Nematic Liquid Crystal Mode Research Articles

Influence of bipyridine isomers and some structurally related compounds on nematic liquid crystal orientation at the aqueous interface

In this paper, orientational anchoring transition of nematic liquid crystal (LC) 4-cyano-4′-pentylbiphenyl (5CB) at the LC-aqueous interface in the presence of bipyridine isomers (2,2′-bipyridine (BPY22) and 4,4′-bipyridine (BPY44)) and some structurally related compounds (1,10-phenanthroline (PANTH110) and aldrithiol-4 (ALD4)) in the aqueous phase is presented. The addition of BPY22, BPY44 and PANTH110 into the aqueous phase causes 5CB to undergo concentration dependent planar-to-homeotropic transition at the LC-aqueous interface. In contrast, ALD4 on addition into the aqueous phase depending on its concentration diminishes the interface between 5CB and the aqueous phase. Furthermore, the LC ordering transitions are related to pH of the solutions containing BPY22 or BPY44 or PANTH110 as revealed by the cross-polarized optical images and the changes in the tilt angle. Density functional theory (DFT) has been used to evaluate stabilization energy and hydrogen bonding strength for understanding the mechanism of LC ordering at the aqueous interface. Our findings provide a new set of compounds that cause homeotropic alignment of 5CB at the LC-aqueous interface, which is a prerequisite for the development LC based biosensors.

Hydrogen bond assisted anchoring transitions in nematic liquid crystals at the aqueous interface

Homeotropic alignment of liquid crystals (LCs) at the aqueous interface and their fundamental understanding is still limited known, which is a prerequisite for the development of new kinds of LC-based sensors for the detection of biomolecules in the aqueous phase. Herein, we have investigated the orientational ordering of nematic LC at LC-aqueous interface in contact with the different hydrogen bonding strength of para-substituted phenol derivatives (X; functional group attached to phenol at para position = -NO2, -CN, -Cl, -F, -CHO, -H) intending to understand how the intermolecular hydrogen bonding interactions at LC-aqueous interface amplified into supramolecular ordering in LCs. The hydrogen bonding interactions between -CN group of 5CB and -OH group of phenol derivatives lead to a change in tilt angle from planar to homeotropic ordering of 5CB molecules at LC-aqueous interface. Raman measurement of different mixtures between 5CB and para-substituted phenol derivatives have been recorded to correlate the experimental observations at the LC-aqueous interface as well as direct evidence of hydrogen bonding at the LC-aqueous interface. Density functional theory (DFT) has been used to calculate hydrogen bonding strength and charge transfer mechanism between 5CB and phenol derivatives. The orientation of LC at the aqueous interface depends on the strength of the interfacial hydrogen bonding between phenol derivatives and LC, which varies on substitution of phenol and concentration of the phenol molecules.

Defects induced by anchoring transitions of nematic fluids at solid and gas interfaces

Temperature triggered anchoring transitions in nematic liquid crystals have been investigated at solid, as well as at air interfaces. We established a searching algorithm based on sessile droplets allowing an efficient discovery of compounds that show orientational transitions. We have identified two types of the transition, one on the coated substrate and another at the free surface bounded by air. As a consequence of the two types of anchoring transitions, we revealed two distinct textures involving topological defects in spherical caps of nematics governed by the reorganisation of the director field. In order to understand this surface induced restructuring of the director field, we performed optical simulations of the sessile droplets. Apart from the known discontinuous interfacial orientational transition we have also identified continuous director restructuring. Moreover we have also detected a double transition, involving a re-entrant behaviour.

Real-time liquid crystal-based creatinine sensor using a micro-patterned flexible substrate

ABSTRACT We propose a novel, simple, cost-effective sensor based on orientational transitions of nematic liquid crystals (LC) for real-time, quantitative, and sensitive detection of creatinine. A micro-well array structure on a polydimethylsiloxane (PDMS) substrate was fabricated to replace traditional copper grids stabilising a thin LC film, which contains 4-cyano-4ʹ-pentylbiphenyl (5CB) LC material and 4ʹ-hexylbiphenyl-4-carboxylic acid (HBA). The principle of the sensor is based on the ordering transition of LCs induced by enzymatic reactions of creatinine and creatinine deaminase (CD), resulting in an optical signal change. The orientational response of LCs was quantitatively monitored in the presence of different amounts of creatinine by cross-polarised optical microscopy and light transmittance measurement system in real time. Concentrations from 50 to 1100 μM of creatinine were successfully detected.

