A new type of electrohydrodynamic instability in nematic liquid crystals with positive dielectric anisotropy

Abstract

A new type of electrohydrodynamic instability originally reported in nematic liquid crystal mixtures with positive dielectric anisotropy and as moderately thick samples is further studied. The ability of homogeneously aligned nematics with positive dielectric anisotropy, in the presence of a magnetic field, to exhibit Williams domains as a threshold effect is numerically investigated. The variation of the threshold voltage for domain formation and dielectric alignment with dielectric anisotropy is calculated theoretically and compared with the experimental results as moderately thick and thin samples.     

Instabilities in electric fields of nematic liquid crytals with positive dielectric anisotropy: domains,loop domains and reorientation

Instabilities in electric fields are reported for p,p′-di-n-butyl azoxybenzene (DIBAB), a new room temperature nematic with a positive dielectric anisotropy. The threshold voltage is found to be almost independent of the frequency up to 100 kHz. However, unlike in nematics with negative dielectric anisotropy usually no normal domains are observed. The instabilities take the form of a reorientation, while loop domains are observed as a transiet effect. These loop domains indicate that the instability is not purely dielectric in origin.     

Nematic nanocomposites with enhanced optical birefringence

The proper performance of electro-optical devices utilising liquid crystals (LCs) requires materials with high diffraction efficiency, i.e. with high optical/dielectric anisotropy, low threshold voltage and fast switching. One can achieve increase of dielectric anisotropy by using chemical synthesis or mixing LC materials. However, in most cases, this causes an increase in the threshold voltage and switching times. Therefore obtaining materials with high dielectric anisotropy and keeping threshold voltage and switching times low is a challenging task. We achieved promising results by making binary mixtures of a polar nematic LC 4'-hexyl-4-biphenylcarbonitrile (HBPCN) with low percentage (1–10% by weight) gold nanoparticles. We report that for the mixtures with 1% and 2% gold the dielectric anisotropy increases by 100% and the birefringence by about 50% of their values for pure nematic. We also report that the increase of the dielectric anisotropy in the mixtures only slightly affects threshold voltage and switching times. We propose that this increase is caused by cluster formation in the mixtures.     

Dielectric and visco-elastic properties of laterally fluorinated liquid crystal single compounds and their mixture

The physical properties of three laterally fluorinated liquid crystalline compounds with negative dielectric anisotropy have been studied from static dielectric permittivity, optical birefringence, bend elastic constant, relaxation time and rotational viscosity measurements. Such negative dielectric anisotropy materials find use as components of mixtures for application in vertically aligned mode liquid crystal displays. Moreover, the physical properties of one phenyl cyclohexane compound with positive dielectric anisotropy have also been studied. A five-component mixture comprising these four mesogens and a non-mesogenic component has been formulated and its physical properties have been thoroughly investigated. An attempt has been made to strike a balance between the optical birefringence of the mixture to adjust the cell gap and the dielectric anisotropy and threshold voltage (V_th ) to ensure low driving voltages. The pretilt angle effect on the threshold voltage and the relaxation time has also been studied. At T?=?20°C, the response time decreases to 22% and 41% for the mixture for 2° and 5° pretilt as compared to zero pretilt. On the other hand, at the same temperature the V_th values are decreased by 5% and 9%, respectively.     

Dielectric studies of tetraethylene glycol-bis(3-methylimidazolium) dichloride (TEGDC) exhibiting large negative dielectric anisotropy

A dielectric anisotropy property of a TEGDC (tetraethylene glycol-bis(3-methylimidazolium) dichloride) is investigated as a function of frequency. TEGDC showed an extremely large negative dielectric anisotropy (-10.95 to -4753.73). Variation of dielectric anisotropy (delta epsilon) with respect to the spot frequencies reveals that liquid crystal (LC) orientation has an n-type property at low frequencies and as the frequency increases dielectric anisotropy character shifts from negative dielectric anisotropy type (n-type) to positive dielectric anisotropy type (p-type). Consequently, the TEGDC is a liquid crystal with large negative dielectric anisotropy.     

