Confinement Of Liquid Crystals Research Articles

Dynamic Microparticle Assembly at the Interface of Chemoresponsive Liquid Crystal Droplets.

The confinement of liquid crystals (LCs) in spherical microdroplets results in exotic internal configurations and topological defects in response to physical and chemical stimuli. Recent exploration into the placement of colloids on the surface of LC microdroplets has led to the design of a new class of functional materials with patterned surface properties. It is established that the placement of a colloid on a LC droplet surface can pin the topological defect at the interface, thereby restricting changes in the LC configuration. Herein, we build upon the handful of reports published to provide a fundamental understanding of the colloid positioning in response to external stimuli. Using polystyrene (PS) colloids, we explored the dynamics of particle self-assembly in response to an interfacial enzymatic breakdown of poly-l-lysine by trypsin. We found that for a significant population of droplets, the positioning of the colloid is unaffected by the changes in the internal ordering of LC. Inspired by the new observations, we delved deeper to understand the role of interfacial stabilizers in modulating the preferential alignment of LC and the placement of colloidal microparticles. We also demonstrated that for a certain population of droplets, the positioning of the colloids remains unperturbed in response to multistep reversible adsorption of interfacial amphiphiles. Our findings reveal interesting possibilities of correlating the stimuli-responsive switching of internal configurations of LC with colloid placement on the particle-decorated LC droplets.

Chiral monomer template for designing Low-Driving-Field blue phase liquid crystals

Blue phase liquid crystals (BPLC) are known to be useful for various non-trivial applications due to their nanoscale self-assembled cubic crystalline structures. Even though the BPLC exhibits an efficient optically isotropic phase due to the double helical confinement of liquid crystal (LC) molecules within a few hundred-nanometer range, this hinders the orientation of LC molecules along the applied field direction. Consequently, the BPLC demands larger driving fields to operate, degrading the device’s performance in real-world applications. In this context, we provide a chiral monomer template for BPLC that decreases the driving field by 52 % while the electro-optical responses remain unaffected. Furthermore, our approach provides higher thermal stability, with a broader BPLC phase range (42 °C), including near ambient temperature. As this approach potentially overcomes the fundamental challenges of BPLC, the proposed system could be useful for next-generation display devices.

Design of a Super-Liquid Crystal-Phobic Coating for Immobilizing Liquid Crystal μ-Droplets─Without Affecting Their Sensitivity.

The aqueous interface of nematic liquid crystal (LC) that undergoes a triggered change in ordering transition of mesogens under an appropriate stimulus has emerged as an important tool for various relevant applications. Further, the confinement of LC into a micrometer dimension appeared to be a facile approach for improving their relevant features and performance. However, the optical characterization of ordering transition in a single micrometer-sized, bare, and free-floating LC droplet in the aqueous phase is an extremely challenging task due to unavoidable Brownian motion, which limits its scope for practical applications. Here, we exploited the 1,4-conjugate addition reaction to report a multilayer coating of a reactive nanocomplex that displayed an extreme repellence to beaded LC droplets with tailored adhesive force through the association of adequate orthogonal chemical modifications with glucamine and selected alkyl acrylates. Further, a spatially selective underwater adhesive super-LC-phobic pattern on a hydrophobic background was developed for immobilizing bare and micrometer-sized LC droplets from their aqueous dispersion without having any arbitrary spillage of the aqueous medium. The settled micrometer-sized LC droplets remained efficient for the triggered change in ordering transition from bipolar (having boojum defects at poles) to radial (with a single defect in the center) configuration. Eventually, a simple and fundamentally distinct chemical strategy of immobilizing a soft and functional material by associating bio-inspired wettability allowed to demonstrate the repetitive triggered LC ordering transition in a single and bare LC droplet.

Surfactant Modulated Phase Transitions of Liquid Crystals Confined in Electrospun Coaxial Fibers.