Electric Freedericksz transition in nematic liquid crystals with graphene quantum dot mixture

A theoretical model to explain the Freedericksz transition for a nematic liquid crystal with graphene quantum dots adding in electric field below the instability area is proposed. A mathematical formula for the critical voltage was obtained by elastic continuum theory combined with Burilov - Zakhlevnykh anchoring model, classical electrostatic principles and temperature dependence of elastic constant. We compared the results with the experimental data recorded for three LC samples and a good agreement between them was found encouraging us to propose this model as a tool for theoretical analysis of graphene quantum dot's dispersions in liquid crystals.

Curvature-induced structural transition of nematic liquid crystals in coaxial cylinder [formula omitted] cell: From bulk to surface order reconstruction

The structural transition of asymmetric π cell under a certain voltage was investigated to understand the display mode and performance of flexible substrates. Nematic liquid crystals show a splay texture at ground state (off state) and bend texture at on state induced by electric field in planar π and coaxial cylinder π cells. Changes in the topological discontinuous transformation of liquid crystal directors can appear between on and off states, and the symmetric splay state can be directly transited to the bend state by order reconstruction under high voltage. This phenomenon cannot be explained by Frank theory. Thus, we used Landau–de Gennes theory to study the effects of curvature on order reconstruction and the splay–bend transition through the detailed process of time dynamics. Results show that bulk-order reconstruction appears in planar π cell, whereas surface-order reconstruction appears in coaxial cylinder π cell because of the existence of a finite curvature radius. Moreover, the results will not change when we exchange the inner and outer boundary conditions under coaxial cylinder π cell. By changing the setting of pretilt angle and comparing the results in planar π and coaxial cylinder π cells, we found that the position of order reconstruction is close to the boundary when the curvature is large.

Investigation of Phase Transitions in Nematic Liquid Crystals by Fractional Calculation

In this study, we investigate nematic-isotropic phase transitions in liquid crystals using fractionally generalized form of the Maier-Saupe Theory (MST). MST is one of the mean-field theories commonly used in the nematic liquid crystals which proved to be extremely useful in explaining nematic-isotropic phase transitions. Fractionally obtained results compared with those of the experimental data for p-azoxyanisole (PAA) in the literature. In this context, the dependence of fourth rank order parameters on second rank order parameters is handled by being a measure of fractality of space. It is observed that the variation of second-rank and fourth rank order parameters versus temperature are in accordance with some values of fractal dimensions. As a result, we can conclude that there is a close relationship between temperature and fractional derivative order parameters.

Surface-induced transition of nematic liquid crystals on graphene/SiC substrate

A nematic liquid-crystal director aligns along the armchair direction of graphene grown on a SiC substrate. The temperature-dependent textural change in the nematic phase is different from the usual texture on the alignment layer. The isotropic-nematic phase transition temperature decreases over time after cell fabrication. Tiny domains occur with the phase transition and appear to be merged from a critical temperature with decreasing temperature in the nematic phase. This is thought to be due to the transition occurring at the interface. On the other hand, the nematic liquid crystal in graphene grown on a Cu foil does not show textural changes, and the phase transition temperature does not decrease even after a long time has elapsed. X-ray fluorescence measurements indicate that silicon atoms exist in the liquid crystal possibly extracted from the SiC substrate. A model of first-order phase transition on the graphene surface has been proposed. This transition is accompanied by an inhomogeneous distribution of scalar order parameters and the transformation from a small multi-grained structure to a large domain distribution in the director field. This structural transformation takes place at a temperature different from that corresponding to the bulk transition. Such behavior may be explained by the adsorption process of silicon atoms in contact with the SiC-graphene interface.

