Thin Nematic Films Research Articles

Microscopic Observations and Simulations of Bloch Walls in Nematic Thin Films

We study Bloch wall defects formed by quenching nematic thin films from planar anchoring to homeotropic anchoring through a temperature-driven anchoring transition. The director profiles of the walls are directly visualized using fluorescence confocal polarizing microscopy, and shown to agree well with the simulation based on the Frank elasticity theory. A pure twist wall exists if the ratio of sample thickness to surface extrapolation length p is smaller than or close to 1; while a diffuse Bloch wall is obtained if p is much greater than 1.

Optical Phase Conjugation and Wavefront Correction by Thin Nematic Liquid Crystal Cells

ABSTRACT We report a study of optical phase conjugation in thin nematic cells by using low intensity of the incident beams and without any external applied field. The reported effect exploits the colossal optical nonlinearity recently observed in thin nematic liquid crystals films doped with a small amount of methyl red. The high conjugated reflectivity values obtained with the analysed samples, allows successful correction of severely aberrated wavefronts of very weak light beams.

Sculptured nematic thin films with periodically modulated tilt angle as rugate filters

The reflectances and transmittances of linearly polarized light are calculated using the piecewise homogeneity approximation for a sculptured nematic thin film (SNTF) with a sinusoidally varying (local) columnar tilt angle. Empirical data on TiO 2, available in the literature, are used to realistically model the variation of the permittivity dyadic with the tilt angle. Wave propagation is taken to occur in the morphologically significant plane of the SNTF. Several Bragg peaks occur, with the fundamental peak making a tilt-modulated SNTF suitable for exploitation similarly to a rugate filter. Additionally, the appearance of a spectral hole in the fundamental Bragg peak is demonstrated with the inclusion of a central defect layer.

Slow dynamics of thin nematic films in the presence of adsorbed nanoparticles

Recent experiments indicate that liquid crystals can be used to optically report the presence of biomolecules adsorbed at solid surfaces. In this work, numerical simulations are used to investigate the effects of biological molecules, modeled as spherical particles, on the structure and dynamics of nematic ordering. In the absence of adsorbed particles, a nematic in contact with a substrate adopts a uniform orientational order, imposed by the boundary conditions at this surface. It is found that the relaxation to this uniform state is slowed down by the presence of a small number of adsorbed particles. However, beyond a critical concentration of adsorbed particles, the liquid crystal ceases to exhibit uniform orientational order at long times. At this concentration, the domain growth is characterized by a first regime where the average nematic domain size LD obeys the scaling law LDt approximately t1/2; at long times, a slow dynamics regime is attained for which LD tends to a finite value corresponding to a metastable state with a disordered texture. The results of simulations are consistent with experimental observations.

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.

Role of Delocalized Electrons in Polyaniline−Nematogen Cyanobiphenyls Interaction

Applying an electric field across a nematic liquid crystal display changes the texture of the thin nematic film. When the field is switched off, the texture relaxes to the initial one within seconds. Experimental evidence that very fast relaxation times could appear in some particular cases is reported. A thin film of plasma polymerized aniline induces a stronger planar anchoring of cyanobiphenyl liquid crystals. This strong anchoring appears when, due to an asymmetric conduction of the liquid crystal cell, alternating voltages create a transient electrically charged layer at the polyaniline/liquid crystal interface. This layer favors an expansion of anchoring toward the bulk, thus lowering dramatically the relaxation time and increasing the contrast ratio.

Interference textures of defects in a thin nematic film: an applet presentation

We have developed an applet presentation showing different structures of a nematic liquid crystal confined to a thin transparent plan-parallel cell. The nematic structures possessing single defects or their pairs may be studied. The corresponding interference textures are shown simulating the polarization optic microscopy experiment. Several parameters defining the experimental set-up can be interactively varied enabling observation of subsequent changes in the interference pattern within a reasonable real time. The basic physics of liquid crystal defects necessary for understanding the limitations of the applet presentation is given.

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.

Morphology and structure of thin liquid-crystalline films at nematic-isotropic transition.

We review the main features of very thin nematic liquid-crystalline films on solid substrates, focusing on 5CB on oxidized silicon wafers. By discussing the theoretical aspects of the observed structures, we show that the phenomena at work include isotropic capillary condensation and that the coexistence of isotropic and nematic terraces in thin films is a result of the interplay of several mechanisms. Further theoretical as well as experimental efforts are needed to completely understand the wetting behavior of these systems.

Thermal Fluctuations of Disclination Lines in a Thin Nematic Film

Polarisation microscopy and dynamic light scattering were used to study the dynamical behaviour of disclination lines in a thin nematic film of 5CB. Our experiments show that the lines exhibit thermally excited diffusive motion with the ends of disclination lines being pinned to the surface. In the dynamic light scattering experiments a process with a broad distribution of relaxation times was observed, which was due to the diffusion of the disclination lines with different lengths and orientations. Polarisation microscopy was used to follow the time dependence of the displacement of a single disclination line, which enabled the measurements of the diffusion coefficient and the line tension of the disclination lines in the samples.

Simulations of Topological Defects in Nematic Liquid Crystal Films

We present a detailed computer simulation study of the formation and evolution of topological defects in thin 3D nematic films with schlieren geometry. The differences between the uniaxial and the biaxial case and the influence of the strength of anchoring to the surfaces are analyzed.

