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Alignment of Nematic Liquid Crystals and Their Mixtures
... In view of this, 70 there have been several studies in quest of better approaches for the alignment of rod-and disc-like molecules on ITO. Among these, silanes have attracted a considerable interest for LC alignment from a long ago and a detailed account of different silane compounds 75 (commercially obtained) studied for the alignment of various LCs can be found in the review by Cognard [28]. Apart from these, some of other methods reported for the alignment of LCs are mentioned below. ...
... The sample therefore appears bright between crossed polarisers © ( Figure 5(g)) unlike in the PL Instec cell where this angle may be influenced by the rubbing direction (RD) resulting in a dark field of view when the polariser © is parallel to the RD ( Figure 5(h)). 860 As mentioned earlier, various commercially available alkoxys and chlorosilanes have been investigated in detail for the alignment of nematic LCs of rod-like compounds [28]. A comprehensive detail of the alignment by these © silylated substrates under various treatment conditions 865 (liquid phase, vapour phase, solution phase and curing at different temperatures) can be found in the review by Cognard [28]. ...
... 860 As mentioned earlier, various commercially available alkoxys and chlorosilanes have been investigated in detail for the alignment of nematic LCs of rod-like compounds [28]. A comprehensive detail of the alignment by these © silylated substrates under various treatment conditions 865 (liquid phase, vapour phase, solution phase and curing at different temperatures) can be found in the review by Cognard [28]. The data showed that monodimethyldichlorosilane showed a strong effect compared to other silanes used for the surface treatment. ...
Indium tin oxide (ITO) substrates are chemically modified with bent-shaped molecules and influence of these surface immobilised molecules on the alignment of bulk liquid crystal (LC) samples has been investigated. Bent-core (BC) compounds containing a dimethylchlorosilane moiety at the end of a terminal alkyl chain and a lateral alkyl chain are successfully attached to transparent conducting ITO substrates in an end-on (ITO-BC-t) and side-on (ITO-BC-l) oriented fashion to create stable permanent self-assembled BC layers on the substrates. Surface functionalization is thoroughly characterised and aligning ability of these BC-modified ITO substrates for orientation of bulk LC sample is evaluated for nematic phase of both rod-like and BCLC samples. The BC-modified substrates are effective in aligning the rod-like compounds in vertical (ITO-BC-t), planar orientations (ITO-BC-l) and the BC compounds in homeotropic (ITO-BC-t) alignment.
... CTAB was used as an ion-forming surfactant providing normal (homeotropic) boundary conditions (Cognard, 1982;Proust and Ter Minassian Saraga, 1972) at a certain concentration. When dissolved in LC, this compound dissociates into a negatively charged Brion and a positive CTA + ion. ...
... It is known that the nematic 5CB is aligned tangentially at the surface of PVA even in the presence of glycerin additions (Drzaic, 1995). The CTA + ions, being adsorbed on the interface, can form nanosized layers that specify the homeotropic alignment of the LC molecules at a certain concentration (Cognard, 1982). The samples of the composite films of 30 μm thickness were placed on a glass substrate with electrodes allowing a DC electric field to be applied along the film plane (Figure 7.7). ...
... Polymer films based on PVA and glycerin solved in water were formed by the method of spin coating of the solution with subsequent drying. It is known that such films specify planar boundary conditions for the nematic 5CB (Cognard, J. 1982). The easy orientation axis was formed by the mechanical rubbing of the polymer surface in a required direction. ...
Two conceptually different approaches can be used to operate liquid
crystal (LC) materials. One approach is based on numerous variants of the Frederiks
effect, which enables the LC reorientation caused by external stimuli without
a change of boundary conditions. All modern optoelectronic LC devices function
on the basis of this effect. The other exploits the anchoring transitions through
the modification of the surface anchoring strength with a change in the tilt and
(or) azimuthal anchoring angle(s) under the external influence (temperature, irradiation,
electric field, and so on). This chapter provides an overview of a novel
method to control LC materials by electrically induced anchoring transitions. The
key element entailed in our method is an ionic surfactant dissolved in LC. The
surfactant is adsorbed partially on the LC cell substrate, thus specifying certain
boundary conditions. Its concentration at the interface varied under the action of
DC electric field resulting in the modification of the surface anchoring. Following
this, changing boundary conditions makes the whole bulk of LC reoriented into a
different state. The modification of boundary conditions can be realized in both
the normal and inverse modes, depending on the content of the ionic surfactant
in LC. This ionic-surfactant operation (ISO) method is applicable to both polymerdispersed
LC (PDLC) films and nematic and cholesteric LC layers. Dynamical parameters
of the electro-optical response of various LC structures are considered.
Response times of the ISO optical cells of twisted -nematic layers are decreased to
tens or hundreds of milliseconds at some volts of applied electric voltage. The implementation
of the ISO method into operating PDLC devices allows observations
of the novel bistability effects in cholesteric droplets. The most impressive feature
of the ISO method is the possibility to reorient LC with dielectric anisotropy Δε of
any sign and value, including Δε = 0.
... There are several different causes of the nature of alignment of the LCs, such as the creation of the roughfinished surface during rubbing technique, the creation of the charges at the surface during rubbing [1][2], photoalignment [3][4][5][6][7][8] or ion/plasma techniques [9,10]. However, the main cause of aligning properties of the surface there is the creation of the anisotropic distribution of the roughness [1][2][3], charges [1][2][3]9,10] and cross-linking/destruction of the polymer [3]. ...
... There are several different causes of the nature of alignment of the LCs, such as the creation of the roughfinished surface during rubbing technique, the creation of the charges at the surface during rubbing [1][2], photoalignment [3][4][5][6][7][8] or ion/plasma techniques [9,10]. However, the main cause of aligning properties of the surface there is the creation of the anisotropic distribution of the roughness [1][2][3], charges [1][2][3]9,10] and cross-linking/destruction of the polymer [3]. Each of these methods of the treatment of surface has their own advantages and drawbacks. ...
... There are several different causes of the nature of alignment of the LCs, such as the creation of the roughfinished surface during rubbing technique, the creation of the charges at the surface during rubbing [1][2], photoalignment [3][4][5][6][7][8] or ion/plasma techniques [9,10]. However, the main cause of aligning properties of the surface there is the creation of the anisotropic distribution of the roughness [1][2][3], charges [1][2][3]9,10] and cross-linking/destruction of the polymer [3]. Each of these methods of the treatment of surface has their own advantages and drawbacks. ...