Induced orientational molecular ordering in nematic liquid crystals with magnetic nanoparticles

Magneto-induced molecular orientational transitions in nematic liquid crystals doped by ferromagnetic nanoparticles have studied. On the basis of a microscopic approach of tensor operators it is sequentially considered the influence of the ferromagnetic dopes on the sensitivity of the orientational structure and the optical activity of the nematic liquid crystals.

The influence of surface effects on Frederiks transition in nematic liquid crystal doped with ferroelectric nanoparticles

Motivated by our recent work, in this work, we present the numerical study of the anchoring effect on the Frederiks threshold field in a nematic liquid crystal doped with ferroelectric colloidal nanoparticles. Assuming weak anchoring conditions, we employ the relaxation method and Maxwell construction to numerically solve the Euler–Lagrangian differential equation for the total free energy together the Rapini–Papoular surface energy to take into account anchoring of nematic liquid crystal molecules at the substrates. In this study, we focus our attention on obtaining the phase diagrams of Frederiks transition for different values of anchoring strength which have been not computed in our previous work. In this way, the effect of nanoparticle radius, nanoparticle volume fraction, nanoparticle polarization, and cell thickness on the Frederiks transition for different values of anchoring conditions are summarized in the phase diagrams. The numerical results show that by increasing the nanoparticles size and nanoparticle volume fraction in the ferronematic system, the Frederiks threshold field is strongly reduced.

Magneto-induced orientation transitions in nematic liquid crystals

ABSTRACTIn tensor operator representation, the intermolecular potential and the total energy functional of nematic liquid crystals with elastic deformation and magnetic impurities has studied. The impact of the external field on the orientation molecular structure is considered as the variational problem of the energy functional minimum taking into account indirect interaction via magnetic impurities between the external field and anisotropy molecules. The threshold fields of onset of the collective reorientation of anisotropy molecules are determined by the deviation of the energy from the mentioned minimum corresponding to an initial equilibrium state of the nematic liquid crystal.

Continuous Viewing Angle Distribution Control of Liquid Crystal Displays Using Polarization-Dependent Prism Array Film Stacked on Directional Backlight Unit

We present a polarization-dependent prism array film for controlling the viewing angle distribution of liquid crystal (LC) display panels without loss of light efficiency. On a directional backlight unit, our polarization-dependent prism array film, made into a stacked bilayer with a well-aligned liquid crystalline reactive mesogen (RM) layer on the UV-imprinted prism structure, can continuously control the light refraction function of the prism array by electrically switching incident polarization states of a polarization-controlling layer prepared by a twisted nematic LC mode. The viewing angle control properties of the polarization-dependent prism array film are analyzed under different prism angle and refractive index conditions of the RM layer. A simple analytic model is also presented to describe the intermediate viewing angle distributions with continuously varying applied voltages and incident polarization states.

Pattern-induced anchoring transitions in nematic liquid crystals

In this paper we revisit the problem of a nematic liquid crystal in contact with patterned substrates. The substrate is modelled as a periodic array of parallel infinite grooves of well-defined cross-section sculpted on a chemically homogeneous substrate which favours local homeotropic anchoring of the nematic. We consider three cases: a sawtooth, a crenellated and a sinusoidal substrate. We analyse this problem within the modified Frank–Oseen formalism. We argue that, for substrate periodicities much larger than the extrapolation length, the existence of different nematic textures with distinct far-field orientations, as well as the anchoring transitions between them, are associated with the presence of topological defects either on or close to the substrate. For the sawtooth and sinusoidal cases, we observe a homeotropic to planar anchoring transition as the substrate roughness increases. On the other hand, a homeotropic to oblique anchoring transition is observed for crenellated substrates. In this case, the anchoring phase diagram shows a complex dependence on the substrate roughness and substrate anchoring strength.

Dynamics of magnetic (electric) field induced perturbation in homeotropic smectic A structure in liquid crystal cell with substrates having periodic microrelief.

The dynamics of a magnetic (electric) field induced perturbation in a homeotropic smectic A (SmA) structure in liquid crystal cells with substrates having periodic microrelief has been investigated. It has been shown that relaxation times of such perturbation in both switching-on and switching-off regimes are of the order of 10-5-10-4s. The results obtained show that previously proposed novel electro (magneto) optic effect in such SmA cell can be utilized in manufacturing of liquid crystal displays with operating times much shorter than those of ordinary modern displays based on the Freedericksz transitions in nematic liquid crystals.

