Nematic Liquid Crystalline Films Research Articles

Non-equilibrium ordering of liquid crystalline (LC) films driven by external gradients in surfactant concentration

HypothesisGradients in the concentration of amphiphiles play an important role in many non-equilibrium processes involving complex fluids. Here we explore if non-equilibrium interfacial behaviors of thermotropic (oily) liquid crystals (LCs) can amplify microscopic gradients in surfactant concentration into macroscopic optical signals. ExperimentsWe use a milli-fluidic system to generate gradients in aqueous sodium dodecyl sulfate (SDS) concentration and optically quantify the dynamic ordering of micrometer-thick nematic LC films that contact the gradients. FindingsWe find that the reordering of the LCs is dominated by interfacial shearing by Marangoni flows, thus providing simple methods for rapid mapping of interfacial velocities from a single optical image and investigating the effects of confinement of surfactant-driven interfacial flows. Additionally, we establish that surface advection and surfactant desorption are the two key processes that regulate the interfacial flows, revealing that the dynamic response of the LC can provide rapid and potentially high throughput approaches to measurement of non-equilibrium interfacial properties of amphiphiles. We also observe flow-induced assemblies of microparticles to form at the LC interface, hinting at new non-equilibrium approaches to microparticle assembly. We conclude that dynamic states adopted by LCs in the presence of surfactant concentration gradients provide new opportunities for engineering complex fluids beyond equilibrium.

A Ginzburg--Landau-Type Problem for Highly Anisotropic Nematic Liquid Crystals

We carry out an asymptotic analysis of a variational problem relevant in the studies of nematic liquid crystalline films when one elastic constant dominates over the others, namely, $ \inf E_\varepsilon(u)$, where $E_\varepsilon(u) := \frac{1}{2}\int_\Omega \left\{\varepsilon\,|\nabla u|^2 + \frac{1}{\varepsilon} \,(|u|^2 - 1)^2 + L \,(\mathrm{div}\, u)^2\right\} \,dx. $ Here $u: \Omega \to \mathbb{R}^2$ is a vector field, $0 < \varepsilon \ll 1 $ is a small parameter, and $L > 0$ is a fixed constant, independent of $\varepsilon$. We identify a candidate for the $\Gamma$-limit $E_0$, which is a sum of a bulk term penalizing divergence and an Aviles--Giga-type wall energy involving the cube of the jump in the tangential component of the $\mathbb{S}^1$-valued nematic director. We establish the lower bound and provide the recovery sequence for this candidate within a restricted class. Then we consider a set of variational problems for $E_0$ arising from various choices of domain geometry and boundary conditions. We demonstrate that the criticality conditions for $E_0$ can be expressed as a pair of scalar conservation laws that share characteristics. We use the method of characteristics to analytically construct critical points of $E_0$ that we observe numerically.

Dimension Reduction for the Landau--de Gennes Model: The Vanishing Nematic Correlation Length Limit

We study nematic liquid crystalline films within the framework of the Landau--de Gennes theory in the limit when both the thickness of the film and the nematic correlation length are vanishingly small compared to the lateral extent of the film. We prove $\Gamma$-convergence for a sequence of singularly perturbed functionals with a potential vanishing on a high-dimensional set and a Dirichlet condition imposed on admissible functions. This then allows us to prove the existence of local minimizers of the Landau--de Gennes energy in the spirit of [R. V. Kohn and P. Sternberg, Proc. Roy. Soc. Edinburgh Sect. A, 111 (1989), pp. 69--84] despite the lack of compactness arising from the high-dimensional structure of the wells. The limiting energy consists of leading order perimeter terms, similar to Allen--Cahn models, and lower order terms arising from vortex structures reminiscent of Ginzburg--Landau models.