Confinement of liquid crystals (LCs) in polymeric fibers offers a promising strategy to control liquid crystal response to external stimuli. Here, the confinement of 4-cyano-4'-pentylbiphenyl (5CB), a nematic liquid crystal, within the core of coaxially electrospun fibers composed of poly(vinylpyrrolidone) (PVP) containing different surfactants is discussed. The effects of surfactant type, surfactant concentration, and core flow rate (confinement) on the LC behavior were demonstrated using polarized optical microscopy, scanning electron microscopy, differential scanning calorimetry, Raman, and dielectric spectroscopy. Introduction of surfactant dopants of varying hydrophilic and hydrophobic components into the sheath altered the interfacial interaction between the PVP sheath and the 5CB core of the fibers. Significant effects on the LC nematic to isotropic phase transition were attributed to changes in surface anchoring between the sheath and core. Confinement of nematic LCs in surfactant doped polymeric fibers demonstrates a facile method for tuning LC phase behavior.

27‐3 2‐D Confinement of LCs with Virtual Walls for a Fast Response LCD

We report a simple method for reducing the response time of homogeneously aligned liquid crystal (LC) cells by using two‐dimensional confinement of the LCs. We have shown that the switching speed can be increased by serval fold in in‐plane switching and fringe‐field switching cells by introducing interdigitated electrodes parallel to the LC alignment direction. We also report an interdigitated pixel electrode structure with alternating tilts for a much wider viewing angle by aligning the LCs without a pretilt. In addition to a short response time and wide viewing angle, this device allows a much larger deviation of the LC alignment direction which is essential for mass production. Moreover, LCs with negative dielectric anisotropy can be used to minimize the transmittance decrease.

Effect of curvature on cholesteric liquid crystals in toroidal geometries.

The confinement of liquid crystals inside curved geometries leads to exotic structures, with applications ranging from biosensors to optical switches and privacy windows. Here we study how curvature affects the alignment of a cholesteric liquid crystal. We model the system on the mesoscale using the Landau-de Gennes model. Our study was performed in three stages, analyzing different curved geometries from cylindrical walls and pores, to toroidal domains, in order to isolate the curvature effects. Our results show that the stresses introduced by the curvature influence the orientation of the liquid crystal molecules, and cause distortions in the natural periodicity of the cholesteric that depend on the radius of curvature, on the pitch, and on the dimensions of the system. In particular, the cholesteric layers of toroidal droplets exhibit a symmetry breaking not seen in cylindrical pores and that is driven by the additional curvature.

Spontaneous emergence of chirality in achiral lyotropic chromonic liquid crystals confined to cylinders.

The presumed ground state of a nematic fluid confined in a cylindrical geometry with planar anchoring corresponds to that of an axial configuration, wherein the director, free of deformations, is along the long axis of the cylinder. However, upon confinement of lyotropic chromonic liquid crystals in cylindrical geometries, here we uncover a surprising ground state corresponding to a doubly twisted director configuration. The stability of this ground state, which involves significant director deformations, can be rationalized by the saddle-splay contribution to the free energy. We show that sufficient anisotropy in the elastic constants drives the transition from a deformation-free ground state to a doubly twisted structure, and results in spontaneous symmetry breaking with equal probability for either handedness. Enabled by the twist angle measurements of the spontaneous twist, we determine the saddle-splay elastic constant for chromonic liquid crystals for the first time.

Dispersion of γ-Alumina Nano-Sized Spherical Particles in a Calamitic Liquid Crystal. Study and Optimization of the Confinement Effects.

We report an experimental study on confined systems formed by butyloxybenzylidene octylaniline liquid crystal (4O.8) + γ-alumina nanoparticles. The effects of the confinement in the thermal and dielectric properties of the liquid crystal under different densities of nanoparticles is analyzed by means of high resolution Modulated Differential Scanning Calorimetry (MDSC) and broadband dielectric spectroscopy. First, a drastic depression of the N-I and SmA-N transition temperatures is observed with confinement, the more concentration of nanoparticles the deeper this depression is, driving the nematic range closer to the room temperature. An interesting experimental law is found for both transition temperatures. Second, the change in shape of the heat capacity peaks is quantified by means of the full width half maximum (FWHM). Third, the confinement does not noticeably affect the molecular dynamics. Finally, the combination of nanoparticles and the external applied electric field tends to favor the alignment of the molecules in metallic cells. All these results indicate that the confinement of liquid crystals by means of γ-alumina nanoparticles could be optimum for liquid crystal-based electrooptic devices.

