid crystal, are reported. Volume changes at the isotropic–nematic transition in these two liquid crystals are similar at low pressure, but behave differently at high pressure. Volume changes at the nonreentrant nematic–smectic C transition are an order of magnitude larger than at the reentrant nematic–smectic A transition. The volume dependence along these two nematic–smectic coexistence curves also reveals the stronger role played by attractive forces in these transitions as compared to the isotropic–nematic transition. Finally, these data do not support a previously reported nematic–smectic C-solid triple point in 70AB.
Reentrancy
Biaxial nematic
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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.
Fréedericksz transition
Homeotropic alignment
Flexoelectricity
Biaxial nematic
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Most liquid crystals consist of molecules shaped like the rod. This chapter focuses on calamitic liquid crystals as they are the most important for applications. Calamitic and thermotropic liquid crystals are important for LCDs. Their nematic phase is the basis for both the most widely used Twisted Nematic cell with active matrix addressing, and for the Super Twist Nematic cell with passive matrix addressing. The liquid crystals used are calamitic and thermotropic in the nematic phase. The operation of this most widely applied LCD is phenomenologically described in order to give an overview over the entire flat panel display system, including the addressing scheme.
Thermotropic crystal
Biaxial nematic
Matrix (chemical analysis)
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Abstract The electric field induced nematic order reconstruction is a way to achieve controlled transitions between uniaxial nematic textures with different topologies, hence allowing intrinsic textural bistability. The order reconstruction connects perpendicular nematic director orientations by exchanging two eigenvalues of the nematic order tensor Q , implying intermediate transient biaxial order. This phenomenon avoids the full nematic melting; in fact the nematic scalar order parameter does not vanish. The Biaxial Order Reconstruction in a calamitic Nematic (BORN) is mainly governed by the biaxial coherence length ξ b . Therefore, by varying ξ b , one can favour or inhibit the transient biaxial order. Recently, it has been demonstrated that ξ b can be controlled by suitable dopants and in this work we study the BORN transition threshold for liquid crystal mixtures made of the commercial nematic liquid crystal E7 and three different metallomesogens with board-like shape. Acknowledgments The authors thank Mauro Ghedini and Daniela Pucci for materials synthesis and helpful discussion and Alfredo Pane for technical assistance.
Biaxial nematic
Bistability
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Abstract An approximate solution is derived for the structure of a twist wall between two regions of opposite 180° twist in a nematic liquid crystal film. The result is used to show that a twist wall is unstable unless the elastic constants of the nematic are such that K22≤ ½(K11 + K33). Thus the observation of such walls in nematic liquid crystals is evidence of this elastic anisotropy. Experimental evidence is given for the validity of the approximate solution obtained.
Biaxial nematic
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Biaxial nematic
Nonane
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Abstract Miscibility phase diagrams of mixtures of side-on side chain liquid crystalline polymers (s-SCLCP) and low molar mass liquid crystals (E48 and E44) have been established by means of polarized optical microscopy and light scattering. E48 and E44 are cyanobiphenyl-based eutectic nematic liquid crystal (LC) mixtures with nematic-isotropic transition temperatures of 93 and 105 C, respectively. The phase diagram of the s-SCLCP/E48 system reveals the coexistence of an isotropic nematic region and a single nematic phase in order of descending temperature. The single nematic phase suggests that the pair is miscible in the nematic region. On the other hand, the s-SCLCP/E44 mixture shows liquid liquid and nematic nematic coexistence phases, suggestive of the immiscibility character of the pair. These nematic phase diagrams of the s-SCLCP/E48 and s-SCLCP/E44 have been analysed in the context of the combined Flory-Huggins (FH) free energy for isotropic mixing and the Maier-Saupe (MS) free energy for nematic ordering of the mesogens. This combined FH/MS theory is capable of predicting the observed nematic phase diagrams consisting of liquid liquid, liquid nematic, nematic nematic, and the pure nematic regions. The change of colour accompanying the appearance and disappearance of the inversion walls may be attributed to the temperature dependence of birefringence.
Biaxial nematic
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Abstract The crucial role of the smectic A-nematic transitional order for the formation of the smectic A, B and G phases from an electrically deformed nematic phase of the liquid crystal 4-n-hexyloxy-benzylidene-4′-n-butylaniline (6O.4) with a typical smectic A-nematic first order transition and the formation of the smectic A and B phases from an electrically deformed nematic phase of the liquid crystal (4-n-butyloxy-benzylidene-4′-n-octylaniline (40.8) with a smectic A-nematic second order transition has been demonstrated. The nematic phase was deformed by an AC voltage of 2U,th 5U th and 10U th, where U th is the threshold voltage which causes the appearance of the Fréedericksz transition in the homeotropic nematic layer. The smectic textures have been observed on free cooling of the nematic phase or after the use of an oven. The smectic A phase of the liquid crystal 60.4 was observed with the formation of a clear smectic A-nematic phase boundary while the smectic A phase of the liquid crystal 40.8 has been formed from intermediate pretransitional stripes, observed by Cladis and Torza [1]. The homeotropic anchoring of the direction was crucial for the formation of the smectic phases of the liquid crystal 40.8 but not significant for the liquid crystal 60.4.
Homeotropic alignment
Biaxial nematic
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