Light propagation in a photonic crystal fiber infiltrated with mesogenic azobenzene dyes

In this paper, light propagation in an isotropic photonic crystal fiber as well in a silica-glass microcapillary infiltrated with a mesogenic azobenzene dye has been investigated. It appeared that light spectrum guided inside the photonic crystal fiber infiltrated with the investigated azobenzene dye depends on the illuminating wavelength of the absorption band and on linear polarization. Also, alignment of the mesogenic azobenzene dye molecules inside silica glass microcapillaries and photonic crystal fibers has been investigated. Results obtained may lead to a new design of optically tunable photonic devices. Full Text: PDF ReferencesP. Russell. St. J. "Photonic-Crystal Fibers", J. Lightwave Technol. 24, 4729 (2006). CrossRef T. Larsen, A. Bjarklev, D. Hermann, J. Broeng, "Optical devices based on liquid crystal photonic bandgap fibres", Opt. Exp. 11, 2589 (2003). CrossRef D. C. Zografopoulos, A. Asquini, E. E. Kriezis, A. d'Alessandro, R. Beccherelli, "Guided-wave liquid-crystal photonics", Lab Chip, 12, 3598 (2012). CrossRef F. Du, Y-Q. Lu, S-T. Wu, "Electrically tunable liquid-crystal photonic crystal fiber", Appl. Phys. Lett 85, 2181 (2004) CrossRef D. C. Zografopoulos, E. E. Kriezis, "Tunable Polarization Properties of Hybrid-Guiding Liquid-Crystal Photonic Crystal Fibers", J. Lightwave Technol. 27 (6), 773 (2009) CrossRef S. Ertman, M. Tefelska, M. Chychłowski, A. Rodriquez, D. Pysz, R. Buczyński, E. Nowinowski-Kruszelnicki, R. Dąbrowski, T. R. Woliński. "Index Guiding Photonic Liquid Crystal Fibers for Practical Applications", J. Lightwave Technol. 30, 1208 (2012). CrossRef D. Noordegraaf, L. Scolari, J. Laegsgaard, L. Rindorf, T. T. Alkeskjold, "Electrically and mechanically induced long period gratings in liquid crystal photonic bandgap fibers", Opt. Expr. 15, 7901 (2007) CrossRef M. M. Tefelska, M. S. Chychlowski, T. R. Wolinski, R. Dabrowski, W. Rejmer, E. Nowinowski-Kruszelnicki, P. Mergo, "Photonic Band Gap Fibers with Novel Chiral Nematic and Low-Birefringence Nematic Liquid Crystals", Mol. Cryst. Liq. Cryst. 558(1), 184 (2012). CrossRef S. Mathews, Y. Semenova, G. Farrell, "Electronic tunability of ferroelectric liquid crystal infiltrated photonic crystal fibre", Electronics Letters, 45(12), 617 (2009). CrossRef V. Chigrinov, H-S Kwok, H. Takada, H. Takatsu, "Photo-aligning by azo-dyes: Physics and applications", Liquid Crystals Today, 14:4, 1-15, (2005) CrossRef A. Siarkowska, M. Jóźwik, S. Ertman, T.R. Woliński, V.G. Chigrinov, "Photo-alignment of liquid crystals in micro capillaries with point-by-point irradiation", Opto-Electon. Rev. 22, 178 (2014); CrossRef D. Budaszewski, A. K. Srivastava, A. M. W. Tam, T. R. Woliński, V. G. Chigrinov, H-S. Kwok, "Photo-aligned ferroelectric liquid crystals in microchannels", Opt. Lett. 39, 16 (2014) CrossRef J-H Liou, T-H. Chang, T. Lin, Ch-P. Yu, "Reversible photo-induced long-period fiber gratings in photonic liquid crystal fibers", Opt. Expr. 19, (7), 6756, (2011) CrossRef T. T. Alkeskjold, J. Laegsgaard, A. Bjarklev, D. S. Hermann, J. Broeng, J. Li, S-T. Wu, "All-optical modulation in dye-doped nematic liquid crystal photonic bandgap fibers", Opt. Exp, 12 (24), 5857 (2004) CrossRef K. Ichimura, Y. Suzuki, T. Seki, A. Hosoki, K. Aoki, "Reversible change in alignment mode of nematic liquid crystals regulated photochemically by command surfaces modified with an azobenzene monolayer", Langmuir, 4, 1214 (1988) CrossRef http://www.beamco.com/Azobenzene-liquid-crystals DirectLink K. A. Rutkowska, K. Orzechowski, M. Sierakowski, "Wedge-cell technique as a simple and effective method for chromatic dispersion determination of liquid crystals", Phot. Lett, Poland, 8(2), 51 (2016). CrossRef L. Deng, H.-K. Liu, "Nonlinear optical limiting of the azo dye methyl-red doped nematic liquid crystalline films", Opt. Eng. 42, 2936-2941 (2003). CrossRef J. Si, J. Qiu, J. Guo, M. Wang, K. Hirao, "Photoinduced birefringence of azodye-doped materials by a femtosecond laser", Appl. Opt., 42, 7170-7173 (2008). CrossRef