Electro-optical Characteristics of Ferroelectric Liquid Crystal Embedded in Photonic Crystal Fiber

The confinement of liquid crystals in a small space induces interesting phenomena that deviate from the bulk properties. We report on the electro-optical properties of ferroelectric liquid crystals (FLCs) confined in transparent photonic crystal fibers (PCFs). A PCF has a large number of cylindrical air holes of micrometer size and they are filled with FLCs. Optical measurements indicate that the directors are parallel to the axis of PCF and the layers are inclined relative to the cylindrical wall with the tilt angle of SmC*. The electro-optical response depends on the surface condition and shows the unusual double hysteresis loop. We discuss the possible configuration of director arrangement and the electro-optical properties of FLC on the basis of a simple model.

Nematic Liquid Crystals Embedded in Cubic Microlattices: Memory Effects and Bistable Pixels

AbstractThe confinement of liquid crystals in geometries with frustrating boundary conditions gives rise to nontrivial effects such as bistability and memory. It is shown that large memory effects arise when nematic liquid crystals are embedded in cubic micrometer‐sized scaffolds made by two‐photon polymerization. The electric field alignment of the liquid crystals inside the porous medium is maintained when the applied field is above a threshold (approximately 2 V per micrometer of cell thickness). The onset of the memory is an on/off type process for each individual pore of the scaffold, and the memory typically starts emerging in one region of the structure and then propagates. The global memory effects in porous structures with controlled geometry are enhanced with respect to the case of random porous structures. This work is a proof of the “memory from topology” principle, which was previously suggested by computer simulations. These new materials can pave the way to new types of bistable displays.

Memory and topological frustration in nematic liquid crystals confined in porous materials

Orientational ordering is key to functional materials with switching capability, such as nematic liquid crystals and ferromagnetic and ferroelectric materials. We explored the confinement of nematic liquid crystals in bicontinuous porous structures with smooth surfaces that locally impose normal orientational order on the liquid crystal. We find that frustration leads to a high density of topological defect lines permeating the porous structures, and that most defect lines are made stable by looping around solid portions of the confining material. Because many defect trajectories are possible, these systems are highly metastable and efficient in memorizing the alignment forced by external fields. Such memory effects have their origin in the topology of the confining surface and are maximized in a simple periodic bicontinuous cubic structure. We also show that nematic liquid crystals in random porous networks exhibit a disorder-induced slowing-down typical of glasses that originates from activated collisions and rearrangements of defect lines. Our findings offer the possibility to functionalize orientationally ordered materials through topological confinement.

Influence of confinement on molecular and director reorientational dynamics of liquid crystal

We present the qualitative results of investigations of influence of confinement of liquid crystal in random porous medium on relaxation due to molecular reorientations investigated by dielectric spectroscopy and fluctuations of director orientations (collective relaxation) investigated by photon correlation spectroscopy. The relaxation times of the process due to the molecular rotations in deeply supercooled state of confined liquid crystal were slower than at the temperatures corresponding to nematic phase by a factor of 10 6. This slowing down was accompanied by anomalous broadening of the dielectric spectra. We attribute this qualitative change in molecular relaxation to coupling of molecules to the pore walls and among themselves via molecule-wall interactions. In the nematic phase of bulk liquid crystals, the relaxation process due to director orientation fluctuations (collective mode) was single exponential. The dynamics of this process was also drastically modified by random confinement. The slow relaxation process, which does not exist in the bulk liquid crystal, and a broad spectrum of relaxation times, appear for confined liquid crystal. These results on the influence of confinement on relaxation properties of liquid crystal were complimented by differential scanning calorimetry experiments.