In spite of the fact that there are different techniques in the creation of the high-quality liquid crystals (LCs) alignment by means of various surfaces, the azimuthal and polar anchoring energies as well as the pre-tilt angle are important parameters to all of them. Here, the modified by a certain manner aligning layers, previously formed by nonlinear laser lithography (NLL), having high-quality nano-periodic grooves on Ti surfaces, recently proposed for LC alignment was studied. The change of the scanning speed of NLL in the process of nano-structured Ti surfaces and their further modification by means of ITO-coating, and deposition of polyimide film has enabled different aligning layers, whose main characteristics, namely azimuthal and polar anchoring energies, were measured. For the modified aligning layers, the dependencies of the twist and pre-tilt angles for LC cells filled by nematic E7 (Δε > 0) and MLC-6609 (Δε < 0) were obtained. Also, the contact angle for droplets of isotropic liquid (glycerol), and nematic LCs was measured for the various values of the scanning speed during the laser processing.
... It is obvious that the orientation of the LCs depends on the AE between molecules of LC and aligning layer. Furthermore, any surfaces, including the aligning layers, are also characterised by surface tension γ S [38]. Consequently, some surfaces possess a hydrophobic property while other surfaces are characterized by a hydrophilic feature. ...
... According to empirical Creagh-Kmetz's rule [38,39] the homeotropic alignment of nematic LC is obtained at γ S < γ LC while the planar alignment occurs at γ LC < γ S (γ S is the surface tension of the substrate and γ LC is the surface tension of LC). Recently it was shown that on the one hand, there has been certain exposure dependence of the contact angle of the nematic droplet E7 at the photosensitive PVCN-F surface [40] and on the other hand, the irradiation time dependence of the AAE W φ of PVCN-F surfaces was found [36]. ...
... The measurement of twist angle φ of the combined LC cell allows us to calculate the value AAE W φ of the aligning layer of tested substrate. The twist angle φ is related to the AAE W φ as follows [36][37][38]: ...
In spite of the fact that there are different techniques in the creation of the high-quality liquid crystals (LCs) alignment by means of various surfaces, the azimuthal and polar anchoring energies as well as the pre-tilt angle are important parameters to all of them. Here, the modified by a certain manner aligning layers, previously formed by nonlinear laser lithography (NLL), having high-quality nano-periodic grooves on Ti surfaces, recently proposed for LC alignment was studied. The change of the scanning speed of NLL in the process of nano-structured Ti surfaces and their further modification by means of ITO-coating, and deposition of polyimide film has enabled different aligning layers, whose main characteristics, namely azimuthal and polar anchoring energies, were measured. For the modified aligning layers, the dependencies of the twist and pre-tilt angles for LC cells filled by nematic E7 ({\Delta}{\epsilon} > 0) and MLC-6609 ({\Delta}{\epsilon} < 0) were obtained. Also the contact angle for droplets of isotropic liquid (glycerol), and nematic LCs was measured for the various values of the scanning speed during the laser processing.
... to align rod-like calamitic LCs [28]. The challenge, however, is to align LCs with molecules of complex shapes, such as bent-core and flexible dimer molecules, for which alignment is not always trivial [35,57]. ...
... A typical LC cell is comprised of two sandwiched glass plates separated by spacers. Prior the assembly, the inner side of each glass plate is treated with an aligning agent, such as a thin polymer layer, to induce the desired LC director orientation [28,41]. ...
... For example, in the absence of the alkyl branches, the pretilt is very small: α ∼ 2 • , whereas low and high density of alkyl chains results in an increase of the tilt angle (~5°-20°) [108]. Control of the tilt angle is crucial in applications in which there is a pre-requisite of molecular realignment along a single predetermined direction [28]. In order to achieve relatively high pretilt (~30°), generally, an oblique evaporation of silicon oxides (SiO x ) is used [59,93]. ...
Alignment of liquid crystal (LC) molecules is an important topic of physics of anisotropic fluids. The boundary conditions and the surface properties of the material in contact with an LC dictate the preferred orientation of the molecules at the interface.
... The creation, study and characterization of the alignment surface, obtained by means of different methods, are important tasks to the LC practical application. For this purpose, both the different aligning materials and various methods of their processing can be used, as demonstrated in many references in book [1] and reviews [2][3][4][5]. ...
... The first method, used extensively for different applications in industry, is the rubbing technique with various materials [1][2][3][4][5][6][7][8][9] of different surfaces [1,10]. However, in spite of the fact that the rubbing technology is widely used in LCD technology, this technique has some shortcomings, among which are accumulation of both the static charges and dust particles [4]. ...
... The first method, used extensively for different applications in industry, is the rubbing technique with various materials [1][2][3][4][5][6][7][8][9] of different surfaces [1,10]. However, in spite of the fact that the rubbing technology is widely used in LCD technology, this technique has some shortcomings, among which are accumulation of both the static charges and dust particles [4]. ...
It is well known that today two main and well studied methods for alignment of liquid crystals has been used, namely: rubbing and photoalignment technologies, that lead to the change of anisotropic properties of aligning layers and long-range interaction of the liquid crystal molecules in a mesophase. In this manuscript, we propose the usage of the nonlinear laser lithography technique, which was recently presented as a fast, relatively low-cost method for a large area micro and nanogrooves fabrication based on laser-induced periodic surface structuring, as a new perspective method of the alignment of nematic liquid crystals. 920 nm periodic grooves were formed on a Ti layer processed by means of the nonlinear laser lithography and studied as an aligning layer. Aligning properties of the periodic structures of Ti layers were examined by using a combined twist LC cell. In addition, the layer of the nanostructured Ti was coated with an oxidianiline-polyimide film with annealing of the polymer film followed without any further processing. The dependence of the twist angle of LC cells on a scanning speed and power of laser beam during processing of the Ti layer was studied. The azimuthal anchoring energy of Ti layers with a periodic nanostructure was calculated. The maximum azimuthal anchoring energy for the nanostructured Ti layer was about 4.6 × 10⁻⁶ J/m², which is comparable to the photoalignment technology. It was found that after the deposition of a polyimide film on the periodic nanostructured Ti layer, the gain effect of the azimuthal anchoring energy to ~1 × 10⁻⁴ J/m² is observed. Also, AFM study of aligning surfaces was carried out.
... The alignment of liquid crystals (LCs) is important and a key condition for their application as a liquid with anisotropic properties. For this purpose, both different aligning materials and various methods of their processing can be used, as demonstrated, for instance, in the book [1] and many reviews [2][3][4][5]. However, in present two main methods can be emphasised, which are used for the creation of the aligning layers and were well studied for further application in display technology, etc. ...
... However, in present two main methods can be emphasised, which are used for the creation of the aligning layers and were well studied for further application in display technology, etc. The first method, used extensively for different applications in industry, is the rubbing technique with various materials [1,[6][7][8][9] of different surfaces [1,10]. Despite the fact that the rubbing technology is widely used in liquid crystal display (LCD) technology, however this technique has some shortcomings, among which are accumulation of both the static charges and dust particles [4]. ...
... However, in present two main methods can be emphasised, which are used for the creation of the aligning layers and were well studied for further application in display technology, etc. The first method, used extensively for different applications in industry, is the rubbing technique with various materials [1,[6][7][8][9] of different surfaces [1,10]. Despite the fact that the rubbing technology is widely used in liquid crystal display (LCD) technology, however this technique has some shortcomings, among which are accumulation of both the static charges and dust particles [4]. ...