Relationship between order parameter and physical constants in fluorinated liquid crystals

We have investigated the relationship between order parameter and physical constants (dielectric anisotropy and splay elastic constant) for fluorinated liquid crystals with a small temperature coefficient of the threshold voltage. In these special fluorinated liquid crystals, the dielectric anisotropy is proportional to the square of the order parameter. We have found that this anomalous dielectric behaviour affects the small temperature coefficient of the threshold voltage.     

Threshold flexoelectric deformations in homeotropic nematic layers

Elastic deformations of nematic liquid crystal layers subjected to a d.c. electric field were studied numerically. The flexoelectric properties of the nematic material and the presence of ionic space charge were taken into account. Homeotropic alignment with finite surface anchoring strength was assumed. The director orientation and the electric potential distribution were calculated; the space charge density was also determined. It was found that the threshold voltage strongly depended on the parameters of the system. In particular, a threshold as low as a few tenths of a volt occurred under suitable circumstances. In the case of a negative dielectric anisotropy, Δ ε, such low values of the threshold voltage existed when the ion concentration was sufficiently high, and given sufficiently large magnitudes of the flexoelectric coefficients and a sufficiently small anchoring energy. If the ion concentration was low or if the flexoelectric coefficients were small or if the surface anchoring was strong, the threshold was equal to several volts. In the case of positive dielectric anisotropy, the threshold amounted to several tenths of a volt for a weakly anisotropic and highly conductive material. If the dielectric anisotropy was sufficiently high or if the ion concentration was sufficiently low, the threshold voltage increased with Δ ε and reached tens of volts. These results can be explained as the effect of the inhomogeneous electric field arising in the vicinity of the surfaces, due to the ionic space charge redistributed by the external voltage. They are qualitatively consistent with earlier experiments which show the effect of the ion concentration on the elastic deformations in flexoelectric nematics. They correspond also with theoretical results concerning the effect of the electric field produced by the surface polarization or by the adsorption of ions.     

Rod-shaped dopants for flexoelectric nematic mixtures

We report the synthesis and liquid crystalline behaviour of two series of para-substituted terphenyls as dopants with a rigid rod-like shape, rather than a wedge-, pear- or banana-shape, for guest–host nematic mixtures with flexoelectric properties. One series of liquid crystalline dopants is of low-to-strongly negative dielectric anisotropy and the other is of low-to-strongly positive dielectric anisotropy. The usefulness of apolar and polar rod-like dopants as components of flexoelectric nematic mixtures of positive dielectric anisotropy for use in LCDs is investigated in general and the dependence of the flexoelectric properties of the doped nematic mixtures on the polarity of the dopants is studied in particular. The correlation between the concentration of the dopant and the magnitude of the flexoelastic ratio of several guest–host nematic mixtures is investigated.     

Dielectric and elastic properties of fluorine-containing tricyclic isothiocyanate liquid crystal

The dielectric and elastic properties of liquid crystals (LCs) generally depend on the molecule structure, such as polar group and carbon chain length. For further investigation of the influence of molecular structure on the dielectric and elastic properties of fluorine-containing tricyclic isothiocyanate LC, the experimental temperature was controlled at 25°C by precision hot stage, and a precision LCR meter was used to measure the capacitance of six LC cells under the voltage from 0.1 to 20 V at 1 kHz. An LC cell capacitance model and a dual-cell model were adopted to obtain the dielectric anisotropy, and the capacitance–voltage curves of six LC materials were plotted. The threshold voltage of Fréedericksz transition was analysed, and a finite difference iterative method was used to attain specific values of three elastic constants. The influence of molecular structure on the dielectric and elastic constants was finally analysed. Experimental results showed that the introduction of meta-difluoro group would increase the dielectric anisotropy and reduce the threshold voltage of LC. As the length of the alkyl carbon chain increased, the dielectric anisotropy would have an odd–even alternation effect, which would lead to changes in the elastic constants of LC.     

