Homeotropic Liquid Crystal Alignment Research Articles

Titania Laden Liquid Crystal Based Sensor as Photocatalyst.

The present research work elucidates the photocatalytic effect of titania on liquid crystal based sensor used to detect sodium arsenate in water. The titania nanoparticles with an average size of 18-20 nm were synthesized by hydrothermal method and calcined up to 500 °C to obtain pure anatase phase. The catalytic activity of titania nanoparticles was enhanced by surface modification with (3-aminopropyl) trimethoxysilane. The sensing experiment was performed under visible light irradiation while keeping the concentration of substrate, analyte, the thickness of the liquid crystal layer, temperature and pH values constant. In addition, span 80 (a non-ionic surfactant) was used to obtain homeotropic alignment of liquid crystals. The TiO₂ nanoparticles gave dark to bright texture within 53 min while the functionalized titania responded in less than 25 min under similar sensing condition. The detection results showed that the functionalized titania nanoparticles possess greater photocatalytic activity under visible light and enhanced the sensing response than unfunctionalized titania nanoparticles.

Photo‐Tunable Supramolecular Ultrathin Surfaces for Simultaneous Homeotropic Anchoring and Superfast Switching of Liquid Crystals

AbstractPhoto‐tunable supramolecular ultrathin surfaces functionalized with a soft ultraviolet (UV) treatment of star‐shaped reactive amphiphiles are fabricated to simultaneously afford homeotropic anchoring of liquid crystals (LCs) and improve the LC switching performance. Photochemically functionalized ultrathin surfaces are generated by cross‐linking star‐shaped amphiphilic monomers in the self‐assembled monolayers with retention of cluster structure under electric field. Since these star‐shaped amphiphiles can construct the close‐packed self‐organized molecular layer with an alkyl brush at the interface between indium tin oxide and LCs through intermolecular hydrogen bonding, they might make elegant alignment surfaces for the homeotropic anchoring of LCs to provide a new type of vertical alignment layer. It is experimentally revealed that supramolecular ultrathin layers with vertical nanopillars form hydrophobic surface, and afford a stable and uniform homeotropic alignment of LCs even in high‐temperature environment. Furthermore, a soft UV irradiation on the self‐assembled ultrathin surfaces under electric field yields superfast responsive cross‐linked ultrathin surfaces with low power consumption compared to conventional polyimide layer. Notably, this controlled methodology offers advantages of simplicity, low‐cost, and scalability in the fabrication of functional alignment surfaces without the need of complex manufacturing steps.

Influence of head group and chain length of surfactants used for stabilising liquid crystal shells

ABSTRACTWe investigate the stability and textural development in nematic liquid crystal (LC) shells, with aqueous interior and exterior, as a function of the type and concentration of surfactant stabiliser of the shell interfaces. The LC is the common thermotropic nematic 5CB and the surfactants are commercial, of cat- as well as of anionic type, with varying alkyl chain length. In addition to stabilising the shell interfaces, surfactants are generally assumed to promote radial (homeotropic) LC alignment, based on prior studies where the surfactant concentration was well above the critical micelle concentration (CMC). Here, we focus on the low-concentration range, below CMC. We find that both cat- and anionic surfactants can stabilise shells, although the higher water solubility of cationics can render stabilisation more difficult. We also conclude that surfactants do not necessarily impose homeotropic alignment; if the surfactant concentration is very low, the director may adopt planar alignment at the 5CB–water interface. Interestingly, the threshold concentration, where the surfactant takes control of alignment, is different for the shell inside and outside. Shells stabilised by solutions of surfactant with concentration near the threshold may therefore adopt a hybrid configuration, with homeotropic inside and planar outside.

Homeotropic Self-Alignment of Discotic Liquid Crystals for Nanoporous Polymer Films.

