Thin-film Liquid Crystal Research Articles

INTERPLAY BETWEEN ART AND SCIENCE IN EDUCATION: “MUSIC” BASED APPROACH IN NANOSCIENCES

Music can be viewed as a structure formed by notes. Different structures in music have potential to yield enormously rich diversity of different melodies. Music is a typical example where a structure defines a property. Similar concepts could be also exploited in education, in presented case in nano-sciences, which are typical representatives of soft materials the structure of which can be strongly manipulated with local geometry and presence of appropriate nanoparticles. The objects of study, named also LC shells, will be exploited as basic unit elements for future soft colloidal crystals. A different arrangement of colloids within the crystal would result in different physical properties in a similar way as different packing of atoms results in different crystals made of real atoms. In presented research will be demonstrated, how relevant basic mechanisms in thin films of nematic liquid crystals could be explained in a classroom and used as a case study, also for explanation of many other physical properties. This research topic is still in its infancy. At this stage only various defect structures in relatively simple geometries (spherical and elliptical) will be analyzed. There is a need to find simple ways to control sensitively the valence of LC shells and in particular to develop strategies to assemble them in crystal structures of desired symmetry. This would allow tailoring specific optical dispersion relations or other physical property of interest and make new ways to teach different physical properties on the »music« based approach. Key words: teaching/learning strategies, »music« based approach, topological defects, localized states, nanoscience, soft materials, liquid crystalline shells.

A Liquid Crystal-Based Passive Badge for Personal Monitoring of Exposure to Hydrogen Sulfide

A new liquid crystal (LC)-based passive dosimeter badge for personal monitoring of exposure to hydrogen sulfide (H2S) gas is reported. When a thin film of LC supported on a surface functionalized with lead perchlorate Pb(ClO4)2 (the LC sensor) is exposed to H2S, the orientation of LC molecules in the film changes from perpendicular to parallel. This reorientation induces a change in the appearance of the LC film when viewed between crossed polarizers. A H2S dosimeter was fabricated by pairing a LC sensor with a glass substrate forming a headspace between the two surfaces, to control diffusion of H2S across the LC film. When the dosimeter is exposed to H2S, a bright front appears as a function of exposure time. An algorithm has been developed to correlate this response length and exposure dose. The dosimeters are functionally stable when subjected to extreme temperature and humidity fluctuations, and are immune to a number of potentially interfering chemicals, except mercaptans. These dosimeters detect H2S at 0.2 ppm TWA (8 hr) with ±20% overall accuracy. The dosimeters were used to monitor the personal exposure of personnel working in an oil refinery. The TWA concentrations measured by the LC-based dosimeters correlate strongly with the NIOSH 1063 method that uses a sorbent tube and a pump followed by laboratory analysis. Thus, the LC-based dosimeters can provide a sensitive tool for on-site assessment of personal exposure to H2S in different environments.

Negative dispersion of birefringence in two-dimensionally self-organized smectic liquid crystal and monomer thin film.

We suggest a method to obtain a negative dispersion (ND) of birefringence using a two-dimensional self-organization of smectic liquid crystal (LC) and monomer molecules. The averaged orientation of the smectic LC was the layer normal direction with the extraordinary refractive index n(e). Meanwhile, the orientation of the monomer molecules was templated by the host-smectic LC and parallel to the layer plane corresponding to the ordinary refractive index n(o). We selected the LC molecules absorbing a shorter wavelength of UV light rather than the polymerized monomers, hence n(e) was more smoothly decreased than n(o) in the visible-wavelength range. Consequently, the birefringence Δn≡n(e)-n(o) was increased with a longer wavelength, thus giving a ND of birefringence. Using the proposed method, the ND of birefringence could be obtained in a single layer, which is desirable for thin flexible applications.

Effect of the synthetic procedure on the properties of thin films of liquid crystals

Liquid-crystal films prepared by different procedures and having different thicknesses were studied. A structural phase transition was found in these films at ∼75°C. The nature of this transition was examined by investigating the temperature dependences of the electric capacity and conductivity of the films; the optical absorption and reflection spectra were also studied. It was concluded that the samples contained a ferroelectric phase. The structure of the films was studied by atomic force microscopy. As a result of the structural studies, the peculiarities of the temperature dependences of conductivity and capacity, in particular, the temperature range in which the phase transition was observed were explained.

