Properties Of Nematic Liquid Crystals Research Articles

Influence of Periodic Non-Uniformities of Well-Structured Sapphire Surface by LIPSS on the Alignment of Nematic Liquid Crystal.

In this study, we report on the alignment properties of nematic liquid crystals on various transparent structured sapphire layers formed by laser-induced periodic surface structures (LIPSS). One-dimensional LIPSS (1D-LSFL) are generated by infrared femtosecond laser pulses along parallel lines covering an area of 5 × 5 mm2, with a line spacing that is varied between 7 and 17 µm. These periodic structures, employed as alignment layers, have a spatial periodicity of about 980 nm, a modulation depth of about 100 nm, and exhibit a high quality due to being characterized by a high degree of homogeneity and parallelism of the structured features. It is found that such alignment layers of the sapphire surface lead to a decreasing azimuthal anchoring energy, when the width of the unstructured gap is increased. Modifying the sapphire surface by an ITO-coating with further deposition of a polyimide film increases the azimuthal anchoring energy by a factor of about four up to Wφ ~ 4.25 × 10−6 J/m2, when the minimum width of the unstructured gap is 7 µm. Comprehensive measurements and comparisons of the azimuthal anchoring energy as well as the pretilt angle for the 1D-LSFL, unstructured gaps, and entire areas depending on the width of unstructured gaps are presented and discussed.

Dielectric and conductivity properties of liquid crystal MBBA doped with SWCNTs

The effect of single-walled carbon nanotubes (SWCNTs) on the dielectric and conductivity properties of nematic liquid crystal 4-methoxybenzylidene-4′-butylaniline (MBBA) has been studied. It was shown that the additive of SWCNTs with concentration of 0.3% leads to an increase in the order parameter of MBBA. In this case, the clearing point is raised, the longitudinal component of the dielectric permittivity decreases while the transverse component increases as well as dielectric anisotropy. The percolation effect promotes to the dominance of hopping electron conductivity over ionic conductivity, leading to an increase in electric conductivity.

Characterization of thermal, electro optical and photoluminescent properties of nematic liquid crystal doped with gold nano particles

A complete study based on the thermodynamical, electro-optical photoluminescent concert of gold nano-particle (G-NPs) doped nematic liquid was investigated in the investigate work. The effect of gold nano-particles with concentration dispersion on thermal, electro-optical and dielectric behaviour successfully investigated. The small amount of diffusion of gold nano-particles in EBBA nematic liquid crystal reduced the transition temperature of nematic-isotropic phase. The gold nano-particles as a dopant increases the functional surface area of the material and strong the excitation reflections. Gold nano-particles enhanced the electro-optical and photoluminescence properties at the different biasing.

The influence of polymer on optical and thermal properties of nematic liquid crystals

Polymer is substance which consists of large number of molecules and plays a very important in our everyday life. In the present study the effect of different concentration of polymer Poly Methyl Methacrylate on the optical and thermal properties of nematic liquid crystal were investigated. The polymer was dispersed in the liquid crystal by polymerization induced phase separation method. The physical properties were investigated using differential scanning calorimetry and polarized optical microscopy. We found various textures according to the molecular order and orientation of the liquid crystal and polymer in constrained regions separated by phase boundaries. These composite materials can be used to enhance contrast ratio with more stability for display application and smart window.

Impact of dispersed graphene oxide on thermodynamical, optical, electro optical and dielectric properties of nematic liquid crystal

ABSTRACT Colloidal suspension of 8CB dispersed with graphene oxide has been prepared. Differential scanning calorimetry scans have been performed for both pure and graphene oxide dispersed samples. The Cr-SmA, SmA-N and N-I transition temperature have been reduced in graphene oxide dispersed sample. Textures have also been recorded in nematic and smectic phases for pure and dispersed samples. Optical studies have been carried out with UV-Visible spectrophotometer in order to determine optical band gap of the samples. Increased optical band gap for nanocomposite has been observed. Increased threshold voltage, splay elastic constant and rotational viscosities have been observed for dispersed nanocomposite. Furthermore, dielectric studies have been carried out for both homeotropic and planar alignment of molecules. Dielectric anisotropy has been found to be increased in the dispersed sample. Observed value of conductivity for graphene oxide dispersed nanocomposite is lower in the smectic phase while it is higher in the nematic phase. Relaxation phenomenon shifts to low frequency for graphene oxide dispersed nanocomposite as compared to the pure sample.