It is well known that the alignment of liquid crystals (LCs) can be realised by rubbing or photoalignment technologies. Recently, nonlinear laser lithography (NLL) was introduced as a fast, relatively low-cost method for large area nano-grating fabrication based on laser-induced periodic surface structuring. In this letter for the first time, the usage of the NLL as a perspective method of the alignment of nematics was presented. By NLL, nanogrooves with about 0.92 μm period were formed on Ti layer. The nanostructured Ti layer (NSTL) was coated with oxidianiline-polyimide film with annealing of the polymer followed without any further processing. Aligning properties of NSTLs were examined with combined twist LC cell. The dependencies of the twist angle of LC cells and azimuthal anchoring energy (AE) of layers on scanning speed and power of laser beam during processing of the Ti layer were the focus of our studies as well. The maximum azimuthal AE, obtained for pure NSTL, is comparable with photoalignment technology. It was found that the deposition of polyimide film on NSTL leads to the gain effect of the azimuthal AE. Also, atomic force microscopy (AFM) study of aligning surfaces was carried out.
... Optical properties of LC materials depend mainly on the orientational ordering of LC molecules, which, in turn, is specified by the anchoring of its molecules at the interface. Surface anchoring of LC can be controlled by the specially chosen and treated orienting layers of specific chemical composition [4,[7][8][9]. For example, in flat capillaries the using of thin polymer film as orienting layer allows obtaining the homogeneous or structured interface and the appropriate anchoring by the rubbing or photoalignment method. ...
... The homeotropic boundary conditions both in the conventional LC cells and in another more complex LC systems such as droplets dispersions, colloids, capillaries, etc. can be realized using some surfactants. In this case the ordered long alkyl chains of surfactant molecules orient the LC molecules [7,10,11]. The parameters of orienting layer and, consequently, the surface anchoring can be varied by changing the concentration of surfactant dissolved in LC [12] or in the ambient solution [13], pH of the ambient medium [14,15], temperature [16] or upon UV irradiation [17,18]. ...
In search of a substance able to function as an anionic surfactant applied for the electrically-induced anchoring transitions in liquid crystals, the compound 4-heptyloxybenzoate benzyl dodecyldimethylammonium (HOBBDDA) has been synthesized. Its orienting influence on nematic liquid crystal 4-pentyl-4′-cyanobiphenyl (5CB) has been tested. HOBBDDA dissolved up to 1.4% in 5CB droplets dispersed in polymer film does not change the tangential surface anchoring inherent to polyvinyl alcohol matrix used. Homeotropic surface anchoring is realized if the HOBBDDA content exceeds 1.7%. The molecules of the surfactant in 5CB dissociate into anions and cations. The anions modify the surface anchoring under the action of DC electric field both in the normal and inverse mode. The mixture of HOBBDDA and cetyltrimethylammonium bromide can function as a binary ionic-cationic surfactant which significantly expands the prospects for using the ionic-surfactant method to control liquid crystal materials.
... Two cells with a layer thickness of 35 and 45 μm were manufactured for measurements. To create the planar orientation for the director of a liquid crystal, the glass substrates precoated with a polymeric (polyvinyl alcohol) layer were subjected to parallel mechanical rubbing by the method of [12]. The director pretilt angle was estimated by measuring the interference of polarized beams by the method of [13]. ...
... Equation (1) provides the possibility to find the Frank constant K 1 from known Freedericksz fields for the two studied cells. Setting χ a = 12.6 × 10 -7 according to [1] and averaging the results of two measurements, we find that K 1 = 4.3 × 10 -12 N (at a temperature of 25°C), which is close to the values given in [1,12]. A certain difference between the values of K 1 is known to be due to the degree of purity of a used substance and the specific features of a cell, in which measurements are performed (small nonparallelism of the plates and pretilt of the director on them). ...
The magnetic Freedericksz transition in a ferronematic representing a diluted suspension of magnetic nanoparticles in an N-(4-methoxybenzylidene)-4-butylaniline (MBBA) nematic liquid crystal was experimentally studied. The transition point was determined by means of dielectric capacitance measurements in cells of different thickness for samples with various volume fractions of ferroparticles. The effect of weak coupling between the magnetic and liquid-crystal subsystems was studied. Low concentrations of quasispherical magnetic particles were shown to decrease the magnetic Freedericksz transition threshold in comparison with a pure liquid crystal. The obtained results were theoretically substantiated. The phase diagram of the suspension was plotted, and the method of estimating the energy of coupling between magnetic particles and a liquid-crystal matrix was proposed.
... The LC cell with the POSS A film also becomes homeotropically aligned after the melting treatment as shown in Figure 6a. The semi-empirical rule called Friedel-Creagh-Kmetz (FCK) rule can be used to partially explain the mechanism of POSS-induced homeotropic alignment [22]. The FCK rule states that γ S < γ LC -homeotropic alignment γ S > γ LC -homogenous alignment where γ S describes the surface tension of the solid alignment film, and γ LC is the surface tension of the LC. ...
... The LC molecules prefer to align perpendicularly to the solid film when the surface energy of the film is relatively low and the inter-molecular interaction of LC molecules is stronger than the interaction across the interface. The FCK rule has been widely supported by experimental data [22,23]. The contact angles and the calculated surface energy of the various POSS films, ITO glass substrates, and the commercial homeotropic (AL60101, JSR Corp. Tokyo, Japan) and homogeneous PI (AL-58, Daily Polymer Corp. Kaohsiung, Taiwan) films are summarized in Table 3. ...
Polyhedral oligomeric silsesquioxanes (POSSs) with nano-size cage structures have been conventionally incorporated into polymers to improve the polymers’ physical properties. In this work, POSS films formed by using POSS nanomaterials with different thermal treatments have been implemented as liquid crystal (LC) alignment films instead of using conventional polyimide alignment films adopted in the LC displays industry. The homeotropic alignment of LCs anchored on POSS films was observed. The morphology and surface energy of POSS films were measured to study their effects on LC orientation anchored on the POSS films.
... There are certain surface modifications which are used to guide LC molecules to orient vertically or planar using techniques like rubbed polyimide [37,38], oblique evaporation of silicon oxides [39], deposition of surfactants [40], shearing technique [41], etc. The electromagnetic field has also been utilized to align LC molecules via photoalignment [42][43][44][45], UV irradiation [46], and polarized laser light [47]. ...