Direct observation of liquid crystals using cryo-TEM: specimen preparation and low-dose imaging.

Liquid crystals (LCs) represent a challenging group of materials for direct transmission electron microscopy (TEM) studies due to the complications in specimen preparation and the severe radiation damage. In this paper, we summarize a series of specimen preparation methods, including thin film and cryo-sectioning approaches, as a comprehensive toolset enabling high-resolution direct cryo-TEM observation of a broad range of LCs. We also present comparative analysis using cryo-TEM and replica freeze-fracture TEM on both thermotropic and lyotropic LCs. In addition to the revisits of previous practices, some new concepts are introduced, e.g., suspended thermotropic LC thin films, combined high-pressure freezing and cryo-sectioning of lyotropic LCs, and the complementary applications of direct TEM and indirect replica TEM techniques. The significance of subnanometer resolution cryo-TEM observation is demonstrated in a few important issues in LC studies, including providing direct evidences for the existence of nanoscale smectic domains in nematic bent-core thermotropic LCs, comprehensive understanding of the twist-bend nematic phase, and probing the packing of columnar aggregates in lyotropic chromonic LCs. Direct TEM observation opens ways to a variety of TEM techniques, suggesting that TEM (replica, cryo, and in situ techniques), in general, may be a promising part of the solution to the lack of effective structural probe at the molecular scale in LC studies.

Equilibrium configurations of nematic liquid crystals on a torus.

The topology and the geometry of a surface play a fundamental role in determining the equilibrium configurations of thin films of liquid crystals. We propose here a theoretical analysis of a recently introduced surface Frank energy, in the case of two-dimensional nematic liquid crystals coating a toroidal particle. Our aim is to show how a different modeling of the effect of extrinsic curvature acts as a selection principle among equilibria of the classical energy and how new configurations emerge. In particular, our analysis predicts the existence of stable equilibria with complex windings.

Effect of phase shift between geometrical and chemical patterning in nematic liquid crystal cells: A Monte Carlo study

Thin films of nematic liquid crystals confined to geometrically as well as chemically patterned substrate at one end and a planar substrate with strong anchoring at the other are studied employing non-Boltzmann Monte Carlo methods. We investigate the effect of temperature on the director structures as the system goes through the isotropic-nematic phase transition. The low temperature results show significant deviations from the phase diagram predicted within the continuum approximation, depicted as a function of the tilt angle at the top substrate and the thickness of the cell. Onset of phase biaxiality is observed at very low temperatures, and it increases as the tilt angle at the top substrate is increased, moving away from the normal to the substrate. A phase shift introduced between the geometrical and chemical patterns at the other end also enhances the phase biaxiality of the system to a fairly high value. This seems to provide a convenient experimentally tunable parameter for controlling the symmetry of the director structures.

Multiple chiral topological states in liquid crystals from unstructured light beams

It is shown experimentally that unstructured light beams can generate a wealth of distinct metastable defect structures in thin films of chiral liquid crystals. Various kinds of individual chiral topological states are obtained as well as dimers and trimers, which correspond to the entanglement of several topological unit cells. Self-assembled nested assemblies of several metastable particle-like topological states can also be formed. Finally, we propose and experimentally demonstrate an opto-electrical approach to generate tailor-made architectures.

Rupture mechanism of liquid crystal thin films realized by large-scale molecular simulations

The ability of liquid crystal (LC) molecules to respond to changes in their environment makes them an interesting candidate for thin film applications, particularly in bio-sensing, bio-mimicking devices, and optics. Yet the understanding of the (in)stability of this family of thin films has been limited by the inherent challenges encountered by experiment and continuum models. Using unprecedented large-scale molecular dynamics (MD) simulations, we address the rupture origin of LC thin films wetting a solid substrate at length scales similar to those in experiment. Our simulations show the key signatures of spinodal instability in isotropic and nematic films on top of thermal nucleation, and importantly, for the first time, evidence of a common rupture mechanism independent of initial thickness and LC orientational ordering. We further demonstrate that the primary driving force for rupture is closely related to the tendency of the LC mesogens to recover their local environment in the bulk state. Our study not only provides new insights into the rupture mechanism of liquid crystal films, but also sets the stage for future investigations of thin film systems using peta-scale molecular dynamics simulations.

