Nematic Liquid Crystalline Materials Research Articles

Topologically frustrated structures in inkjet printed chiral nematic liquid crystal droplets - experiments and simulations.

Director field alignment in inkjet printed droplets of chiral nematic liquid crystalline materials is investigated using both experiments and numerical simulations. Experimental investigations are performed by depositing droplets of varying sizes and pitches on homeotropic alignment layers. The competition between the bulk behaviour of the chiral nematic liquid crystal and the boundary conditions imposed by the droplet surface leads to the formation of a range of possible internal director configurations. Numerical investigations are performed using a free energy minimisation approach, and the resultant simulated polarising optical microscope images are found to agree well with experimental observations.

Dissipation in nonequilibrium thermodynamics and its connection to the Rayleighian functional

We examine quantitatively the role of dissipation in nonequilibrium thermodynamics and its connection to variational principles and the Rayleighian functional. The extremum of the Rayleighian is sometimes used to describe the inertialess (dissipation-dominated) dynamics of continuum systems, and it has been applied recently for the modeling of soft matter dynamics. We discuss how dissipation is considered within one of the modern complete descriptions of nonequilibrium thermodynamics, namely the single generator bracket formalism. Within this formalism, dissipation is introduced through the use of the dissipation bracket, describing irreversible dynamics, which is added to a Poisson bracket that describes the reversible dynamics of the system. A possible connection with the Rayleighian functional is then demonstrated that in all cases considered herein, the Rayleighian is equal to minus one half of the effective dissipation rate of the Lagrangian functional. The effective dissipation rate is obtained starting with an inertial (i.e., flux-based or velocity-based) system description, involving the Poisson bracket and the primitive part (i.e., without the entropy correction term) of the dissipative bracket. Several examples are discussed in detail, ranging from an algebraic model (damped oscillator) to continuum ones: modeling of fluid flow in porous particle media, viscous Newtonian compressible and incompressible fluid flows, and more interestingly, flow of a nematic liquid-crystalline material.

Optical manipulation of sessile droplets of nematic liquid crystalline materials on the surface of a photovoltaic crystal

Optical manipulation of sessile droplets of nematic liquid crystalline materials on the surface of a photovoltaic crystal

Gold Nanorod-Induced Effects in a Mesogenic Compound 4-(trans-4-n-Hexylcyclohexyl) isothiocyanatobenzene.

Gold nanorods (GNRs) have a capsule-like structure with different optical properties than spherical gold nanoparticles due to surface plasmon resonance. Liquid crystals (LCs) are mesogenic compounds having crystal-like orientation and liquid-like fluidity. They are important materials from a technological point of view. Both GNRs and LC compounds are anisotropic in shape and properties. Different nano entities show interesting results when dispersed in different liquid crystalline materials which are instrumental from the application point of view. In the present work, GNRs have been dispersed in nematic liquid crystalline materials, namely 4-(trans-4-n-hexylcyclohexyl) isothiocyanatobenzene (6CHBT). Calorimetric, texture, spectroscopic, and dielectric studies were carried out for a pure 6CHBT and its composites with GNRs. Different calorimetric and dielectric parameters such as transition temperature, enthalpy, heat flow, permittivity, dielectric strength, dielectric anisotropy, and relaxation frequency have been determined, and the effect of GNRs has been explored. This article gives an insight into the influence of GNRs on the morphology and anisotropic physical properties of the nematic liquid crystalline material.