Nematic Liquid Crystals (NLCs), long-range orientationally ordered fluids, have always been the best choice for the development of various display and non-display devices due to their simplest phase structure, good thermal stability, and easier alignment. The extreme sensitivity of NLCs to the external stimuli such as electric, magnetic, light, and surface forces, etc. has been the key feature for their broad range of applications. For tunable electro-optical device fabrication, the response of NLC molecules to the external stimuli must be coherent. Such coherent stimuli response of NLC molecules could be achieved by using various alignment techniques. For instance, rubbing of polyimides, coating of surfactants, magnetic field, oblique evaporation of SiOx, photoalignment, etc. are being utilized for the uniform and stable alignment of NLCs. Besides these alignment techniques, the use of nanoparticles to align NLCs has also recently been considered as one of the novel techniques to align NLCs without using any alignment layer. In this chapter, we have greatly reviewed the recent progress made in the field of nanoparticle-controlled alignment of NLCs useful for the tunable electro-optical devices and discussed the future perspectives.
... was used as a planar aligning material [28] (Figure 3(a,c)). ...
... surfactants, [43] electric and magnetic field, [44,45] polarized laser light, [46] shearing technique [47] etc. Usually, in the laboratory and industry, the rubbing technique (i.e., rubbed polyimide) is mainly utilized to align LCs in the fabrication of liquid crystal displays (LCDs) and other electro-optical devices. However, this method of alignment requires a lot of processing time, costly, prone to create defects in the surfaces, static charges, etc. ...
Since their inception, liquid crystals (LCs) have been a topic of great interest among researchers around the globe. The alignment of LC molecules remains pivotal to explore the basic and applied aspects of LCs. Various alignment techniques such as rubbing of polyimides, coating of surfactants, magnetic field, oblique evaporation of SiOx, photoalignment, etc. have been widely explored to obtain the uniform alignment of LCs, eventually required in the fabrication of LC based displays and other photonic devices. One has to judiciously select the alignment technique to produce LC displays at industrial scale. However, certain issues have always remained which further stimulated the researchers to explore new sustainable ways of aligning LCs. Under this framework, the nanoparticles-controlled alignment could be one of new methods to align LCs. In this review, we have focused on the nanoparticles (isotropic and anisotropic) controlled alignment of LCs. The alignment of LCs could be achieved by: (i) doping of nanoparticles in the bulk LCs and (ii) patterning or growth of nanostructures on the substrates. Interestingly, the nanostructures grown on the non-conducting substrate are found to work as LC aligning agent and transparent conducting electrode. The nanoparticles doped polyimide alignment layers are also found to significantly improve the alignment of LCs. The quality of LC alignment obtained by using nanoparticles is more or less same as in conventional alignment techniques but less time consuming and cost-effective. Besides the induced or improved alignment of LCs due to nanoparticles, the electro-optical properties of LC devices are also found to be greatly improved as compared to devices using conventional alignment techniques. Moreover, we have discussed the pros and cons and future perspectives of nanoparticles tuned alignment of LCs.
... surfactants, [43] electric and magnetic field, [44,45] polarized laser light, [46] shearing technique [47] etc. Usually, in the laboratory and industry, the rubbing technique (i.e., rubbed polyimide) is mainly utilized to align LCs in the fabrication of liquid crystal displays (LCDs) and other electro-optical devices. However, this method of alignment requires a lot of processing time, costly, prone to create defects in the surfaces, static charges, etc. ...
... As well, polarization and wavelength-dependent light scattering was studied and found 45% of light scattered, when unpolarized white light was incident on the anisotropic gels. In the coming years, Ma et al. introduced another approach using surfactant treated polymer to control surface energy to achieve the VA of LC molecules because the LC molecules exhibit VA on low energy surfaces and homogeneous alignment on high energy surfaces as per the semiempirical Friedel-Creagh-Kmetz (FCK) rule [81,82]. Thus, authors reported that micro-droplets of LCs with negative dielectric anisotropy encased in the treated polymer and during LC droplet formation surfactant must induce the VA of LCs and resulted in 92% of maximum transmission. ...
With the maturation of display devices using liquid crystal (LC) materials, likely, in normally transparent reverse mode polymer-dispersed liquid crystals (PDLCs) devices’ performance and development have been assessed in the present review. The fabrication techniques and various normally transparent reverse mode devices such as polymer-dispersed liquid crystal (PDLC), polymer network liquid crystal, polymer-stabilized liquid crystal, anisotropic LC gels and nematic emulsions are discussed. Further, the simultaneous effects of external stimuli such as external fields, optical effects, temperature and composition of constituents of PDLC mixture are reported for analysing the technological advancement of normally transparent reverse mode LC devices. The performance parameters discussed in this review are texture formation, transmission, response time, contrast ratio, viewing angle, threshold and operating voltages. The interdependency of these parameters on overall performance of the device is also discussed and analysed. Thus, new areas of application in the realms of 2D/3D and multi-plane display devices, normally transparent shutters/smart windows, augmented/virtual reality devices are interesting technological advancements in basic and applied research using these novel anisotropic LC and polymeric materials.
... Silane-coated surfaces, known to generally induce homeotropic ordering of thermotropic nematics, 8 favour instead the planar alignment of RM734, which can be oriented in any direction by gentle surface rubbing. Fig. 2 shows PTOM pictures demonstrating that the N phase exhibits a good homogeneous planar alignment in the direction of rubbing, both right after cell filling (Fig. 2a) and when the N phase is obtained on heating from the N F phase (Fig. 2b). ...
The success of nematic liquid crystals in displays and optical applications is due to the combination of their optical uniaxiality, fluidity, elasticity, responsiveness to electric fields and controllable coupling of the molecular orientation at the interface with solid surfaces. The discovery of a polar nematic phase opens new possibilities for liquid crystal-based applications, but also requires a new study of how this phase couples with surfaces. Here we explore the surface alignment of the ferroelectric nematic phase by testing different rubbed and unrubbed substrates that differ in coupling strength and anchoring orientation and find a variety of behaviors - in terms of nematic orientation, topological defects and electric field response - that are specific to the ferroelectric nematic phase and can be understood as a consequence of the polar symmetry breaking. In particular, we show that by using rubbed polymer surfaces it is easy to produce cells with a planar polar preferential alignment and that cell electrostatics (e.g. grounding the electrodes) has a remarkable effect on the overall homogeneity of the ferroelectric ordering.
... However, such glass substrates, with or without ITO, do not promote any uniform alignment on the liquid crystal in contact with the substrate surface. Usually, it's promoting random planar or tilted alignment, and, at certain wetting conditions, the substrate surface may result in a vertical (homeotropic) alignment, as well [1]. The substrate contact surface may adopt anisotropic physical properties either by direct treatment of the glass substrate surface, such as unidirectionally mechanical rubbing [2][3][4], by oblique deposition of inorganic films, such as SiOx [5], or by organic thin layers deposited onto the substrate surface and properly treated afterwords. ...
To improve the characteristics and quality of the photoalignment of liquid crystals has been a main goal of many relevant studies. We demonstrate in this work, following the general concept of composite materials, that the photoalignment of liquid crystal can substantially be improved when using alignment layers made from photoalignment composite materials. These composites contain at least one photoalignment material. The other components of the photoalignment composite material may or may be not photosensitive, as conventional polyimide materials, for instance.