Multiwall carbon nanotubes doped ferroelectric liquid crystal composites: A study of modified electrical behavior

We systematically investigated the role of carbon nanotubes and their nature of interaction with the high polarization ferroelectric liquid crystal molecules that causes a change in the dynamic behavior of the liquid crystals. The carbon nanotubes were functionalized with carboxyl group (–COOH) before dispersion in order to enhance their stability in the liquid crystal medium. For the systematic investigation of a non linear behavior of dispersed composite systems, results for various physical properties were determined by thermal, morphological and dielectric studies in the planer aligned 5μm thickness cells. An effort has also gone into detail to investigate these properties with varying concentration (0.02wt%, 0.05wt% and 0.1wt%) of multiwall carbon nanotubes. The various carbon nanotubes doped ferroelectric liquid crystal thin film composites have shown enhanced dielectric strength and dielectric permittivity values as compared to the undoped sample.

Suspended Liquid Crystal Thin Film as Phenomenological Model for Cell Membrane

Suspended Liquid Crystal Thin Film as Phenomenological Model for Cell Membrane

Note on the hydrodynamic description of thin nematic films: Strong anchoring model

We discuss the long-wave hydrodynamic model for a thin film of nematic liquid crystal in the limit of strong anchoring at the free surface and at the substrate. We rigorously clarify how the elastic energy enters the evolution equation for the film thickness in order to provide a solid basis for further investigation: several conflicting models exist in the literature that predict qualitatively different behaviour. We consolidate the various approaches and show that the long-wave model derived through an asymptotic expansion of the full nemato-hydrodynamic equations with consistent boundary conditions agrees with the model one obtains by employing a thermodynamically motivated gradient dynamics formulation based on an underlying free energy functional. As a result, we find that in the case of strong anchoring the elastic distortion energy is always stabilising. To support the discussion in the main part of the paper, an appendix gives the full derivation of the evolution equation for the film thickness via asymptotic expansion.

Experimental investigation of one-sided optical transparency in cholesteric liquid crystal thin film

Influence of boundary conditions on optical properties of a layer of cholesteric liquid crystal (CLC) has been investigated. In particular, a sample was obtained which is transparent for circularly polarized light incident from one side and gives losses in the transmission spectrum (within nearly 100 nm wide wavelength range) if light is incident from the opposite side, i.e. exhibits optical diode behavior. Two selective reflection bands were detected in the reflection spectrum of the sample. Spectral position of both selective reflection bands and transmission spectrum are shown to be CLC temperature-controllable.

Observation of two regions of selective light reflection from a thin film of a cholesteric liquid crystal

Two regions of selective light reflection (in the short- and long- wavelength parts of the visible spectrum) from a thin film of a cholesteric liquid crystal (CLC), consisting of the mixture of two CLCs with opposite chirality and a nematic liquid crystal, are experimentally found for the first time. The spectral position of the reflection regions and the separation between them varies depending on the CLC composition and the temperature. The long-wavelength region of reflection corresponds to the region of Bragg reflection from the CLC helix, while the short-wavelength region is probably due to the defects in the structure of the CLC film.

Integrating the NAE Grand Challenges and Holographically-formed Polymer Dispersed Liquid Crystal Thin Films (H-PDLC) into the Kenyan High School Curriculum.

ABSTRACTAccess to cutting-edge technologies in materials science and engineering within K-12 education is a great struggle in developing countries. In this work, a problem-based, hands on set of seven modules for integrating Holographically-formed Polymer Dispersed Liquid Crystal (H-PDLC) Bragg Grating thin films into the Kenyan secondary physics, chemistry and mathematics curriculum is proposed. Through funding provided by the National Science Foundation, a pilot study of the integration of these modules, using the National Academy of Engineering’s (NAE) Grand Challenges for Engineering as a contextual vessel, is carried out. The efficacy of these curriculum-integrated modules in communicating real world materials science and engineering challenges is examined using qualitative and quantitative means. A method for expanding the use of this experience with other graduate students is proposed.