Enhanced stability of the nematic phase of 4-pentyl-4ʹ-cyanobiphenyl due to the dispersion of copper nanoparticles

ABSTRACT Embedding of nanoparticles in nematic liquid crystalline materials has received huge attention in recent years. In most of the work, nanoparticles decrease nematic-isotropic transition temperature along with an undesired increase in conductivity. Interestingly, there are few reports where nematic-isotropic transition temperature has increased along with improvement in other relevant properties due to presence of ferroelectric and/or functionalised nanoparticles of small size. Here, we are reporting enhanced nematic-isotropic transition temperature of 4-pentyl-4′-cyanobiphenyl (5CB) due to dispersion of metallic copper nanoparticles (CuNPs). Low concentrations (0.2 and 0.5 weight%) of CuNPs have been dispersed in 5CB to prepare nanocomposites. Thermodynamic results on unaligned samples and dielectric results on planar and homeotropic aligned samples show enhancement of isotropic to nematic transition temperature of 5CB in the presence of CuNPs without any substantial change of conductivity. Optical studies show a marginal decrease in the band gap of the dispersed samples as compared to those of the pure 5CB which validates conductivity results. The relaxation frequency corresponding to the rotation of molecules about their short axes has increased due to the presence of CuNPs in the nematic matrix of 5CB. These observed results are useful for the electro-optical applications as well as for fundamental aspects.

Optical activity and linear electrooptic response in the paraelectric subphase of an achiral ferroelectric nematic compound

ABSTRACTPolarizing optical microscopy and electro optics are used to study a compound with constituent non-chiral molecules, DIO, known to exhibit three nematic phases, namely N, Nx and ferronematic NF. N is found to be a conventional nematic but shows two additional unusual features: the presence of optically active domains and a linear electro optic response to the applied signal. The optical activity is explained by a segregation of the stable helical conformers of the opposite sense; these preferring to form domains with an optical rotation of ±4°/μm. This is the first example of helical segregation observed in non-chiral molecules in the high temperature nematic phase. Unlike the ordinary nematic liquid crystalline material, this compound shows a linear electro optical response to the applied electric field, thereby confirming the polar nature of this phase. NF is the ferroelectric nematic as reported previously. The strong polar azimuthal surface interaction energy in the NF phase stabilises a homogeneous structure in planar-aligned cells that are rubbed parallel and gives a twisted structure in cells rubbed antiparallel.

Ferroelectric Nematic-Isotropic Liquid Critical End Point

A critical end point above which an isotropic phase continuously evolves into a polar (ferroelectric) nematic phase with an increasing electric field is found in a ferroelectric nematic liquid crystalline material. The critical end point is approximately 30 K above the zero-field transition temperature from the isotropic to nematic phase and at an electric field of the order of 10 V/μm. Such systems are interesting from the application point of view because a strong birefringence can be induced in a broad temperature range in an optically isotropic phase.

Synthesis and characterization of semiconducting copper oxide nanoparticles and their impact on the physical properties of a nematic liquid crystalline material 4-pentyl-4′-cyanobiphenyl

Green synthesis of nanoparticles is a biological, eco-friendly and less expensive method which is free from chemical contaminants. In this work, copper oxide nanoparticles (CuO NPs) have been synthesized via green synthesis method. This method proved to be efficient for preparing CuO NPs at room temperature without using any inert atmosphere. The CuO NPs have been characterized using X-Ray Diffraction, Ultraviolet–Visible Spectroscopy and Fourier Transform Infrared Spectroscopy techniques which confirmed the proper synthesis of CuO NPs of the size 2–3 nm. Two different weight percents (0.2 and 0.5) of the synthesized CuO NPs have been dispersed in the nematogenic liquid crystalline material 4-pentyl-4′-cyanobiphenyl (5CB). The physical properties of pure 5CB and its nanocomposites were studied by differential scanning calorimetry, ultraviolet–visible spectroscopy and impedance spectroscopy. Thermodynamic and dielectric results show enhancement in the nematic to isotropic transition temperature of nanocomposites along with enhancement in conductivity as compared to its value for the pure 5CB. Through optical studies, the band gap has been estimated by the Tauc plots which suggest decrease in the optical band gaps of dispersed samples.