Relationship of pitch length of cholesteric liquid crystals with order parameter and normalized polarizability

It is well-known that a cholesteric liquid crystal (CLC) resembles nematic liquid crystal (NLC) in all physical properties except a tendency to align CLC molecules into a helical structure. Thus, due to these mutual similarities between a CLC and NLC, finding the relationship of pitch length (P) with physical parameters of an NLC can develop the CLC properties. In this work, we introduced the relationship of P with NLC's order parameter, and normalized polarizability as used in CLC. We successfully equationalized the temperature dependence of p with order parameter, and normalized polarizability of NLC. The temperature-dependent p shows linear equational form with order parameter, and normalized polarizability. Based on our results, P decreases at high temperatures, and increases with the order parameter values and molecular normalized extraordinary polarizability parameter. The results exhibit several advantages, such as simplifying the complex experimental works before synthesizing chiral dopants as employed in CLC, developing the selective reflection properties in a CLC with NLCs, and investigating the helical twisting property in CLC with the physical properties of NLCs.

Optofluidic Platform Based on Liquid Crystals in X-Cut Lithium Niobate: Thresholdless All-Optical Response

In this study, we present a new configuration of the recently reported optofluidic platform exploiting liquid crystals reorientation in lithium niobate channels. In order to avoid the threshold behaviour observed in the optical control of the device, we propose microchannels realized in a x-cut crystal closed by a z-cut crystal on the top. In this way, the light-induced photovoltaic field is not uniform inside the liquid crystal layer and therefore the conditions for a thresholdless reorientation are realized. We performed simulations of the photovoltaic effect based on the well assessed model for Lithium Niobate, showing that not uniform orientation and value of the field should be expected inside the microchannel. In agreement with the re-orientational properties of nematic liquid crystals, experimental data confirm the expected thresholdless behaviour. The observed liquid crystal response exhibits two different regimes and the response time shows an unusual dependence on light intensity, both features indicating the presence of additional photo-induced fields appearing above a light intensity of 107 W/m2.

Synthesis of β-cyclodextrin Functionalized silica nanoparticles and their effects on the dielectric, optical and electro-optic properties of nematic liquid crystal

ABSTRACT The present work reported the synthesis and characterisation of β-cyclodextrin (βCD) grafted into silica (SiO2@βCD) nanoparticles (NPs). Morphology analysis shows that the prepared NPs have a spherical shape with 170 nm of size. The electro-optic, dielectric and viscoelastic properties of SiO2@βCD doped nematic 5CB were examined in different concentrations of the NPs. A decrease in clearing temperature, birefringence and the order of the nanocomposite systems as compared to the pure system was observed. Dielectric parameters of pure and doped samples were measured in the frequency range of 1–2 MHz. Ionic conductivity and dielectric anisotropy increase significantly with NPs concentration due to the strong dipole moment and polarisability of SiO2@βCD. The advantage of dopedSiO2@βCDnematic 5CB is attributed to the significant decrements in threshold voltage by 32% and strong additional coupling of the electrical dipole moments in the spherical NPs with the nematic director field. On the other hand, it is found that the NPs exhibit a decreasing effect on the response time of the nematic liquid crystal, which is demonstrated by the decrement in the viscosity. The results indicate that doping SiO2@βCD is appropriate to fabricate a fast response LC device.

Dynamic of orientation processes in liquid crystal systems of technical devices

The relaxation properties of nematic liquid crystals by the pulsed method of a fixed distance in a rotating magnetic field are investigated. The adequacy of the solution of the equation of motion of the nematic director in the conditions of asynchronous motion of the director and the magnetic field induction vector was experimentally confirmed. The values of the rotational viscosity coefficient and the orientation relaxation time that characterize the parameters of devices with a liquid-crystal working medium in a wide range of pressures and temperatures are calculated. In the framework of the free volume theory, the dependence of the rotational viscosity coefficient on temperature and pressure is analyzed.

Enhanced dielectric properties of Nematic liquid crystal doped with ferroelectric nanoparticles

ABSTRACT In this work, we report the electrical and dielectric behaviours of a ferroelectric nanoparticle-doped liquid crystal cell using the impedance and dielectric spectroscopic measurements in the frequency range 1 Hz – 2 MHz. The impact suitable equivalent electric circuit is proposed and the circuit parameters are obtained using the impedance spectroscopy technique. The impact of the nanoparticles on these parameters which attributed to mobile charge carriers accumulating near the electrodes of the cell is demonstrated and discussed. The presence of nanoparticles is found to increase the dielectric anisotropy (Δε) with maintaining a low loss tangent. As a result, a highly Δε = 120 for frequencies <100 Hz and Δε = 27 for f > 100 Hz is achieved. The dielectric measurements show that the presence of the nanoparticles leads to the appearance of low-frequency relaxation (f ≈ 500 Hz) due to the interfacial polarisation, a slight increase in the diffusion coefficient and the ion mobility. This work provides a foundation to quantitatively investigate the interface effect in dielectric composites.