Electrically induced anchoring transition in nematics with small or zero dielectric anisotropy

The orientational transitions induced by electrically controlled ionic modification of surface anchoring in liquid crystal cells based on the nematics with small or zero dielectric anisotropy Δε are considered. The type of director reorientation is shown to be independent of the sign of dielectric anisotropy and can be the same for the nematics with both negative and positive Δε. Besides, the orientational transition and corresponding switchable optical states do not depend on the Δε value and can be effectively realised even for the nematics with zero dielectric anisotropy.     

Polar 2-alkoxyethoxy-substituted nematic liquid crystals

The synthesis is reported of a series of polar nematic liquid crystals incorporating an alkoxyalkoxy chain attached in a terminal position to a molecular core consisting of two aromatic rings. The dependence of the mesomorphism and phase transition temperatures on the nature of this terminal chain was studied. The shape anisotropy of the core is increased by the presence of an additional 1,4-disubstituted phenyl unit in a number of p-terphenyls. A number of highly polar esters with a terminal methoxyethoxy chain have been prepared as dopants to lower the threshold voltage of LCDs. The presence of the methoxyethoxy chain leads to an increase in the molecular dipole moment, the dielectric anisotropy and the dielectric constant perpendicular to the molecular long axis. Several compounds with a four-unit diether central linkage were also prepared.     

Analysis of cell gap-dependent driving voltage in a fringe field-driven homogeneously aligned nematic liquid crystal display

We have studied cell gap-dependent driving voltage characteristics in a homogeneously aligned nematic liquid crystal (LC) cell driven by a fringe electric field, termed the fringe field switching (FFS) mode. The results show that for the FFS mode using a LC with positive dielectric anisotropy, the operating voltage decreases as the cell gap decreases, whereas it increases with a decreasing cell gap when using a LC with negative dielectric anisotropy. The difference between LCs is explained by simulation and experiment.     

Study on cyanobiphenyl nematic doped by silver nanoparticles

We study the effects of doped (1%wt and 2%wt) silver nanoparticles on material properties of nematic liquid crystal: 4-pentyl-4′-cyanobiphenyl. Using differential scanning calorimetry, electrical and dielectric measurements methods, we show that the doped NPs do not affect the nematic’s phase clearing point, lower the dielectric anisotropy, viscosity, switching-off time, and increase the threshold voltage and elasticity of the nematic. We report that the doped materials temperature behaviour of Frederick’s transition threshold voltage and switching-off time, deviates from the expected behaviour for pristine nematics. To explain this anomalous behaviour, we perform data analysis of the governing Frederick’s transition material parameters of studied samples. We show that the elastic parameter of doped samples is not following the predictions of Maier–Saupe theory, which is valid for conventional nematics. We report that the doped samples temperature behaviour of the elastic parameter follows the predictions of the Gelbart and Ben-Shaul theory.     

Analysis of cell gap-dependent driving voltage in a fringe field-driven homogeneously aligned nematic liquid crystal display

We have studied cell gap-dependent driving voltage characteristics in a homogeneously aligned nematic liquid crystal (LC) cell driven by a fringe electric field, termed the fringe field switching (FFS) mode. The results show that for the FFS mode using a LC with positive dielectric anisotropy, the operating voltage decreases as the cell gap decreases, whereas it increases with a decreasing cell gap when using a LC with negative dielectric anisotropy. The difference between LCs is explained by simulation and experiment.     

Some Novel Nematic Materials Bearing Two Lateral Nitrile Functions

More than fifty esters of 3-alkyl-6-hydroxy-1,2-benzenedicarbonitrile have been prepared. The new esters exhibit stable nematic phases of high negative dielectric anisotropy. The relationship between the clearing points of the esters and the various rings they incorporate is the same as that normally observed for nematic materials of positive dielectric anisotropy.     