We show that by a proper choice of the components, the properties of the photoalignment materials, made by photoalignment composites, can be tailored in a broad range, resulting in higher sensitivity, and thus shorter exposure time, uniform and smooth photoalignment topography (without domains and sandy structures) resulting in high image contrast. High-quality photo-patterning of the liquid crystal alignment is demonstrated in this work with a photoalignment composite material containing one of our photoalignment materials and a conventional polyamide alignment material.
... LCs in general are strongly influenced by the boundary conditions of the substrates, confinement and anchoring as a result of the interaction between the LC molecules and the bounding surfaces [39]. A variety of polymer layers, spin coated and rubbed are employed to promote alignment of thermotropic LC phases [39,40]. However, for the systems investigated here, none of these procedures have provided any particular alignment as commonly observed for thermotropic LCs. ...
Graphene oxide (GO) forms well ordered liquid crystal (LC) phases in polar solvents. Here, we map the lyotropic phase diagram of GO as a function of the lateral dimensions of the GO flakes, their concentration, geometrical confinement configuration and solvent polarity. GO flakes were prepared in water and transferred into other polar solvents. Polarising optical microscopy (POM) was used to determine the phase evolution through the isotropic-biphasic-nematic transitions of the GO LC. We report that the confinement volume and geometry relative to the particle size is critical for the observation of the lyotropic phase, specifically, this determines the low-end concentration limit for the detection of the GO LC. Additionally, a solvent with higher polarisability stabilises the LC phases at lower concentrations and smaller flake sizes. GO LCs have been proposed for a range of applications from display technologies to conductive fibres, and the behaviour of LC phase formation under confinement imposes a limit on miniaturisation of the dimensions of such GO LC systems which could significantly impact on their potential applications.
... Homeotropic alignment is produced even by the polyimide PI2555, which is widely used for planar orientation of the conventional nematics such as 5CB. Homeotropic alignment by clean glass can be related to the hydrophilic nature of the substrate; many thermotropic nematics align similarly on clean glass [35]. The polyimide SE-1211 shows multiple regimes of alignment. ...
Liquid crystals formed by acute-angle bent core (ABC) molecules with a 1,7 naphthalene central core show an intriguing phase behavior with the nematic phase accompanied by poorly understood additional phases. In this work, we characterize the physical properties of an ABC material, such as birefringence, dielectric permittivities, elastic constants, and surface alignment and present x-ray diffraction and transmission electron microscopy studies of their ordering. The ABC molecular shape resembling the letter λ yields a very small splay elastic constant in the uniaxial nematic phase and results in the formation of a tetragonal positionally ordered columnar phase consisting of alternating polar and apolar molecular columns with a uniform uniaxial director that can be bent but not splayed.
... All but one produce homeotropic orientation of 1Cl-N(1,7)-O6, with the director perpendicular to the bounding plates, is produced even by the polyimide PI2555, which is widely used for planar orientation of the conventional nematics such as 5CB. Homeotropic alignment by clean glass can be related to the hydrophilic nature of the substrate; many thermotropic nematics align similarly on clean glass[34]. The polyimid SE-1211 shows multiple regimes of alignment. ...
Liquid crystals formed by acute-angle bent-core (ABC) molecules with a 1,7 naphthalene central core show an intriguing phase behavior with the nematic phase accompanied by poorly understood additional phases. In this work, we characterize the physical properties of an ABC material, such as birefringence, dielectric permittivities, elastic constants, and surface alignment and present X-ray diffraction and transmission electron microscopy studies of their ordering. The ABC molecular shape resembling the letter $\lambda$ yields a very small splay elastic constant in the uniaxial nematic phase and results in the formation of a tetragonal positionally ordered columnar phase consisting of molecular columns with a uniform uniaxial director that can be bent but not splayed.
... Alignment of liquid crystals (LCs) has long been attracted significant focus since many interesting applications of LCs in various areas, such as the fabrication of LC display devices [1,2], chemical and biological sensing [3,4], organic thin-film transistors [5] etc., demand a perfect alignment of LCs. Different kinds of surface modifications, namely, rubbed or photo-aligned polymer films [6], Langmuir blodgett (LB) films [7][8][9], self-assembled monolayers (SAMs) [10,11] have been investigated in detail for the better LC alignment. ...
An azobenzene derivative containing a dimethylchlorosilane end group was chemically attached to a flexible polymer substrate to create permanent photo alignment layer. The commercially obtained polymer substrate (overhead projector film) was characterised and found to contain cellulose acetate as the main component. Surface functionalization of the substrate was confirmed through X-ray photoelectron spectroscopy (XPS), attenuated total reflectance fourier transform infrared (ATR-FTIR) spectroscopy, water contact angle (CA) and atomic force microscopy (AFM) techniques. Photo-isomerisation behaviour of the azo-layer on the polymer substrate was additionally confirmed by UV-diffuse reflection spectroscopy (UV-DRS). Further, the azo-modified flexible substrate was analysed for the orientation of the bulk liquid crystal (LC) sample and tuning the LC orientation by utilising photo-switching behaviour of the azo-layer on the substrate.
... Although, individual molecules may be polar, the nett anti parallel ordering in the system ensures that macroscopically the nematic phase is non-polar. Nanoparticles have been shown to increase, the polymerability of polymer siloxane in a liquid crystalline phase to interact with the electrolytic field (Cognard, 1982), leading to an improvement in the properties of the polymer in a direction that reduces the J. Eng. Applied Sci., 14 (Special Issue 9): 10700-10705, 2019 ...
... In this case, the W → ∞ 1 and b « L connections take place. By such coupling, near the lower and upper reference surfaces of sandwich-cell, the boundary layers are arisen [25,26,[36][37][38]. Therefore, in this case the thermo-morphologic and thermotropic properties of N mesophase are determined by these boundary layers in the sandwich-cell. ...
Important peculiarity of phase transitions in liquid crystalline materials is availability of the heterophase regions, in which low temperature and high temperature phases simultaneously take place. In this work, peculiarities of the the thermo-morphologic and thermotropic transformations of the heterophase regions of phase transitions between nematic mesophase and isotropic liquid vs. boundary conditions have been investigated. 4-n-alkyl-4’-cyanobiphenyl liquid crystals with different length of the alkyl chain were objects of our investigations. Effect of thickness of liquid crystalline layer in the sandwich-cells on the thermo-morphologic and thermotropic properties of liquid crystals under investigations has been observed. Temperature and linear widths of the heterophase regions of phase transitions were determined with high accuracy. Change of the temperature and linear widths of the heterophase regions as function of thickness of liquid crystalline layer was found.
... POM textures also show that the doped concentration of NP was adequate to achieve complete VA without any aggregation. The generation of VA with doped NPs may an establishment with the empirical FriedeleCreagheKmetz (FCK) rule [63], which state that if excess energyðg s Þ of the solid sub- occurred, however if g s > g LC , the surface will give a parallel alignment of LC. In fact, the adsorption of ZnO NPs on the substrates mediates and lowers the surface tension of the substrates thus lowering the excess energy of substrate resulting in VA [55]. ...