Enhanced Electro-optical Properties of Low Viscous Nematic Liquid Crystal Doped with Mixed Phase Anatase/Rutile TiO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; Nanoparticles for Display Applications

Organic–inorganic composite based on liquid crystalline and TiO2 nanoparticles were obtained and investigated taking into account the crystallographic form of TiO2 i.e., anatase/rutile mixed phase. TiO2 is an important class of material having various dielectric and electro-optical properties. The existent research presents the electro-optical properties of nematic liquid crystal (NLC) E204 and TiO2 nanocomposites over an extensive range of frequencies. Various important display parameters such as response time, threshold voltage, pretilt angle and activation energy of pristine as well as TiO2 doped composites systems were measured and analyzed. In comparison with the pure, TiO2-doped composite systems has approximately 50% faster response time, owing to its remarkable decline in the relaxation time and activation energy of the LCs. The alteration in the optical intensity of the NLC composites as a function of the concentration of TiO2 nanoparticles (NPs) was also examined. It was established that the optical intensity in nano-nematic composites was decreasing with the concentration of TiO2 NPs. Also, it was observed that an escalation in the TiO2 NPs concentration in NLC composites indicates to an escalation in the birefringence. Probable mechanisms of the interactivity between NLC molecules and TiO2 NPs have been discussed. The current work shows that the TiO2 NPs doping has an encouraging application in the display devices, including other electro-optical as well as photonic applications.

Influence of 4-(2-S(+)-Methylbutyloxy)Benzoic Acid on Mesomorphic and Physico-Chemical Properties of 4-Butyl-4'-Octanoyloxyazobenzene

Mesomorphic, optical, dielectric, and orientation properties of nematic liquid crystal 4-butyl-4'-octanoyloxyazobenzene doped with chiral 4-(2-S(+)-methylbutyloxy)benzoic acid have been studied. The clearing temperatures of the mixtures were measured by polarizing microscopy. An estimation of the mesophase destabilization with the introduction of the dopant was carried out. The helix pitch of the induced chiral nematic mesophase was measured. The effect of the chiral dopant on anisotropic characteristics of the mesophase has been studied. An insignificant increase in the optical and dielectric anisotropy is associated with the formation of liquid crystal – dopant supermolecules due to hydrogen bonds –COOH…OCO–.

The scientific duo of TiO2 nanoparticles and nematic liquid crystal E204: Increased absorbance, photoluminescence quenching and improving response time for electro-optical devices

The present investigation on the dielectric and electro-optical properties of nematic liquid crystal (NLC) E204 and TiO2 nanocomposites has been carried out with a wide range of frequencies. The low-frequency spectra of these composites with a concentration of 0.1 wt/wt%, 0.5 wt/wt%, and 1.0 wt/wt% are recorded in the frequency range of 50–1000 Hz, and the content of impurity ions in these samples has been analyzed. The ion trapping by adsorbing sites of TiO2 and that effect on DC conductivity and diffusion constant have also been investigated. TiO2-doped LC cell results in a two-fold faster response time of the pure LC cell, as a consequence of a remarkable reduction in the rotational viscosity of the LCs. The change in the intensity of the photoluminescence (PL) spectra of NLC composites as a function of the concentration of TiO2 nanoparticles (NPs) has also been investigated. PL spectra reveal that the PL-quenching increases in nano-nematic composites with increasing the concentration of TiO2 NPs following to the Stern-Volmer quenching equation. The pure NLC exhibits a strong absorption band nearly at 314 nm, attributing to π-π* transitions in the phenyl rings which has been slightly blue-shifted after the addition of TiO2 NPs. This blue shift was to be attributed to the n → π⁎ electronic transitions in the systems. The doping of TiO2 NPs has a promising application to the field of LC devices and displays.

Effect of fullerenes C60 on dielectric relaxation, electric conductivity, and electro-optic properties of 4-cyano-4′-pentylbiphenyl

Effect of fullerenes C60 on dielectric relaxation, electric conductivity, and electro-optic properties of nematic liquid crystal 4-cyano-4′-pentylbiphenyl is investigated. It is shown that the inclusion of fullerenes into liquid crystal leads to decreasing of the clearing temperature and the order parameter. In this case, the transverse component of the real part of dielectric permittivity increases while the longitudinal component decreases. A maximum of dielectric absorption shifts to the low-frequency region. The capture of moving ions by fullerenes leads to a decrease of electric conductivity. The elastic constant, rotational viscosity and switching times decrease while the threshold voltage of Freedericksz effect increases.