Ferroelectric Particles in Nematic Liquid Crystals with Soft Anchoring

A theoretical evaluation of the electric Freedericksz transition threshold and saturation field is proposed for a liquid crystals composite with ferroelectric particles. Existing models consider a strong anchoring of nematic molecules on the glass support of the cell, but in this paper a soft molecular anchoring of molecules on the glass support and also on the ferroelectric nanoparticle’s surface is assumed. Thus, a finite saturation field was obtained in agreement with real systems. Calculations are made for planar configuration of positive dielectric anisotropy liquid crystals. The results are compared with data obtained on similar systems from different publications and the differences are discussed.     

Four unit linking groups III. Liquid crystals of negative dielectric anisotropy

A series of 1-n-alkoxy-2,3-difluoro-4-[3-(trans-4-pentylcyclohexyl)propyloxy]-benzenes has been synthesized. Several homologues exhibit an enantiotropic nematic phase of moderately strong negative dielectric anisotropy (Δε ≈ 5) at or just above room temperature. An analogous series of three-ring diethers also including some (E)-3-allyloxy derivatives possesses enantiotropic, wide range nematic phases also of moderately strong negative dielectric anisotropy, but at elevated temperatures. The synthesis and liquid crystal transition temperatures of these 2,3-difluoroquinone derivatives are described and comparisons are made with the transition temperatures of the corresponding materials incorporating standard central linkages (-, C₂H₄, CH₂O and COO).     

Polar 2‐alkoxyethoxy‐substituted nematic liquid crystals

The synthesis is reported of a series of polar nematic liquid crystals incorporating an alkoxyalkoxy chain attached in a terminal position to a molecular core consisting of two aromatic rings. The dependence of the mesomorphism and phase transition temperatures on the nature of this terminal chain was studied. The shape anisotropy of the core is increased by the presence of an additional 1,4‐disubstituted phenyl unit in a number of p‐terphenyls. A number of highly polar esters with a terminal methoxyethoxy chain have been prepared as dopants to lower the threshold voltage of LCDs. The presence of the methoxyethoxy chain leads to an increase in the molecular dipole moment, the dielectric anisotropy and the dielectric constant perpendicular to the molecular long axis. Several compounds with a four‐unit diether central linkage were also prepared.     

Proprietes dynamo-optiques du bromure de poly-N-butyl-4-vinylpyridinium dans differents solvants

Flow birefringence measurements on polybutylvinylpyridinium bromide have confirmed the change of the sign of the optical anisotropy previously observed by the electric birefringence method. The birefringence is negative in aqueous media, but positive in some organic solvents such as nitromethane. The estimation of the intrinsic optical anisotropy shows that this parameter is negative in water as well as in nitromethane; this is consistent with an orientation of the pyridinium lateral group perpendicular to the long axis of the particle. The positive sign of the birefringence displayed in nitromethane can be attributed both to an important contribution of form anisotropy, and to a pronounced decrease of the intrinsic optical anisotropy; the latter is explained by a conformational change of the polyelectrolyte which becomes more coiled and more flexible in solvents of lower dielectric constant.     

Gradient flexoelectric switching response in a nematic phenyl benzoate

We report on the inverse flexoelectric effects observed in a nematic liquid crystal with a small positive dielectric anisotropy subject to static and very low frequency (<1 Hz) a.c. fields. The Bobylev–Pikin flexobands appear at a temperature‐dependent d.c. threshold. Under square wave excitation, a new type of transient optical response occurs soon after each polarity reversal, and we ascribe it to the gradient flexoelectric distortion explicable on the basis of the presence of intrinsic double layers. This instability is characterized by a threshold voltage that decreases with temperature. Its maximum amplitude increases linearly with voltage close to threshold, and occurs after polarity reversal at a time τ_m that scales inversely as the voltage; τ_m decreases exponentially with frequency and temperature. After each polarity change, the ionic current following the charging current decreases almost exponentially to a non‐zero value; the residual current increases monotonically with the applied bias.