Liquid crystal-based sensors for selective and quantitative detection of nitrogen dioxide

A highly sensitive nitrogen dioxide (NO2) sensor based on orientational transition of a thin film of liquid crystal (LC) supported on a gold surface is reported. Transport of NO2 molecules through the LC film to the LC–gold interface induces an orientation transition in the LC film. The dynamic behavior of the sensor response exhibits a concentration-dependent response rate that is employed to generate an algorithm for quantitative determination of unknown concentrations. Sensitive, selective and reversible detection with minimal effects of environmental fluctuations suggest that these sensors can be used for quantitative NO2 detection for a number of applications.

Analysis of Nematic Liquid Crystals with Disclination Lines

We investigate the structure of nematic liquid crystal thin films described by the Landau--de Gennes tensor-valued order parameter with Dirichlet boundary conditions of nonzero degree. We prove that as the elasticity constant goes to zero a limiting uniaxial texture forms with disclination lines corresponding to a finite number of defects, all of degree 1/2 or all of degree -1/2. We also state a result on the limiting behavior of minimizers of the Chern-Simons-Higgs model without magnetic field that follows from a similar proof.

Interface Induced Crystal Structures of Dioctyl-Terthiophene Thin Films

Temperature dependent structural and morphological investigations on semiconducting dioctyl-terthiophene (DOTT) thin films prepared on silica surfaces reveals the coexistence of surface induce order and distinct crystalline/liquid crystalline bulk polymorphs. X-ray diffraction and scanning force microscopy measurements indicate that at room temperature two polymorphs are present: the surface induced phase grows directly on the silica interface and the bulk phase on top. At elevated temperatures the long-range order gradually decreases, and the crystal G (340 K), smectic F (348 K), and smectic C (360 K) phases are observed. Indexation of diffraction peaks reveals that an up-right standing conformation of DOTT molecules is present within all phases. A temperature stable interfacial layer close to the silica-DOTT interface acts as template for the formation of the different phases. Rapid cooling of the DOTT sample from the smectic C phase to room temperature results in freezing into a metastable crystalline state with an intermediated unit cell between the room temperature crystalline phase and the smectic C phase. The understanding of such interfacial induced phases in thin semiconducting liquid crystal films allows tuning of crystallographic and therefore physical properties within organic thin films.

Ultrathin Reflective Flexible Liquid Crystal Display Film

In this investigation, we present a thin reflective liquid crystal film with a mixed-mode twisted nematic mode. A low-temperature alignment process is used to obtain good orientation of the liquid crystal molecules. A microcell process is utilized to maintain a uniform cell gap of the flexible display under various bending conditions. To simplify the structure of a flexible mixed-mode twisted nematic liquid crystal display film, a layer of Zeonor optical plastic material is used in place of conventional plastic substrates; this material exhibits quarter wave retardation. The ultrathin device exhibits good cell gap uniformity, a high reflectivity of 35%, and a short response time of 6.67 ms.

Microfluidic sensing devices employing in situ-formed liquid crystal thin film for detection of biochemical interactions

Although biochemical sensing using liquid crystals (LC) has been demonstrated, relatively little attention has been paid towards the fabrication of in situ-formed LC sensing devices. Herein, we demonstrate a highly reproducible method to create uniform LC thin film on treated substrates, as needed, for LC sensing. We use shear forces generated by the laminar flow of aqueous liquid within a microfluidic channel to create LC thin films stabilized within microfabricated structures. The orientational response of the LC thin films to targeted analytes in aqueous phases was transduced and amplified by the optical birefringence of the LC thin films. The biochemical sensing capability of our sensing devices was demonstrated through experiments employing two chemical systems: dodecyl trimethylammonium bromide (DTAB) dissolved in an aqueous solution, and the hydrolysis of phospholipids by the enzyme phospholipase A(2) (PLA(2)).