Viscoelastic and dielectric properties of 5CB nematic liquid crystal doped by magnetic and nonmagnetic nanoparticles.

In this article we show how spherical nanoparticles (NPs) imposing planar anchoring can strongly impact the viscoelastic, dielectric, and electro-optical properties of a nematic liquid crystal when they are not aggregated. We also demonstrate that when the NPs are magnetic, most nematic properties are more impacted than when they are nonmagnetic. With magnetic NPs a molecular disorder is induced that decreases the nematic order parameter, this decrease impacting the values of elastic constants, viscosity, and response time. The impact on 5CB liquid crystal (LC) has been investigated with spherical nanoparticles (NPs) of identical size around 6 nm, magnetic (γFe_{2}O_{3}), and nonmagnetic (CeO_{2}) ones that are both surface functionalized by poly(aminopropylmethylsiloxane-b-dimethylsiloxane) (PAPMS-b-PDMS) block copolymer ligands to promote planar anchoring. In the presence of nonmagnetic NPs, despite an almost constant nematic order parameter, a significant decrease of elastic constants (25.4%), viscosity (22%), and response time (23%) is measured. It suggests a dilution effect for the intermolecular interactions in the presence of NPs. This hypothesis is supported by the observation of an enhanced decrease of the same nematic parameters in the presence of magnetic NPs that can be fully explained by the corresponding order parameter decrease. This finally leads to a remarkable decrease of the splay elastic constant by 51% in the presence of magnetic NPs. The decrease of the nematic order parameter by 18% in the presence of magnetic NPs demonstrates that the NP magnetic moments are only weakly coupled to the nematic director and consequently only induce a disorder in the composite system. A significant influence of the expected large LC structural modifications in the presence of magnetic NPs is, however, shown by a particularly large increase of the diffusion coefficient 43% and large decrease of the dielectric anisotropy (43%). We believe that the observed impact of NPs with planar anchoring on nematic properties could be extended to most spherical NPs if their aggregation can be avoided. In particular, the difference between magnetic and nonmagnetic NPs could be extended to ferroelectric and nonferroelectric NPs.

Magnified charge carrier conduction, permittivity, and mesomorphic properties of columnar structure of a room temperature discotic liquid crystalline material due to the dispersion of low concentration ferroelectric nanoparticles.

Liquid crystal nanocomposites have been a hot topic of research due to optimization of physical properties with such blending. There are several reports on enhancement of physical properties of nematic liquid crystals due to the blending of the nanomaterials. L. M. Lopatina and J. V. Selinger [Phys. Rev. Lett. 102, 197802 (2009)]10.1103/PhysRevLett.102.197802 have even proposed a theory based on experimental results for the enhancement of the properties of the nematic mesophase in the presence of ferroelectric nanoparticles. However, discotic liquid crystal nanocomposites are less studied. In the present experimental work, we have studied the effect of ferroelectric (BaTiO_{3}) nanoparticles on a room temperature discotic liquid crystalline material, namely 1,5-dihydroxy-2,3,6,7-tetrakis(3,7-dimethyloctyloxy)-9,10-anthraquinone. We investigated the physical properties of low concentration ferroelectric nanoparticle dispersed discotic columnar structure, using calorimetric, optical, x-ray diffraction, and dielectric spectroscopy tools. Results show that inclusion of ferroelectric nanoparticles in the discotic matrix consolidates the stability of the columnar matrix of the Col_{h} phase by virtue of their ferroic nature. An enhancement in charge carrier conductivity by several orders of magnitude at ambient conditions has been observed which makes such systems highly appropriate for one-dimensional conductors. Low concentration of BaTiO_{3} nanoparticles substantially enhanced permittivity of the system also. A molecular relaxation mode has been observed in the middle frequency range of the dielectric spectra. Enhancement of these important parameters could be possible due to the ferroelectric nature of the dispersed nanoparticles.

