Thin Film Of Nematic Liquid Crystal Research Articles

Quantifying spatial alignment and retardation of nematic liquid crystal films by Stokes polarimetry.

Recently developed alignment techniques for liquid crystals enable the definition of arbitrary alignment patterns. We present a method to determine these two-dimensional spatial alignment distributions as well as the retardation of thin nematic liquid crystal films. The method is based on quantifying the influence of the birefringence of such a film on light with known input polarization by measuring the Stokes parameters of light. We show that we are able to distinguish arbitrary alignment patterns unambiguously. In addition, we demonstrate the ability to evaluate the homogeneity of the alignment as well as the thickness or birefringence of the film.

Effects of spatially-varying substrate anchoring on instabilities and dewetting of thin nematic liquid crystal films.

Partially wetting nematic liquid crystal (NLC) films on substrates are unstable to dewetting-type instabilities due to destabilizing solid/NLC interaction forces. These instabilities are modified by the nematic nature of the films, which influences the effective solid/NLC interaction. In this work, we focus on the influence of imposed substrate anchoring on the instability development. The analysis is carried out within a long-wave formulation based on the Leslie-Ericksen description of NLC films. Linear stability analysis of the resulting equations shows that some features of the instability, such as emerging wavelengths, may not be influenced by the imposed substrate anchoring. Going further into the nonlinear regime, considered via large-scale GPU-based simulations, shows however that nonlinear effects may play an important role, in particular in the case of strong substrate anchoring anisotropy. Our simulations show that instability of the film develops in two stages: the first stage involves formation of ridges that are perpendicular to the local anchoring direction; and the second involves breakup of these ridges and formation of drops, whose final distribution is influenced by the anisotropy imposed by the substrate. Finally, we show that imposing more complex substrate anisotropy patterns allows us to reach basic understanding of the influence of substrate-imposed defects in director orientation on the instability evolution.

Electric-field-mediated instability modes and Fréedericksz transition of thin nematic films

Instabilities at the deformable free surface of a thin nematic liquid crystal film can develop interesting patterns when exposed to an external electrostatic field. A general linear stability analysis is performed involving the Ericksen–Leslie governing equations for the dynamics of the nematic film coupled with the anisotropic Maxwell stresses for the electric field to uncover the salient features of these instabilities. The study reveals the coexistence of twin instability modes: (i) long-wave interfacial mode – stimulated when the sole destabilizing influence of the electric field overcomes the Frank bulk elasticity and surface tension force, and (ii) finite-wavenumber mode – engendered by the combined destabilizing influence originating from the anisotropic electric field and Ericksen stress, for the films with positive dielectric anisotropy and weaker Frank bulk elasticity. The results reported here are in contrast with the same obtained from the more frequently employed long-wave approach. The air-to-liquid-crystal filling ratio between the electrodes as well as thermodynamic parameters such as the dielectric anisotropy, Frank elasticity, and director orientations across the film and boundaries are found to play crucial roles in the selection of modes, whereas kinetic parameters such as Leslie viscosity coefficients influence only the time scale of instability. Importantly, at higher field intensities a symmetry-breaking Fréedericksz-type transition of director orientations is found to happen, which also causes the transition of the dominant mode of instability from the long-wave to the finite-wavenumber mode for films with relatively lower values of Frank bulk elasticity and positive dielectric anisotropy.

Optical Acetone Vapor Sensors Based on Chiral Nematic Liquid Crystals and Reactive Chiral Dopants

Accurate monitoring of exposure to organic vapors, such as acetone, is an important part of maintaining a safe working environment and adhering to long‐ and short‐term exposure limits. Here, a novel acetone vapor detection system is described based on the use of a reactive chiral dopant in a nematic liquid crystal thin film, which can accurately monitor the total exposure over a given time frame through changes in the reflection band of the film. It is found that films exposed to 1000 parts per million by volume (ppmv) of acetone vapor for 2 h exhibit a shift in reflection band of 12 nm, with smaller shifts observed at lower concentrations of acetone. Such sensors have potential applications in industrial hygiene.

Electric field generation of Skyrmion-like structures in a nematic liquid crystal.