... A planar aligning surface reduces the director orientation degeneracy but does not eliminate it. Introducing an unambiguous in-plane direction requires additional steps, such as mechanical unidirectional rubbing [7], irradiation with polarized light [8], or bombardment with ions at glancing angles [9]. Typically, the result is rigid anchoring of the LC director along a direction that is not truly in-plane, but may point at a certain small angle away from it depending on the technological parameters of the surface preparation routine [10]. ...
Background: The alignment of liquid crystals by surfaces is crucial for applications. It determines the director configuration in the bulk, its stability against defects and electro-optical switching scenarios. The conventional planar alignment of rubbed polymer layers can be locally flipped to vertical by irradiation with a focused ion beam on a scale of tens of nanometers.
Results: We propose a digital method to precisely steer the liquid crystal director tilt at polymer surfaces by combining micrometer-size areas treated with focused ion beam and pristine areas. The liquid crystal tends to average the competing vertical and planar alignment actions and is stabilized with an intermediate pretilt angle determined by the local pattern duty factor. In particular, we create micrometer-sized periodic stripe patterns with this factor gradually varying from 0 to 1. Our optical studies confirm a predictable alignment of a nematic liquid crystal with the pretilt angle continuously changing from 0° to 90°. A one-constant model neglecting the difference between the elastic moduli reproduces the results quantitatively correctly.
Conclusion: The possibility of nanofabrication of polymer substrates supporting an arbitrary (from planar to vertical) spatially inhomogeneous liquid crystal alignment opens up prospects of “imprinting” electrically tunable versatile metasurfaces constituting lenses, prisms and q-plates.
... POM textures also show that the doped concentration of NP was adequate to achieve complete VA without any aggregation. The generation of VA with doped NPs may an establishment with the empirical FriedeleCreagheKmetz (FCK) rule [63], which state that if excess energyðg s Þ of the solid sub- occurred, however if g s > g LC , the surface will give a parallel alignment of LC. In fact, the adsorption of ZnO NPs on the substrates mediates and lowers the surface tension of the substrates thus lowering the excess energy of substrate resulting in VA [55]. ...
In this work, vertically aligned electro-optical (E-O) cells were fabricated using plane Indium–Tin-Oxide (ITO) substrates without any surface treatment of ITO for the alignment of liquid crystal (LC) molecules. Experimentally, using a specific amount (0.3% wt/wt) of Zinc oxide (ZnO) nano particles (NPs) in NPs-LC mixture, a high quality vertically aligned liquid crystal (VALC) was achieved in cells without compromising physical properties of LC. Results reveal much improved morphological and E-O characteristics of display cells by lowering the threshold voltage to 1.68 V for NPs induced VALC compared with 1.86 V for polyimide induced vertically aligned liquid crystal (PIVALC) cell. The operating voltages were observed 1.96 V and 2.2 V for NPs induced VALC and PIVALC cells, respectively. The contrast ratio (CR) was obtained ∼12.4% better for NPs induced VALC cell compared with PIVALC cell. Further, isotropic temperature (Tiso) behavior was studied with polarized optical microscope (POM) and found in same order of value of temperature for both NPs induced VALC as well as PIVALC cells. Additionally, using UV–Visible spectroscopy study, optical band gap (Eg) was calculated and found in agreement with reported values of LC system.
... Во-первых, ионная сила раствора ЖК (I) принималась равной единице. Во-вторых, не учитывалось влияние ориентирующих покрытий [24]. ...
On the basis of the dielectric spectroscopy experimental results of 4-n-pentyl-4'-cyanobiphenyl (5CB), an empirical model describing the formation of double electrical layers (DEL) for the cells with the homeotropic and planar orientation is constructed. The model allows to determine the parameters of DEL, such as thickness and capacity, taking into account physical parameters of liquid crystal. It is also shows the empirical dependence of the parasitic capacitance, which is due to the presence of conduction ions, on the thickness of these layers.
... Consequently, anchoring effects on a solid boundary have been extensively studied, and various techniques have been established to enforce specific molecular orientations at interfaces between liquid crystals and solids. 1,2 These orientations are referred to as planar, when the molecules lie in the plane of 1 the substrate; homeotropic, when they are perpendicular to it; or conical, when the angle that they make with the plane is between 0 • and 90 • . ...
Controlling the anchoring of liquid crystal molecules at an interface with a water solution influences the entire organization of the underlying liquid crystal phase, which is crucial for many applications. The simplest way to stabilize such interfaces is by fabricating single emulsions of liquid crystal in water; however, a greater sensitivity to interfacial effects can be achieved using double emulsions, i.e. liquid crystal shells. Anchoring transitions on those systems are traditionally triggered by the adsorption of surfactant molecules onto the interface, which is neither an instantaneous nor a reversible process. In this study, we report the ability to change the anchoring of 5CB, one of the most widely used liquid crystals, at the interface with dilute water solutions of PVA, a polymer surfactant commonly used for stabilizing liquid crystal shells, simply by controlling the temperature in the close vicinity of the liquid crystal clearing point. A quasi-static increase in temperature triggers an instantaneous change in the film's boundary conditions from parallel to perpendicular, owing to the disordering effect of PVA, prior to the phase transition of the bulk 5CB. We study this anchoring transition on both flat suspended films and spherical shells of liquid crystals. Switching anchoring entails a series of structural transformations involving the formation of transient structures in which topological defects are stabilized. The type of defect structure depends on the topology of the film. This method has the unique ability to influence both interfaces of the film nearly at the same time, and can be applied to transform an initially polydisperse group of nematic shells into a monodisperse population of bivalent shells.
... The sensitive response of LC orientation to the interface conditions and external stimuli is the basic principle for LC-based biosensing. In a typical LC-based biosensing platform, the order of LC molecules is usually induced by an aligning reagent, for example, dimethyloctadecyl [3-(trimethoxysilyl)propyl]ammonium chloride (DMOAP) to orient LC homeotropically (i.e., vertically), at a LC-glass interface [1][2][3]. The disturbance in the homeotropic alignment of LC caused by the presence of biomolecules at the interface, as illustrated in Fig. 1, results in changes in optical texture that can be readily observed under a polarizing optical microscope [3][4][5]. ...
In comparison with the conventional approach entailing sandwiched cells for LC-based biosensing, here we demonstrate a simpler platform for label-free biodetection. This unique LC- on-single-substrate technique is capable of rapid screening and bioassay of DNA hybridization.
... Для завершения процесса полимеризации стекло со слоем ПВС помещалось в сушильный шкаф, разогретый до температуры 80 ºС на 30 мин. После этого поверхность натиралась ворсистой тканью в одном направлении для создания планарной ориентации ЖК в ячейке [8]. ...