A 3D Optical Surface Profilometer Using a Dual-Frequency Liquid Crystal-Based Dynamic Fringe Pattern Generator.

We propose a liquid crystal (LC)-based 3D optical surface profilometer that can utilize multiple fringe patterns to extract an enhanced 3D surface depth profile. To avoid the optical phase ambiguity and enhance the 3D depth extraction, 16 interference patterns were generated by the LC-based dynamic fringe pattern generator (DFPG) using four-step phase shifting and four-step spatial frequency varying schemes. The DFPG had one common slit with an electrically controllable birefringence (ECB) LC mode and four switching slits with a twisted nematic LC mode. The spatial frequency of the projected fringe pattern could be controlled by selecting one of the switching slits. In addition, moving fringe patterns were obtainable by applying voltages to the ECB LC layer, which varied the phase difference between the common and the selected switching slits. Notably, the DFPG switching time required to project 16 fringe patterns was minimized by utilizing the dual-frequency modulation of the driving waveform to switch the LC layers. We calculated the phase modulation of the DFPG and reconstructed the depth profile of 3D objects using a discrete Fourier transform method and geometric optical parameters.

Lattice model for biaxial and uniaxial nematic liquid crystals.

We use a lattice gas model to describe the phase transitions in nematic liquid crystals. The phase diagram displays, in addition to the isotropic phase, the two uniaxial nematics, the rod-like and discotic nematics, and the biaxial nematic. Each site of the lattice has a constituent unit that takes only six orientations and is understood as being a parallelepiped brick with the three axes distinct. The possible orientations of a brick are those in which its axes are parallel to the axes of a Cartesian reference frame. The analysis of the model is performed by the use of a mean-field approximation and a Landau expansion of the free energy.

12.4: A Liquid Crystal Biosensor for Liver Diseases

The urinary concentration level of bile acids is a useful indicator for the diagnosis of liver diseases. Here we present a sensor platform based on the anchoring transition of nematic liquid crystals (LCs) at the surfactant‐laden LC/aqueous interfaces for the detection of bile acids in urinary solution.

Phase diagrams of orientational transitions in absorbing nematic liquid crystals

A theory of orientational transitions in nematic liquid crystals (NLCs), which employs the expansion of optical torques acting on the NLC director with respect to the rotation angle, has been developed for NLCs with additives of conformationally active compounds under the action of optical and low-frequency electric and magnetic fields. Phase diagrams of NLCs are constructed as a function of the intensity and polarization of the light field, the strength of low-frequency electric field, and a parameter that characterizes the feedback between the rotation of the NLC director and optical torque. Conditions for the occurrence of first- and second-order transitions are determined. The proposed theory agrees with available experimental data.

Improved Vertically-Aligned Nematic Mode for High Performance Displays

This paper presents an improved vertical alignment nematic liquid crystal mode characterized by the protrusions or slits of the top substrate and additional stripe type common electrodes with polarity switching of the bottom substrate to improve multi-domain vertically aligned (MVA) and patterned vertically aligned (PVA) nematic modes. MVA and PVA modes have disadvantages such as an LC disclination in the vicinity of the middle region of electrodes between the top and bottom protrusions in MVA mode or the top and bottom slits in PVA mode. Therefore, the stripe type common electrode generating a horizontal electric field and the protrusion or slit producing some pretilt of liquid crystals (LCs) were used to improve the LC disclination, which influences the transmittance and response speed. The simulation results showed that the proposed VA mode has higher transmittance than the MVA and PVA modes. As a result, the proposed VA mode can improve the response speed and transmittance remarkably, which makes it useful for upgrading the LCD display quality.

Electrical Freedericksz transitions in nematic liquid crystals containing ferroelectric nanoparticles

A new theoretical approach was elaborated to explain the contradictions reported in many papers about the electric threshold for Freedericksz transition in nematic liquid crystal (NLC) with ferroparticles additives. The free energy density of the mixture was estimated and the contributions of the interaction energy of NLC molecules with ferroparticles surface were calculated. Experimental results for 5CB-BaTiO3 mixture are given.