Running streams of a ferroelectric nematic liquid crystal on a lithium niobate surface

ABSTRACTSessile droplets of a ferroelectric nematic liquid crystalline material were exposed to surface electric fields produced by pyroelectric and photogalvanic (photovoltaic) effects in X-cut iron-doped lithium niobate crystals. The resulting dynamic processes were monitored by polarisation optical (video)microscopy (POM). During heating/cooling cycles, at first, the droplets change their shape from spherical to extended ellipsoidal. Then, they start to move rapidly along the surface electric field, i.e. along the crystal’s polar axis (c-axis). During this motion, several droplets merge into running streams (tendrils) extending towards the edges of the top surface area. Finally, practically all liquid crystalline material is transported from the top surface to the side surfaces of the crystal. At stabilised temperatures, laser illumination of the assembly causes dynamic processes that are localised to the illuminated area. Also, in this case, the LC droplets merge into several tendril-like formations that are preferentially oriented along the c-axis of the crystal. The pattern of tendrils fluctuates with time, but it persists as long as the illumination is present. In this case, the LC material is transported between the central and the edge regions of the illuminated area.

Light-induced dynamics of liquid-crystalline droplets on the surface of iron-doped lithium niobate crystals

We investigated the effect of a photovoltaic field generated on the surface of iron-doped lithium niobate crystals on sessile droplets of a ferroelectric nematic liquid crystalline and a standard nematic liquid crystalline material present on this surface. When such an assembly is illuminated with a laser beam, a wide range of dynamic phenomena are initiated. Droplets located outside the laser spot are dragged in the direction of the illuminated area, while droplets located inside the illuminated region tend to bridge each other and rearrange into tendril-like structures. In the ferroelectric nematic phase (NF), these processes take place via the formation of conical spikes evolving into jet streams, similar to the behavior of droplets of conventional dielectric liquids exposed to overcritical electric fields. However, in contrast to traditional liquids, the jet streams of the NF phase exhibit profound branching. In the nematic phase (N) of both the ferroelectric nematic and the standard nematic material, dynamic processes occur via smooth-edged continuous features typical for conventional liquids subjected to under-critical fields. The difference in dynamic behavior is attributed to the large increase of dielectric permittivity in the ferroelectric nematic phase with respect to the dielectric permittivity of the nematic phase.

Enhancement of birefringence for liquid crystal with the doping of ferric oxide nanoparticles

Electro-optical (E-O) property of nematic liquid crystalline (NLC) materials is the key parameter of display and non-display applications. Optical anisotropy (i.e., birefringence) is one of the critical parameters of these LCs via which E-O properties are governed. In this article, we have investigated the effect of ferric oxide nanoparticles (Fe2O3 NPs) on the birefringence and the order parameter of a NLC when doped into different weight percentages (wt%). The birefringence of the NLC has been examined via the phase modulation optical transmittance technique. It is noticed that the value of birefringence increases with the concentration of NPs in the LC matrix which is analogous to an improved order parameter of NLC after the addition of NPs. The improved ordering of LC molecules has been verified using polarized optical microscopy. This study helps in understanding the guest-host interactions for improved optical applications of LC materials.

Novel smectic phases and orientational order switching in two-dimensional liquid crystalline system

ABSTRACT Liquid crystalline (LC) materials are ubiquitous in our modern life. Most of the applications where LC materials find their use depend on maintaining the orientation of the constituent LC particles. In this article, we report a recipe to control the molecular orientation. We have done a computer simulation study of a two-dimensional (2D) liquid crystalline material consisting of soft ellipses. The pair interaction is modelled by using the Gay–Berne potential. The 2D LC material is set to interact with an underlying substrate which varies periodically in one-dimension (1D) only. By controlling the substrate parameters like periodicity and strength, we showed that a nematic phase LC material transitions into different novel phases. We finally show that by applying a substrate with large periodicity, the substrate strength can be used as a control parameter to steer the LC director.