... In addition, the suitability of polymers as LC alignment materials also depends on the following factors: (1) uniformly align the LC molecules, (2) ease of processing (to form thin films on the substrate surface by several conventional methods such as spin coat, rolling coat, dipping coat, and so on; (3) not reacting with the LC molecules; (4) strong adhesion with the substrate; and (5) to be hydrophobic. Therefore, one of the most recently developed and important electronics applications of PIs is their use in liquid crystal (LC) alignment layers for LC flat-panel display devices [100,101]. ...
... The basic principle of LC-based biosensing originates from the sensitive response of LC orientation to external stimuli as a result of its optical anisotropy (i.e., birefringence). In a typical LC-based biosensing platform, the order of LC molecules is usually induced by an aligning reagent, such as dimethyloctadecyl [3-(trimethoxysilyl)propyl] ammonium chloride (DMOAP) to orient LC homeotropically (i.e., vertically) at a LC-glass interface [1][2][3][4][5]. The disturbance in the homeotropic alignment of LC due to the presence of biomolecules at the interface is reflected through the change in optical texture, which can be readily observed under a polarizing optical mi croscope [3][4][5]. ...
A quantitative protein assay and label-free immunodetection based on dye-doped liquid crystal (DDLC) was developed in this study. The black dye incorporated as the dopant in the DDLC mixture features a high dichroic ratio and wide visible-light absorption band so that the level of disturbance in DDLC orientation in the presence of biomolecules can be readily observed with the naked eye. Due to the absorption and anisotropic features of DDLC, transmission spectrometry was exploited to establish a quantitative method for DDLC-based biosensing. Our results indicate that the transmittance of DDLC decreased with increasing concentration of bovine serum albumin (BSA), a common protein standard, and the detection sensitivity can be improved by using linearly polarized light during spectral measurement. A linear correlation between the spectral parameters of DDLC and BSA concentration was thus derived to facilitate protein quantitation. As a demonstration of clinical relevance, a label-free DDLC-based immunoassay for the cancer biomarker CA125 was manifested by reacting CA125 with immobilized anti-CA125 antibody. The transmittance of DDLC decreased with increasing concentration of CA125, but when a nonspecific protein such as BSA was reacted with the anti-CA125 antibody, the transmission spectrum was similar to that in the absence of CA125, suggesting that immunodetection via the DDLC-based biosensing platform was specific. It is evident from this study that with its unique dichroic and optical properties, DDLC is a potential biosensing material with capabilities of both qualitative detection as required in rapid screening as well as quantitative bioassay as in cancer screening.
... The goal of the study is to study intermolecular interactions of the dyes CD-1 and SD-1 with the oligodimethysiloxane surface. Many derivates from oligodimethysiloxane or silicones are known as the materials that orientate liquid crystals [7][8][9][10][11][12][13]. ...
We carried out the modeling orientation of the dyes CD–1 and SD–1 relative to the surface of oligodimethysiloxane using the atom–atom potentials method. We have discovered the dependence of the interaction energy in dyes molecules on the angles which characterizes their orientation relative to the surface of the oligodimethysiloxane crystal. It was found out that the obtained energy value of interaction with the surface can explain weak adhesive qualities of the dyes and the orientation type relative to the surface. We identified the break– loose force for the dyes on the oligodimethysiloxane crystal surface.
The article deals with the influence of the
radiation of light-emitting diode (LED) light sources on
the photobiological hazard for the retina. It has been
shown that LED lamps with a directional diffuser coating
and luminaires with diffuser and directional diffuser
diffusers generally do not exceed the zero risk group
(RG0) blue light hazard limits. The specified light
sources at brightness not higher than 104 kd/m2 for any
values of the correlated color temperature (CCT) in the
range of 2700-6500 K refer only to RG0. Most LED
lamps and luminaires for indoor lighting are safe for the
retina in terms of the photobiological hazard of blue light.
Optical properties of polymer-liquid-crystal composites (PLCCs) with controlled surface adhesion during uniaxial deformation have been studied. It has been experimentally shown that in PLCCs based on the 4-nheptyl- 4'-cyanobiphenyl nematic liquid crystal, the phenomenon of optical hysteresis can be observed during uniaxial deformation. The structure of such a film is a polymer matrix, in the volume of which elongated nematode droplets are distributed randomly, the long axes of which are oriented mainly along the direction of film stretching. The results of this work can be used used for creating elements of static memory for recording and long-term storage of information and optical switches of bistable states.
We report the effect of oblique irradiation of thin As2S3 chalcogenide glass films on the photoalignment of Nematic liquid crystals and their electro optical properties. After depositing 60 nm As2S3 films on ITO coated substrates, the substrates were irradiated with polarized blue light (436 nm) at angles 0°, 5°, 20° and 48° of the light propagation direction with respect to the substrate plane. Then, the substrates were assembled and filled with the liquid crystal. The device LC-20° exhibits good extinction, and lower threshold voltage when compared to all other devices. The contrast ratio of LC-20° almost double that of the LC-5° and LC-48° devices. The response time increases with the angle, so for LC-20° slightly longer than for LC-5° but shorter than that of LC-48°. It is evident from the observations that the obliquely irradiated cells show good photoalignment and enhanced electro optical properties, although no pretilt is generated due to the oblique irradiation. Therefore, contrary to the photoalignment using photosensitive polymers, chalcogenide glasses have the advantage of providing photoalignment with little sensitivity to the irradiation angle. This fact led us to propose a model for the photoinduced alignment on chalcogenide glasses based on directional structural changes, not related to oriented dipoles generation as in the case of photoalignment with polymers.
Dependence of the director tilt angle of nematic liquid crystal (LC) under conical anchoring from the two-component polymer mixture composition has been studied. We varied the ratio of poly(isobutyl methacrylate) (PiBMA), which specifies a conical anchoring for the nematic liquid crystal LN-396, and poly(methylmethacrylate) (PMMA) assigning a tangential alignment for the same nematic. An oblique incidence light technique to determine a tilt angle has been used. It has been shown that the tilt angle increases from 0∘ to 47.7∘ when PiBMA:PMMA ratio changes in the range 30:70 to 100:0. The specific optical textures viewed under the polarizing microscope and proper orientational structures have been considered for various compositions of the polymer films. An electric field action on the formed orientational structures has been investigated. The obtained results are promising for the application in various electro-optical LC devices with a conical anchoring in which the director tilt angle is a crucial parameter: a controlled diffraction gratings, an electrically operated achromatic rotators of linear light polarization, etc.
Liquid crystal (LC) alignment methods are very important for manufacturing liquid crystal displays (LCDs). The photoalignment method in which polyimide films are radiated with linearly polarized ultraviolet (LPUV) light is one of the most effective non-rubbing processes to solve problems such as electrostatic charge and dust accumulation. This entry presents a review on photoalignment film of LC using new fluorine-containing polyimides. We have provided detailed spectroscopy study to understand the possible mechanism behind the photoalignment effect for liquid crystal molecules.