Skyrmions are particle-like topological objects that are increasingly drawing attention in condensed matter physics, where they are connected to inversion symmetry breaking and chirality. Here we report the generation of stable Skyrmion-like structures in a thin nematic liquid crystal film on chemically patterned patchy surfaces. Using the interplay of material elasticity and surface boundary conditions, we use a strong electric field to quench the nematic liquid crystal from a fully aligned phase to vortex-like nematic liquid crystal structures, centered on patterned patches, which carry two different sorts of topological defects. Numerical calculations reveal that these are Skyrmion-like structures, seeded from the surface boojum topological defects and swirling towards the second confining surface. These observations, supported by numerical methods, demonstrate the possibility to generate, manipulate and study Skyrmion-like objects in nematic liquid crystals on patterned surfaces.

Voltage-controllable liquid crystal waveguide

This study presents an optical waveguide formed by a self-focusing pump beam in a nematic liquid crystal thin film. The pump beam is a polarized high power Gaussian laser beam. Applying a voltage near the Freedericksz transition threshold enhances the optical field of pump beam to reorientate liquid crystal molecules. The reorientation of LC molecules forms the refractive index as a Gaussian-like distribution in the pump beam region. By the assistance of applying electric field, the pump beam can form an optic waveguide by self-focusing effect. A polarized lower power probe beam near the waveguide can be coupled to the waveguide. The coupling efficiency can be controlled by applying voltage.

Evaluation of Alignment of Nematic Liquid Crystal Thin Film by Spectroscopic Ellipsometry

Generally, macroscopic molecular orientation state in liquid crystal layer can be described by Ossen and Frank's continuum theory. However, the validity of applying the continuum theory to nematic liquid crystal thin film (NLCF) has not been confirmed yet. In this study, we experimentally examined NLCF by employing spectroscopic ellipsometer. As a result, theoretical and experimental result represent a good agreement when NLCF thickness is from 60 to 250 nm. Furthermore, the influence of surface roughness on alignment film surface and glass substrate was also examined.

Evaluation of Alignment of Nematic Liquid Crystal Thin Film by Spectroscopic Ellipsometry

Evaluation of Alignment of Nematic Liquid Crystal Thin Film by Spectroscopic Ellipsometry

Quasistatic electric-field-modulated optical pattern transition in a thin nematic liquid-crystal film with a single feedback mirror

We present here the pattern formation in the transverse profile of a continuous-wave laser beam passing through a parallel planar-aligned nematic liquid crystal (NLC) film biased by a quasistatic electric field and then reflected back to the sample cell by a single planar mirror. The effect of the biasing voltage is studied through the ability to change the nonlinearity by modulating the orientation of the NLC molecules electrically. By suitably modulating the quasistatic electric field the optical pattern transition from the hexagon to the roll is achieved. The pattern transition can be explained by the linear stability analysis for the nonlinear liquid crystal film with optical feedback.

Optical phase conjugation and efficient wave front correction of weak light beams by dye-doped liquid crystals

We report the observation of optical phase conjugation by thin nematic liquid crystal films by using submilliwatt incident power without any external applied field. The light-induced average birefringence evaluated by solving the wave equations for the four interacting waves, is in agreement with that typical of the colossal optical nonlinearity recently observed in the same materials. The high conjugated reflectivity values, obtained with the analyzed samples, allows correction of severely aberrated wave fronts of very weak light beams.

The surface-induced static director distribution in thin nematic liquid crystal films: A deuterium nuclear magnetic resonance spectroscopy study

We have studied the static director distribution in thin nematic liquid crystal cells with different film thicknesses and different surface anchoring strengths using a combination of deuterium nuclear magnetic resonance (NMR) spectroscopy and continuum theory. A nematic liquid crystal, 4-pentyl-d2-4′-cyanobiphenyl (5CB-d2) deuteriated in the α position of the pentyl chain, was confined between two glass plates with both weak and strong anchoring conditions; the anchoring strengths were measured by using a saturation voltage method. A series of deuterium NMR spectra, obtained using a quadrupolar echo sequence, was acquired as a function of the applied electric field, which can be used to explore the director deformation. The deuterium NMR spectra predicted by continuum theory involving the magnetic, electric, elastic, and unified surface anchoring energies are found to be in good agreement with experiment. The investigation also reveals that profiles of the director distribution for the case with a magnetic field have a top-hat shape different to the bowler-hat shape, which is well known as the director profile for the case with no magnetic field.