Effect of iron oxide (γ-Fe2O3) nanoparticles on the morphological, electro-optical and dielectric properties of a nematic liquid crystalline material

The effect of Iron Oxide Nanoparticles (γ-Fe2O3 NPs) on a room temperature nematic liquid crystalline material namely 4-(4′-hexylcyclohexyl) isothiocyanatobenzene (6CHBT) has been studied. The ferronematic composites have been characterized using differential scanning calorimetry, polarizing optical microscopy, dielectric spectroscopy and electro-optical measurements. It is found that the ferronematic with the low concentration of γ-Fe2O3 NPs enhanced the orientational ordering of nematic matrix due to the strong orientational coupling between magnetic dipole moments associated to nanoparticles and nematic liquid crystal director, which is the essential feature of these systems. It resulted the improvement of various parameters of liquid crystals important for the application. In the ferronematics, the nematic to isotropic transition temperature increased as compared to that of the pure material. Dielectric spectroscopy has shown an increase of dielectric anisotropy in the composite systems. Electro-optical results reveal a decrement in both the splay elastic constant and the electric threshold voltage, in line with a noteworthy improvement of the steepness of transmission- voltage curves.

Enhancement of electro-optical and dielectric parameters of a room temperature nematic liquid crystalline material by dispersing multi-walled carbon nanotubes

ABSTRACT We have prepared the composites of a room temperature nematic liquid crystal mixture by dispersing small fractions of Multi-Walled Carbon Nanotubes into its matrix. The electro-optical and dielectric properties of the prepared samples were investigated. The effect of Multi-Walled Carbon Nanotubes on various display parameters of host nematic liquid crystal mixture was observed. The variations in some of the display parameters like dielectric anisotropy, threshold voltage, splay elastic constant and activation energy have been observed for composite systems with respect to pure nematic liquid crystal mixture. Dispersion of Multi-Walled Carbon Nanotubes enhanced the nematic order of liquid crystal host, resulting in the improvement of dielectric and electro-optical parameters of studied composite systems.

Dielectric and electro-optic properties of 6CHBT nematic liquid crystals and silver nanoparticles composites

ABSTRACTWe have prepared the composite of a room temperature nematic liquid crystalline material namely 4-(trans-4′-n-hexylcyclohexyl) isothiocyanatobenzoate (6CHBT) and silver nanoparticles (AgNPs). Thermodynamic, electro-optic and dielectric properties of the prepared composite have been investigated. From these investigations, parameters such as transition temperature, dielectric anisotropy, dielectric permittivity and splay elastic constant have been determined for the pristine as well as composite systems. Threshold switching voltage of the pure and composite samples has also been determined from transmission voltage curve. The effects of dispersion of AgNPs on dielectric and electro-optic parameters of LC-NPs composite have been discussed in terms of existing standard models.

Optical second harmonic generation in a ferromagnetic liquid crystal.

A comparative experimental investigation of the dependence of second harmonic generation (SHG) on an applied external voltage between a standard nematic liquid crystalline material and an analogue ferromagnetic nematic liquid crystalline material was performed by using a fundamental optical beam at an 800 nm wavelength. For the ferromagnetic material, the dependence of SHG on an applied magnetic field was also examined. Three different polarization combinations of the fundamental and the second harmonic radiation were analysed. The SHG signal observed in the former material is attributed to a combination of electric field-induced SHG (EFISHG) and flexoelectric deformation-induced SHG, while the SHG signal observed in the latter material is attributed solely to flexoelectric deformation-induced SHG. The obtained dependences of the SHG signal on the associated optical retardation show that, in the most favourable polarization combination, the two contributions generate about the same effective nonlinear optical susceptibility.

Dielectric and electro-optical properties of a nematic liquid crystalline material with gold nanoparticles

ABSTRACTWe have prepared the composites of a room temperature nematic liquid crystalline material namely 4-(trans-4′-n-hexylcyclohexyl) isothiocyanatobenzoate (6CHBT) and gold nanoparticles (GNPs). Thermodynamic, electro-optical and dielectric properties have been investigated. Effect of dispersion of GNPs on various electro-optical and display parameters of host liquid crystalline material have been studied. Physical parameters such as threshold voltage, dielectric anisotropy and splay elastic constant have altered for composite systems. Due to the dispersion of GNPs, nematic to isotropic transition temperature is significantly increased. Relaxation frequency corresponding to flip-flop motion of the 6CHBT molecules about their short axes has increased due to the presence of GNPs.