Dimension Reduction for the Landau-de Gennes Model on Curved Nematic Thin Films

We use the method of $\Gamma$-convergence to study the behavior of the Landau-de Gennes model for a nematic liquid crystalline film attached to a general fixed surface in the limit of vanishing thickness. This paper generalizes the approach that we used previously to study a similar problem for a planar surface. Since the anchoring energy dominates when the thickness of the film is small, it is essential to understand its influence on the structure of the minimizers of the limiting energy. In particular, the anchoring energy dictates the class of admissible competitors and the structure of the limiting problem. We assume general weak anchoring conditions on the top and the bottom surfaces of the film and strong Dirichlet boundary conditions on the lateral boundary of the film when the surface is not closed. We establish a general convergence result to an energy defined on the surface that involves a somewhat surprising remnant of the normal component of the tensor gradient. Then we exhibit one effect of curvature through an analysis of the behavior of minimizers to the limiting problem when the substrate is a frustrum.

Highly anisotropic thermal conductivity of discotic nematic liquid crystalline films with homeotropic alignment.

Discotic nematic liquid crystal (DNLC) films are prepared by thermal treatment and a photo-crosslinking reaction inside sandwiched glass plates. The DNLC films exhibit outstanding in-plane thermal conductivity which is much larger than cross-plane thermal conductivity. The homeotropic alignment and higher crosslinking density of DNLCs in the films further increase the thermal conductivity anisotropy.

Polarized Phosphorescence of Isotropic and Metal‐Based Clustomesogens Dispersed into Chiral Nematic Liquid Crystalline Films

Polarized Phosphorescence of Isotropic and Metal‐Based Clustomesogens Dispersed into Chiral Nematic Liquid Crystalline Films

Dimension Reduction for the Landau-de Gennes Model in Planar Nematic Thin Films

We use the method of $$\Gamma $$ -convergence to study the behavior of the Landau-de Gennes model for a nematic liquid crystalline film in the limit of vanishing thickness. In this asymptotic regime, surface energy plays a greater role, and we take particular care in understanding its influence on the structure of the minimizers of the derived two-dimensional energy. We assume general weak anchoring conditions on the top and the bottom surfaces of the film and the strong Dirichlet boundary conditions on the lateral boundary of the film. The constants in the weak anchoring conditions are chosen so as to enforce that a surface-energy-minimizing nematic Q-tensor has the normal to the film as one of its eigenvectors. We establish a general convergence result and then discuss the limiting problem in several parameter regimes.

Fluctuation-induced interactions in nematics with disordered anchoring energy

We examine fluctuation-induced (pseudo-Casimir) interactions in nematic liquid-crystalline films confined between two surfaces, where one of the surfaces imposes a strong homeotropic anchoring (ensuring a uniform mean director profile), while the other one is assumed to be a chemically disordered substrate exhibiting an annealed distribution of anchoring energies. We employ a saddle-point approximation to evaluate the free energy of interaction mediated between the two surfaces and investigate how the interaction force is influenced by the presence of disordered surface anchoring energy. It is shown that the disorder results in a renormalization of the effective surface anchoring parameter in a way that it leads to quantitative and qualitative changes (including a change of sign at intermediate inter-surface separations) in the pseudo-Casimir interaction force when compared with the interaction force in the absence of disorder.