Mechanical Theory for Nematic Thin Films

A mechanical model that describes the stability and structure of thin nematic liquid crystal films between two different isotropic fluids is presented. The model provides expressions for the structural disjoining pressure and the generalized Young−Laplace equation and is used to analyze the film as a single membrane and as two separate interfaces (microscopic model). The model is used to find expressions for the film tension (membrane model), the two film surface tensions (microscopic model), the contact angles of the film with the meniscus, and the film spreading coefficient. Asymmetric single-component nematic liquid crystal films are found to be stable because orientation gradients generate positive disjoining pressures. The disjoining pressure in nematic films scales with the inverse of the film thickness square, and this scaling produces an ordering of the surface tensions, such that the single-membrane film tension is greater than the two interface film tensions (microscopic model), which in turn ar...

Light Scattering by Thin Nematic Liquid Crystal Films

The scattering of light by thin nematic liquid crystal films is studied. The distribution of fluctuation wavevectors relevant to the scattering is peaked around the difference between the wavevectors of the scattered and the incident light ray. The width is inversely proportional to the film thickness. Furthermore, only a discrete set of fluctuation wavevectors is allowed due to restrictions imposed by the boundary conditions. In the case of weak anchoring the relaxation time is a linear function of the thickness for sufficiently small film thicknesses. Similar results may be expected for other confined nematic systems, e.g. for nematics confined to porous media.

Theory of light scattering by thin nematic liquid crystal films.

Light scattering by thin nematic liquid crystal films is considered. The confinement has two important consequences. First, fluctuations with wave vectors not equal to the difference between the wave vectors of the scattered and the incident light ray can contribute to the scattering. The distribution of fluctuation wave vectors relevant to the scattering is peaked around this difference and has a width inversely proportional to the film thickness. Second, only a discrete set of fluctuation wave vectors is allowed due to restrictions imposed by the boundary conditions. Consequently, the relaxation times of the different fluctuation modes depend on the film thickness. It appears that the relaxation time decreases due to the confinement. In the limit of vanishing thicknesses the relaxation time goes linearly to zero with the film thickness. The main conclusions are expected to hold qualitatively for other confined nematic systems, e.g., for nematics confined in porous media. \textcopyright{} 1996 The American Physical Society.

Distortions induced by the K13 surfacelike elastic term in a thin nematic liquid-crystal film.

We have investigated some of the consequences of the inclusion of a nonzero (and fairly large) ${\mathit{K}}_{13}$ term in the elastic free energy of a thin nematic liquid-crystal layer, where ${\mathit{K}}_{13}$ is the splay-bend elastic constant. A sufficiently thin film is predicted to deform spontaneously in zero applied field, for large enough values of ${\mathit{K}}_{13}$. This deformation breaks the mirror symmetry of the film around its midplane and disappears at a critical value of the applied field which is a function of sample thickness. In the particular case where the boundaries favor parallel anchoring, the midplane of the spontaneously deformed layer will contain a nonsingular \ensuremath{\pi} wall. The existence of this anomalous distortion mode leads to dramatic changes in the topology of the Fr\'eedericksz phase diagram: the onset of the Fr\'eedericksz transition is predicted to occur at the critical field for infinitely strong anchoring, and the distorted state eventually becomes unstable with respect to the undistorted configuration as the strength of the applied field is further increased. For small ${\mathit{K}}_{13}$ no spontaneously deformed state occurs and the critical field of the Fr\'eedericksz transition is merely found to be shifted from its value for ${\mathit{K}}_{13}$=0. The possibility of experimentally observing some of these effects is also briefly discussed.

Surface Forces of the Electric Field Induced Flowin Nematic Liquid Crystals

In thin nematic liquid crystal films with homeotropic boundaries, surface forces are measured that are produced by an electric field induced flow. The sample is placed between two horizontal circular glass disks in a torsion viscosimeter, where a torque is exerted on the upper disk by these surface forces. The evolution of the induced flow is registered via this torque. The time constants of this evolution, the viscosity term γ1 - α2 2/ηc and the elastic constant K3 are determined.