Nanostructured polymer films with continuous, membrane-spanning pores from polymerizable hexagonal columnar discotic liquid crystals (LCs) were fabricated. A robust alignment method was developed to obtain homeotropic alignment of columns between glass surfaces by adding a small amount of a tri(ethylene glycol) modified analogue of the mesogen as a dopant that preferentially wets glass. The homeotropic LC alignment was fixated via a photoinitiated free radical copolymerization of a high-temperature tolerant trisallyl mesogen with a divinyl ester. Removal of the hydrogen-bonded template from the aligned columns afforded a nanoporous network with pores of nearly 1 nm in diameter perpendicular to the surface, and without noticeable collapse of the nanopores. The effect of pore orientation was demonstrated by an adsorption experiment in which homeotropic film showed a threefold increase in the initial uptake rate of methylene blue compared to planarly aligned films.

Homeotropic alignment behavior of liquid crystal molecules on self-assembled monolayers with fluorinated alkyl chain

This study demonstrates the orientation characteristics of liquid crystals (LCs) on fluorinated self-assembled monolayers (FSAMs) deposited using the gas phase method. The authors confirmed that the FSAMs were properly deposited on indium-doped tin oxide glass substrates using x-ray photoelectron spectroscopy and atomic force microscopy. Compared with conventional polyimide (PI) layers, the authors confirmed that the FSAM is an alternative to the conventional PI layer. Regardless of the positive and negative LC characteristics, the LC molecules were vertically aligned on the FSAMs, with hydrophobic properties. In addition, an LC cell using an FSAM showed faster response times than that with a conventional PI layer when the electro-optical characteristics were examined. Therefore, these results indicate that FSAMs are suitable for LC applications as homeotropic LC alignment layers.

Vertical Alignment of Liquid Crystals Over a Functionalized Flexible Substrate

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.

Homeotropic alignment of liquid crystals on ITO surface using LBL assembly

AbstractIn this work, the very thin layer‐by‐layer (LBL) film that was constructed by dip coating method on indium tin oxide surface can be used in liquid crystal (LC) displays devices. The obtained results indicate that the ultrathin LBL film shows the homeotropic alignment layer, and a uniform vertical alignment of LC molecules was gained very easily. The progress of vertical‐aligned LC cells with a LBL layer was evaluated. The obtained threshold voltage and response time of the LC cell were 2.472 V and 12.5 ms, respectively. So, the competitive performance of the LC cell could allow new sign at a low‐cost budget with rubbing process in LC display technology.

Label-free optical detection of thrombin using a liquid crystal-based aptasensor

We describe a real-time, label-free aptasensor that utilizes the orientational properties of nematic liquid crystals (LCs) for detection of thrombin. The imaging principle is based on the homeotropic orientation of LC films, which can be influenced by thrombin-specific aptamers and N,N-dimethyl-N-(3-(trimethoxysilyl)propyl)-1-octadecanaminiuchloride (DMOAP) immobilized on glass slides. The interactions between thrombin and its specific aptamer, including ionic and hydrogen bonds, can interfere with the homeotropic alignment of LCs, leading to changes from ‘dark’ to ‘bright’ responses that can be visualized simply using crossed polarizers. We describe the condition for maintaining the homeotropic state of LCs, which are suitable for thrombin-specific aptamer experiments, and reactions between aptamer and several enzymes, including thrombin. This novel simple LC-based aptasensor has not only high specificity, but also high sensitivity (low detection limit of 1 pg/mL (26.7 fM)). We also note the blood coagulation reaction between fibrinogen and thrombin, which confirms that the aptamer-bound thrombin is auto-enzymatic. Our result shows that the sensitivity of the proposed sensor increases (1 pg/mL to 0.1 pg/mL) using fibrinogen.

Topographically induced homeotropic alignment of liquid crystals on self-assembled opal crystals.

The surface of multilayered opal crystals resulted in homeotropic alignment of liquid crystal (LC), originated from the surface topography of opal crystals rather than a chemical nature of the nanoparticles. The polar anchoring energy (5.51 × 10-5 J/m2) of the crystal surface for nematic LC molecules was in a similar range to the conventional polyimide alignment layer (2.11 × 10-5 J/m2) used for commercial applications. The critical length scale for anchoring transition was approximately Lw = ~1 μm. If a diameter of particle d << 1 μm for opal crystals, LC molecules preferred to anchor vertically to the surface to minimize elastic free energy of bulk LCs. The LC favored a planar anchoring if d >> 1 μm. The results provide crucial insights to understand the homeotropic alignment of LCs on solid surfaces and therefore offer opportunities to develop novel materials for a vertical alignment of LCs.