The synthesis and dielectric characterization of liquid crystalline hydrogen bonded complex of 3-(4-(dimethyl amino) phenyl)-1-(4-hydroxyphenyl) prop-2-en-1-one with 8-(4-cyanobiphenyl-4′-oxy) octan-1-ol

A new chalcone based nematic schlieren texture liquid crystalline material was prepared from 3-(4-(dimethyl amino) phenyl)-1-(4-hydroxyphenyl) prop-2-en-1-one (DMAC) and 8-(4-cyanobiphenyl-4′-oxy) octan-1-ol (LC8) by molecular self-assembly processes via hydrogen bond formation between tertiary amine group of DMAC and hydroxyl group of the LC8. Hydrogen bond formation was observed by FTIR spectroscopy. Dielectric properties of DMAC has been studied by impedance spectroscopy. The Cole-Cole analysis was used to determine the Resistance-Capacitance (RC) components equivalent circuit situation of doped LC8 on the DMAC. Dependent angular frequency variation of dielectric parameters and ac conductivity properties are non-Debye type relaxation mechanism of interfacial dopant in the liquid crystal molecular structure of the reorientation polarization effects.

Entropic screening preserves non-equilibrium nature of nematic phase while enthalpic screening destroys it.

The present manuscript describes the role of entropic and enthalpic forces mediated by organic non-polar (hexane) and polar (methanol) solvents on the bulk and microscopic phase transition of a well known nematic liquid crystalline material MBBA (N-(4-methoxybenzylidene)-4-butylaniline) through Differential Scanning calorimetry (DSC), UV-Visible (UV-Vis), and Fourier Transform Infrared (FTIR) spectroscopy. DSC study indicates continuous linear decreases in both nematic-isotropic (N-I) phase transition temperature and enthalpy of MBBA in presence of hexane while both these parameters show a saturation after an initial decay in methanol. These distinct transitional behaviours were explained in terms of the "depletion force" model for entropic screening in hexane and "screening-self-screening" model for methanol. Heating rate dependent DSC studies find that non-Arrhenius behaviour, characteristic of pristine MBBA and a manifestation of non-equilibrium nature [Dan et al., J. Chem. Phys. 143, 094501 (2015)], is preserved in presence of entropic screening in the hexane solution, while it changes to Arrhenius behaviour (signifying equilibrium behaviour) in presence of enthalpic screening in methanol solution. FTIR spectra show similar dependence on the solvent induced screening in the intensities of the imine (-C = N) stretch and the out-of-plane distortion vibrations of the benzene rings of MBBA with hexane and methanol as in DSC, further establishing our entropic and enthalpic screening models. UV-Vis spectra of the electronic transitions in MBBA as a function of temperature also exhibit different dependences of intensities on the solvent induced screening, and an exponential decrease is observed in presence of hexane while methanol completely changes the nature of interaction to follow a linear dependence.

Thermal optical non-linearity of nematic mesophase enhanced by gold nanoparticles--an experimental and numerical investigation.

In this work the mechanisms leading to the enhancement of optical nonlinearity of nematic liquid crystalline material through localized heating by doping the liquid crystals (LCs) with gold nanoparticles (GNPs) are investigated. We present some experimental and theoretical results on the effect of voltage and nanoparticle concentration on the nonlinear response of GNP-LC suspensions. The optical nonlinearity of these systems is characterized by diffraction measurements and the second order nonlinear refractive index, n2, is used to compare systems with different configurations and operating conditions. A theoretical model based on heat diffusion that takes into account the intensity and finite size of the incident beam, the nanoparticle concentration dependent absorbance of GNP doped LC systems and the presence of bounding substrates is developed and validated. We use the model to discuss the possibilities of further enhancing the optical nonlinearity.