Enhanced electrochemical and electro-optical properties of nematic liquid crystal doped with Ni:ZnCdS/ZnS core/shell quantum dots

The doping effects of Ni-doped ZnCdS/ZnS (Ni:ZnCdS/ZnS) core/shell quantum dots (QDs) on the electrochemical and electro-optical properties of a nematic liquid crystal (LC) system were demonstrated. The impedance response of pure and QD-dispersed nematic LCs was investigated in the frequency range of 0.1 Hz–100 kHz. By fitting a suitable equivalent electric circuit model to the experimental data, it was concluded that the QDs doping noticeably increases the electrical conductivity and charge-capacitance of nematic LC. The ionic mobility and diffusion coefficient are strictly dependent on the QD concentration and the amount of these parameters decreases when 0.25 wt% of the QDs is introduced to the nematic LC, and then increases on further addition of the QDs. This phenomenon is due to the competition between the generated ionic impurities during assembling and the ion-capturing effect of the QDs. The viscosity of nematic LC increased at first and then decreased with increasing concentration of the QDs. Dielectric permittivity of nematic LC samples was also measured at different temperatures. The QD-doped samples exhibited higher permittivity compared to pure nematic LC. The dielectric anisotropy exhibited an increasing trend upon the addition of the QDs up to 0.75 wt% and decreased with further adding QDs. The dielectric results indicated that the nematic phase LC induced self-assembly on QDs to form one-dimensional arrays along the nematic director. Moreover, QDs dispersed in the LC mixture trapped ionic impurities, suppressed the ionic screen effect, and strengthened the electric field in the nematic LC bulk. Therefore, the QDs doping decreased the threshold voltage of the nematic LC cell. The estimated field-off response time initially increased and then decreased with an increase in the concentration of QDs. These results suggest that Ni:ZnCdS/ZnS QDs doped into an LC system can be beneficial for both the display industry as well as circuits working at the highly sensitive system.

Effect of cobalt oxide nanoparticles on dielectric properties of a nematic liquid crystal material

Recently nanoparticles (NPs) are extensively explored as the dopant in liquid crystal (LC) materials because of their phenomenal properties such as good guest–host interaction and high surface to volume ratio. Keeping this motivation into mind, we have observed the influence of newly synthesized cobalt oxide (CoO) NPs on the dielectric and optical properties of nematic LC (NLC) namely 5 CB. For the observation of dielectric properties, we have performed dielectric spectroscopic measurements in the frequency range from 20 Hz to 1 MHz. The sample cell thickness-dependent studies have been performed to perceive the effect of applied field strength on dielectric parameters in presence of CoO NPs. Along with it, we have observed the alignment of the mesogens through the optical microscopy. So doping of such NPs serves various potential applications in the field of research as well as being proven to be cost-effective for industrial applications. In this article, we report the variation in dielectric properties of nematic liquid crystal (NLC) namely 5CB by the means of cobalt oxide nanoparticles (CoO NPs) doping. These CoO NPs may work as the ion absorbing agent in the NLC medium and can easily modify dielectric properties without influencing molecular alignment. Along with this, we have performed the LC sample cell thickness dependent studies of pure and CoO NPs doped 5 CB.

Dielectric relaxation, electric conductivity, and electro-optic properties of SWCNT-doped liquid crystal 5CB

The effect of single-walled carbon nanotubes (SWCNTs) on the dielectric, conductivity, and electro-optic properties of nematic liquid crystal 4-cyano-4'-pentylbiphenyl (5CB) has been studied. It was shown that the additive of SWCNTs with concentration of 0.5% leads to an increase in the order parameter of 5CB. In this case, the clearing point is raised, the longitudinal component of the dielectric permittivity increases while the transverse component decreases. The incipient percolation effect promotes to the dominance of hopping electron conductivity over ionic conductivity, leading to an increase in specific conductivity. The elastic splay constant of 5CB is enhanced and, accordingly, the threshold voltage of the Freedericksz effect increases in this situation. An increase in distance between LC molecules leads to decreasing of viscosity. As a result, the flip-flop motion of molecules become easier and the switching times are also reduced.

Investigation of electro-optical and dielectric properties of nematic liquid crystal dispersed with biowaste based porous carbon nanoparticles: Increased birefringence for display applications

Porous carbon nanoparticles (PCNPs) of diameter 80–100 nm are obtained from teakwood materials waste, using catalyst free pyrolysis method. These PCNPs are dispersed in a pure nematic liquid crystal (NLC) with different concentrations. Polarizing optical microscopic (POM) textures are recorded for pure NLC and NLC - PCNP composites showed better homogeneous alignment when nanoparticles were dispersed in the composites as compared with the pure NLC. The birefringence measurement using the transmittance technique showed interesting properties. It is found that birefringence is increased for the NLC - PCNP composites when compared to pure NLC. Contrast ratio for the NLC - PCNP composites increased after the dispersion of PCNPs. The Relative permittivity increases for NLC - PCNP composites in comparison with the pure NLC. Dielectric measurements showed that the dielectric anisotropy increases for NLC - PCNP composites when compare to pure NLC. The electro-optical study revealed that the threshold voltage of the system decreases after the dispersion of PCNPs. Presented data of the nano composite mixtures found to be very useful in flat panel displays (FPDs) and phase shutters.