Polyhedral Oligomeric Silsesquioxane Films for Liquid Crystal Alignment

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.

Fabrication of hybrid homeotropically aligned liquid crystal device using self-assembled molecular layers

ABSTRACTThis report demonstrates hybrid homeotropic alignment of liquid crystals (LCs) using a self-assembled molecular (SAM) layer. 4-(4-Hep-tylphenyl)benzoic acid forms a SAM layer through hydrogen bonding with indium-tin oxide (ITO) substrate, and the LCs were aligned vertically without a polyimide (PI) layer on one side of substrate. The proposed hybrid structure featured a more stable homeotropic alignment than the LC device (LCD) with only half of a PI layer, and showed electro-optical characteristics similar to conventional LCDs with full PI layers. The SAM layer showed stable alignment and fast response in the LC cell by a simple doping method.

The effect of surface polarity of glass on liquid crystal alignment

ABSTRACTThe effects of the surface polarity of a glass substrate on the orientation of nematic liquid crystals (LCs) were studied using the polarised optical microscope and Fourier-transform infrared spectroscopy. On the surface of oxygen plasma treated glass, a homeotropic alignment of LCs was induced for LCs with negative dielectric anisotropy. This suggests that vertical orientation of LCs could be induced on a polar glass substrate without using an LC alignment layer. Upon cooling towards the isotropic–nematic transition, E7 with positive dielectric anisotropy changes its LC arrangement to isotropic, homeotropic, planar orientations in order. The nematic LC anchoring transition of E7 was interpreted by considering the competition between van der Waals forces and dipole interactions that control the alignment of LC molecules on a polar glass surface.

41‐3: Novel Pl‐Less Vertical Alignment Technology Using Hydrogen Bonding of Nonionic Amphiphiles

We propose a novel method for Pl‐less vertical alignment of liquid crystals (LCs) using nonionic amphiphiles that form hydrogen bond with the ITO substrates. Excellent homeotropic alignment of LCs was obtained with a simple mixing process, and its electro‐optical performance was similar to that of the conventional polyimide method.

Vertical alignment of liquid crystals using novel self-assembled molecular layer of alkylated benzoic acid derivatives

ABSTRACTThe one-step vertical alignment of liquid crystals (LCs) was achieved by forming a novel self-assembled molecular layer of alkylated benzoic acid derivatives on an indium tin oxide substrate through hydrogen bonding. This method utilizes the simple doping of the alignment material into the LCs. The assembled molecular layer promoted the stable homeotropic LC alignment. The electro-optical characteristics of LC devices exhibited stable vertical alignments and fast response times, depending on the number of rigid aromatic rings in the alignment material. Our simplified alignment method allows the fabrication of LC devices without a separate alignment step for fast LC switching.

In situ self-assembled homeotropic alignment layer for fast-switching liquid crystal devices

ABSTRACTWe propose a novel method for homeotropic alignment of liquid crystals (LCs) utilising in situ self-assembly of a low concentration of 4-(4-heptylphenyl)benzoic acids that form hydrogen bond with the indium tin oxide (ITO) substrates. Stable homeotropic alignment in the LC device is achieved with a simple mixing process of benzoic acid derivative in LC media, and it yields electro-optical performance similar to that achieved with the conventional alignment method using polyimides. It is experimentally confirmed that an ultrathin self-assembled molecular layer of 4-(4-heptylphenyl)benzoic acid formed by hydrogen bonding on ITO substrate makes it possible to attain a reliable homeotropic alignment of LCs. Furthermore, this simple approach provides a cost-effective and stable LC alignment layer with fast response time and thermal stability.