Fast Integrated Electro-Optical Switch and Beam Deflector Based on Nematic Liquid Crystal Waveguides

In this paper, the design, the realization, and the experimental characterization of an integrated electro-optical switch and a beam deflector, both based on planar nematic liquid crystal (NLC) waveguides, are reported. In order to test such devices, a particular waveguide structure consisting of a three-stage device, having a thin nematic LC film as the middle stage and two glass waveguides as the other stages, has been designed and realized. The electro-optical behavior and the response time have been studied for different configurations for both TE and TM polarized light. By a proper choice of the material parameters and because of the presence of an additional constant bias voltage, we got response time in the microsecond range. The experimental results obtained are very promising in the frame of integrated electro-optical devices, even if the effect underlying the results is not yet understood and probably needs some deeper research into liquid crystal physics and surface interactions.

Fluctuation-induced forces in liquid crystals: stability of thin nematic films and fine nematic colloidal dispersions

Fluctuation-induced forces in liquid crystals: stability of thin nematic films and fine nematic colloidal dispersions

Binary separation in very thin nematic films: thickness and phase coexistence.

The behavior as a function of temperature of very thin films (10 to 200 nm) of pentylcyanobiphenyl on silicon substrates is reported. In the vicinity of the nematic-isotropic transition we observe a coexistence of two regions of different thicknesses: thick regions are in the nematic state while thin ones are in the isotropic state. Moreover, the transition temperature is shifted downward following a 1/h(2) law ( h is the film thickness). Microscope observations and small-angle x-ray scattering allowed us to draw a phase diagram which is explained in terms of a binary first-order phase transition where thickness plays the role of an order parameter.

Casimir Interactions and Instability of Thin Nematic Films

We review our theoretical studies of mean-field and fluctuation-induced forces together with van der Waals force in a thin hybrid nematic film. We focus on the stability of nematic films frustrated by competing surface fields. Depending on temperature, anchoring strengths, and frustration, the films can be either stable, or they dewet the substrate through spinodal dewetting.

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.

Fingering instabilities in driven thin nematic films

Motivated by experimental work (Cazabet et al., unpublished), we consider the possibility of fingering instabilities in thin films of nematic liquid crystals. We use lubrication theory on the flow equations for nematic liquid crystals to derive a simple model describing the evolution of the film height. As far as we are aware, this is the first time such a systematically derived, time-dependent thin film model for nematics has been presented. Simple “leading-order” solutions (depending on only one spatial coordinate) are found for two different flow driving mechanisms: (i) gravity perpendicular to the film and (ii) gravity parallel to the film (capillarity is also included in both cases). The effect of imposing two-dimensional perturbations to these solutions is studied. We find that for case (i) instability is possible, depending on whether or not there is complete wetting (i.e., whether or not the equilibrium contact angle of the droplet with the substrate is zero). For case (ii) we always have instability, as we would expect from the analogous result for Newtonian fluids [Phys. Fluids 8, 460 (1996); Europhys. Lett. 10, 25 (1989)].

Fast Electro-Optical Switch Based on a Nematic Liquid Crystal Waveguide

An integrated electro-optical switch based on a planar nematic liquid crystal waveguide has been realized and tested. We present the experimental results obtained with a three-stage device, having as middle stage a thin nematic LC film and two glass waveguides as other stages. The electro-optical behavior and the response times have been studied for different configurations. The experimental findings show that an additional bias voltage can improve both the transmittivity and the response time of the device (faster than 100 μs), leading to very promising results in the frame of new integrated electro-optical switches. The effect underlying the latter result is not yet understood and probably need some deep insight into liquid crystal physics and into surface interactions. A further analysis is in progress in order to explain an anomalous electro-optical behavior occurred for higher applied voltages.

On Instability of a Bend Fréedericksz Configuration in Nematic Liquid Crystals

In this paper we discuss the stability of a uniform Fréedericksz configuration in highly anisotropic thin nematic films. We show that the Fréedericksz transition in a bend geometry is at least two-dimensional, and it occurs at field strengths lower than the known classical threshold.

Saddle-splay elasticity and interfacial nematostatics.

This Brief Report drives the generalized force balance equations of interfacial statics between nematic liquid crystals (NLC) and isotropic fluids (I), using the classical equations of liquid crystal physics, taking into account an important class of gradient surface elasticity, known as saddle-splay elasticity. The objective is to identify the exact nature of the saddle-splay contributions to the fundamental interfacial force balance equations, known as the Laplace-Young equation and the Marangoni force equation. General expressions for the dynamic generalization of these two equations were given by Shih, Mann, and Brown [Mol. Cryst. Liq. Cryst. 98, 47 (1983)], but the specific form of the static terms appearing in these two equations were missing in the literature, and are now given in this paper. It is found that the tensorial order and functional form of the contributions of saddle-splay elasticity to the two force balance equations are congruent with those arising from the interfacial tension. Therefore, to generalize the interfacial equations of nematostatics by including saddle-splay energy, the interfacial tension must be renormalized with the saddle-splay energy contribution. In addition, saddle splay gives rise to distortion stresses, the two-dimensional analog to the bulk Ericksen stresses, which contribute to the tangential Marangoni force. Exact expressions for pressure jumps across NLC/I interfaces and for the tangential Marangoni force are derived and analyzed. These generalized results are expected to be useful in the characterization of nematocapillarity phenomena, such as wetting, spreading, and the mechanics of thin nematic films.