Three types of liquid crystal displays (LCDs) have been developed: transmissive, reflective, and transflective. This chapter focuses on the thin film transistor‐addressed wide‐viewing angle transmissive LCDs which form the mainstream of LCD production and development. It introduces the twisted nematic, in‐plane switching, and fringe field switching displays, as well as the multi‐domain vertical alignment device mode. Although the LC material only occupies a thin layer in a display device, it makes crucial contributions to the device performance. The chapter addresses phase compensation methods for achieving wide viewing angle in LCDs. Contrast ratio is a key display metric affecting the image quality. Organic light‐emitting diodes (OLEDs) exhibit a much higher contrast ratio than LCD. In the presence of surrounding light, the ambient contrast ratio (ACR) rather than the dark room contrast ratio is more representative. ACR has been widely used to evaluate the sunlight readability of LCDs and OLED displays.
A water-soluble aromatic polyamide (WAPA) thin film was formed by coating a newly synthesized poly(2,2′-disulfo-4,4′-benzidine isophthalamide) cesium salt (PDBI-Cs). Its electro-optical properties as an alignment layer in liquid crystal devices were analyzed. Twisted nematic-liquid crystal cells containing WAPA alignment layers annealed at a low temperature of 100 °C showed high voltage holding ratio values above 95 % at a frame rate of 60 Hz, a fast response time of 7.18 ms at ac operation of 1 kHz under LC-applied voltage of 5 V, and excellent contrast ratio of 3284 due to the high optical anisotropy effect of the WAPA surface. Thus, PDBI-Cs-based WAPA can be a promising candidate as a new eco-friendly low-temperature-processable alignment layer for flexible electro-optic devices.
A unique feature of phase transitions in liquid crystalline materials is the availability of heterophase regions. In such regions, the simultaneous coexistence of high temperature and low temperature phases takes place. Thermal mobility and temperature gradient in the heterophase regions lead to the appearance of various types of defective formations in the aligned and non‐aligned textures of liquid crystalline materials. In the present work, the morphologic, structural, and optical peculiarities of defective formations and non‐homogeneities, which appear in the heterophase regions of phase transitions, are investigated. The singular points, disclination, inversion walls, droplets, and confocal formations are objects of the investigations herein. The heterophase regions of the direct and reverse nematic ↔ isotropic liquid and smectic A ↔ isotropic liquid, smectic C ↔ isotropic liquid, and smectic C* ↔ isotropic liquid thermotropic phase transitions are created by the original method, that is, the capillary temperature wedge method. Crystallo‐optics and crystallo‐physics methods are used in this work. Experimental investigations of defects, non‐homogeneities, and various formations, which appear in heterophase regions of thermotropic phase transitions, are carried out. These investigations are possible using the original method of the temperature wedge. In this work, various types of liquid crystals, which exhibit nematic (N), smectic A (SmA), smectic C (SmC), and smectic C* (SmC*) mesophases are objects of the investigations.
To date, azo dyes are widely used for liquid crystal photoalignment in flat panel displays. The relation of dichroism of azo dyes with their orientation under intensive light wave field is considered. Values of a parameter, which characterizes orientation order of both molecules and dimers, were obtained as well as energy of their orientation process. Number of cooperatively oriented dye molecules was estimated. The angles distribution functions for different dimers were derived. Dipole electrical transition moments directed on angles close to 90° to the direction of maximal polarizability of dimers were revealed. The relation of azo dyes dichroism with their orientation under light wave field is considered. Values of an orientation order parameters of both molecules and dimers were obtained. The angles distribution functions for different dimers were found. Dipole electrical transition moments directed on angles close to 90° to the direction of maximal polarizability of dimers were revealed.
A simple and effective approach for vertical alignment of liquid crystals (LCs) over a functionalized transparent flexible substrate is described. Surface characterization of this commercially available plastic substrate through X-ray photoelectron spectroscopy (XPS) and attenuated total reflection infrared spectroscopy (ATR-IR) indicated that cellulose acetate is main component of the transparent substrate. This substrate was chemically functionalized with a suitable LC compound. A trimethoxysilane terminated new rod-shaped mesogen is synthesized and covalently attached to the pre-treated film through silane condensation reaction. LC functionalization of the polymer film is confirmed through contact angle (CA), atomic force microscopy (AFM), XPS and ATR-IR spectroscopy studies. Versatility of the LC modified flexible substrates for the alignment of bulk LC sample at substrate-LC interface was assessed for nematic (N) and smectic A (SmA) phases. Remarkably, LC functionalized cellulose acetate films were found to be highly efficient in assisting a perfect homeotropic alignment of LCs (for both, a room temperature N and a high temperature SmA phase) over the entire area of the LC sample under observation indicating their superior aligning ability in comparison to their unmodified and octadecyltrimethoxysilane (OTS) modified counterparts. The demonstrated method of surface modification of flexible polymer film is easy, surface modified substrates are stable for several months, retained their aligning ability intact and more importantly they are reusable with maximum delivery.
We meet friction processes quite often in everyday life. As for instance, when we move bodies relative to each other ( kinetic friction) or try to put in motion various bodies at rest (static friction).
Development of the efficient lubricants can improve durability of tribosystems and save energy in machinery. An optimum and cost-efficient trend is to improve tribological performance of the available lubricants by introduction of suitable additives. The experience shows that liquid-crystal cholesterol compounds (LCCC) can acquire planar orientation on the bearing surfaces and thus reduce friction of lubricated surfaces and increase their load-bearing capacity. It can be assumed that lubricant turns into a mesomorphous state where the molecules of the additive and lubricant acquire the ordered position.
It is well known that today two main and well studied methods for alignment of liquid crystals has been used, namely: rubbing and photoalignment technologies, that lead to the change of anisotropic properties of aligning layers and long-range interaction of the liquid crystal molecules in a mesophase. In this manuscript, we propose the usage of the nonlinear laser lithography technique, which was recently presented as a fast, relatively low-cost method for a large area micro- and nanogrooves fabrication based on laser-induced periodic surface structuring, as a new perspective method of the alignment of nematic liquid crystals. 920 nm periodic grooves were formed on a Ti layer processed by means of the nonlinear laser lithography and studied as an aligning layer. Aligning properties of the periodic structures of Ti layers were examined by using a combined twist LC cell. In addition, the layer of the microstructured Ti was coated with an oxidianiline-polyimide film with annealing of the polymer film followed without any further processing. The dependence of the twist angle of LC cells on a scanning speed and power of laser beam during processing of the Ti layer was studied. The azimuthal anchoring energy of Ti layers with a periodic microstructure was calculated. The maximum azimuthal anchoring energy for the microstructured Ti layer was about 4.6*10-6 J/m2, which is comparable to the photoalignment technology. It was found that after the deposition of a polyimide film on the periodic microstructured Ti layer, the gain effect of the azimuthal anchoring energy to ~ 1*10-4 J/m2 is observed. Also, AFM study of aligning surfaces was carried out.
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