Measurement of anchoring coefficient of homeotropically aligned nematic liquid crystal using a polarizing optical microscope in reflective mode

Although the homeotropic alignment of liquid crystals is widely used in LCD TVs, no easy method exists to measure its anchoring coefficient. In this study, we propose an easy and convenient measurement technique in which a polarizing optical microscope is used in the reflective mode with an objective lens having a low depth of focus. All measurements focus on the reflection of light near the interface between the liquid crystal and alignment layer. The change in the reflected light is measured by applying an electric field. We model the response of the director of the liquid crystal to the electric field and, thus, the change in reflectance. By adjusting the extrapolation length in the calculation, we match the experimental and calculated results and obtain the anchoring coefficient. In our experiment, the extrapolation lengths were 0.31 ± 0.04 μm, 0.32 ± 0.08 μm, and 0.23 ± 0.05 μm for lecithin, AL-64168, and SE-5662, respectively.

Homeotropic Alignment Properties of Liquid Crystal and Photocurable Monomer System via UV Irradiation

We investigated the homeotropic (or vertical) liquid crystal (LC) alignment properties in a LC and photocurable monomer mixture system through UV irradiation. Different homeotropic LC alignment behaviors were observed depending on the type of photocurable monomers and UV irradiation time. The LC alignment behavior showed a good correlation with the wettability of the alignment films due to surface morphology on the alignment layer surfaces by photopolymerization-induced phase separation. The electro-optical characteristics of the LC cells fabricated with these polymer films were as good as or even better than those fabricated from rubbed polyimide (PI) films, the most commonly used LC alignment layers.

Liquid Crystal Biosensor Based on Cd&amp;lt;sup&amp;gt;2+&amp;lt;/sup&amp;gt; Inducing the Bending of PS-Oligo for the Detection of Cadmium

In this study, a novel liquid crystal (LC) biosensor was developed for the highly sensitive and selective detection of Cd2+ based on Cd2+ inducing the bending of PS-oligo. This strategy makes use of the DNA conformational change to enhance the disruption of orientation of LC leading to an amplified optical signal. DNA containing-SH was bound on the glass slide of the LC cell modified with the DMOAP/APTES. The DMOAP can effectively induce the homeotropic alignment of LC. In the presence of Cd2+, Cd2+ can induce DNA to bend and become a 2 nm spherical structure, which can greatly disrupt the orientational arrangement of LC, resulting in the correspond changes of the optical image of LC cell birefringent under the polarizing microscope. When the Cd2+ concentration is low to 0.1 nM, the optical signal of LC biosensor has an obvious change. But in the absence of Cd2+, there is no orientational response of LC and the optical image under the polarizing microscope is still a uniform dark background. Thus, this LC sensing method has a sensitive and clear distinction between positive and negative results and offers a highly sensitive detection of Cd2+ with a low detection limit down to 0.1 nM.

Highly sensitive bovine serum albumin biosensor based on liquid crystal

A highly sensitive liquid crystal (LC) based bovine serum albumin (BSA) protein biosensor is designed. A uniform homeotropic alignment of nematic LC was observed in BSA free substrate which changed into homogeneous in presence of BSA. The change in the LC orientation is found to depend strongly on BSA concentration. This change in the LC alignment is attributed to the modification in the surface conditions which is verified by contact angle measurements. We have detected an ultra low concentration (0.5 μg/ml) of BSA. The present study demonstrates the utilization of LC in the realization of high sensitivity biosensors.

Liquid Crystal Orientation Mechanism: Competition Between Rubbing and Ion-beam Method

The effect of liquid crystal (LC) alignment on a homeotropic polyimide (PI) surface induced by ion beam (IB) irradiation and rubbing process was studied. LC alignment was not affected by IB irradiation with an exposure time of 10 s, and an IB irradiation with an exposure time of 60 s more effectively oriented the LCs on the PI layer than the rubbing process. It was assumed that the LC alignment depended on the C-O bonds created from the C=O bonds on the PI surface broken by IB irradiation after an exposure time of 60 s, which resulted in a strong surface energy that transformed the homeotropic LC alignment to homogeneous states.