Dielectric relaxation in liquid crystals confined in a quasi-one-dimensional system

Dielectric spectroscopy was used to investigate the influence of confinement of the liquid crystal 4- n-penyl-4′-cyanobiphenyl 5CB (LC) on the nematic-to-isotropic (N–I) phase transition and the dynamics of molecular reorientations via rotation of molecules around their short axis under extremely narrow confinement in pores of 2.3 nm diameter, that may be considered a quasi-one-dimensional (1D) system. We observed that, instead of undergoing the N–I phase transition in the quasi-1D system, 5CB inside the 2.3 nm pores shows dynamic behavior typical for supercooled liquids. Relaxation of molecular origin dramatically slows down upon cooling the sample from the isotropic phase. Molecular reorientations were dielectrically active at temperatures much above the bulk N–I transition temperature but with the relaxation rate much slower and dielectric spectrum much broader than in the bulk 5CB. We suggest that the slowing down of the mode, which is molecular in the bulk material, may be due to enhancement of the effective viscosity of 5CB under such narrow confinement, resulting in supercooled dynamics.

Study of nematic liquid crystals confined in molecular sieves. 4-octyl-4′-cyanobiphenyl (8CB) in AlMCM-41 and cloverite

Abstract The confinement of a nematic liquid crystal (LC), 4-octyl-4′-cyanobiphenyl, in the molecular sieves of AlMCM-41 and cloverite type is investigated by dielectric spectroscopy, thermal analysis and FTIR spectroscopy. We focus herein on the samples containing the LC not only in the pores and cavities, but also on the external surface of the molecular sieve grains. A bulk-like dynamical behaviour was found for these samples. An interaction of the LC molecules with the surface took place by coordination to Lewis sites and by hydrogen bonding with OH groups.

Principle of a compact polarisation mode dispersion controller using homeotropic electroclinic liquid crystal confined single mode fibre devices

In this paper, we discuss the principle of a polarisation controller, with fast response times, combining liquid crystal rotatable waveplates. The device uses a smectic A* liquid crystal material in a homeotropic orientation. This particular structure allows high speed endless control of the polarisation and fine tuning of the induced phase retardation. These characteristics, combined with recent advances in the confinement of liquid crystal between single mode fibre cores, make this solution highly suited to use in polarisation mode dispersion compensation. The design, advantages and limitations of such a solution are briefly discussed.

Kinetics of phase ordering of nematic liquid crystals confined in porous media.

Employing a time-dependent Ginzburg-Landau model, we investigate the influence of a random field on the phase ordering kinetics of nematic liquid crystals. We find that in the scaling regime the effect of random field (slowing down the growth of nematic) dominates over initial conditions for spatial dimensionality d< or =2, whereas for d>2 the random field has all its effect in the "initial-growth" regime. In this last case the mere confinement of liquid crystals is insufficient to produce slow growth of the nematic order.

Kinetics of phase ordering of nematic liquid crystals in a confined geometry

Employing a time-dependent Ginzburg-Landau model, we investigate the phase ordering kinetics of nematic liquid crystals confined in a narrow pore. We consider various orientations of the surface field and compute domain growth laws and scaling functions. In the absence of any anchoring field we find that (and in contrast to a confined binary liquid mixture) mere confinement of the liquid crystals is insufficient to produce slow growth of the nematic domains owing to the vector nature of the order parameter. In the presence of a strong homogeneous anchoring, the system quickly reaches a nematic state. On the other hand, for homeotropic anchoring, the liquid crystal takes on an XY character and the growth law exponent crosses over from 1/2 to 1/4 as the field strength is increased. \textcopyright{} 1996 The American Physical Society.

Nuclear magnetic resonance of confined MBBA

Confinement of liquid crystals in small pores changes their thermodynamic, structural, and dynamical properties. In this study we are concerned with solid, liquid crystalline, and isotropic liquid phase behaviour of the nematogenic compound MBBA confined in small pores of variable diameters. Controlled Pores Glasses (CPG) were used, the morphology of these materials is a three-dimensional connected network of cylindrical pores. The present data investigates the dynamic behaviour of MBBA in porous media over the temperature range from 130 K to 330 K as a function of pores diameters.