Pseudo-Casimir interactions across nematic films with disordered anchoring axis

We study the effective pseudo-Casimir interaction forces mediated by a nematic liquid-crystalline film bounded by two planar surfaces, one of which imposes a random (disordered) distribution of the preferred anchoring axis in the so-called easy direction. We consider both the case of a quenched as well as an annealed disorder for the easy direction on the disordered surface and analyze the resultant fluctuation-induced interaction between the surfaces. In the case of quenched disorder, we show that the disorder effects appear additively in the total interaction and are dominant at intermediate inter-surface separations. Disorder effects are shown to be unimportant at both very small and very large separations. In the case of annealed disorder its effects are non-additive in the total inter-surface interaction and can be rationalized in terms of a renormalized extrapolation length.

Influence of a Change in Helical Twisting Power of Photoresponsive Chiral Dopants on Rotational Manipulation of Micro‐Objects on the Surface of Chiral Nematic Liquid Crystalline Films

Herein we report a group of five planar chiral molecules as photon-mode chiral switches for the reversible control of the self-assembled superstructures of doped chiral nematic liquid crystals. The chiral switches are composed of an asymmetrically substituted aromatic moiety and a photoisomerizing azobenzene unit connected in a cyclic manner through methylene spacers of varying lengths. All the molecules show conformational restriction in the rotation of the asymmetrically substituted aromatic moiety in both the E and Z states of the azobenzene units resulting in planar chirality with separable enantiomers. Our newly synthesized compounds in pure enantiomeric form show high helical twisting power (HTP) in addition to an improved change in HTP between the E and Z states. The molecule with a diphenylnaphthalene unit shows the highest ever known initial helical twisting power among chiral dopants with planar chirality. In addition to the reversible tuning of reflection colors, we employed the enantiomers of these five compounds in combination with four nematic liquid crystalline hosts to study their properties as molecular machines; the change in HTP of the chiral dopant upon photoisomerization induces rotation of the texture of the liquid crystal surfaces. Importantly, this study has revealed a linear dependence of the ratio of the difference between HTPs before and after irradiation against the absolute value of the initial HTP, not the absolute value of the change in helical twisting power between two states, on the angle of rotation of micro-objects on chiral nematic liquid crystalline films. This study has also revealed that a change in irradiation intensity does not affect the maximum angle of rotation, but it does affect the speed of rotational reorganization of the cholesteric helix.

Properties of spatial solitons in chiral nematic liquid crystal cells

In this work we demonstrate the properties of spatial solitary waves in chiral nematic liquid crystalline film. Such self-trapped beams were created due to the optical reorientation nonlinearity for the light power of a few tenths of milliwats at the distances of a few millimeters. Additionally, it is shown that the direction of propagation of such nematicons can be modified by changing the light beam input polarization as well as by applying an external electric field. Full Text: PDF

Spatial Solitons in Chiral Nematics

In this work we present for the first time experimental results of measurements of spatial solitons (nematicons) in chiral nematic liquid crystalline film. We measured the propagation of light beam at the distance of a few millimeters and the nonlinear self-focusing was observed for a light power of order of a few tenths of milliwats. Additional, in a waveguide induced by the nematicon, other low-power light beam (signal beam) with different wavelength was trapped. There were also observed the changes of the nematicon direction by changing the position across the nematics layer. The experimental results are in a good agreement with theoretical predictions.

Spatial Solitons in Twisted Nematic Layer

In this work we investigate light beam propagation in twisted nematic liquid crystalline film. Due to the optical reorientational nonlinearity light beam is self-focusing and finally the spatial solitary wave is created. With increasing the nonlinear effect the direction of light beam propagation is also changing. The samples were filled with 6CHBT nematic liquid crystal and the propagation of light beam at the distance of few millimeters was measured. Nonlinear self-focusing was obtained for a light power of few tenths of milliwats.

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.

Nonlinear optical limiting of the azo dye methyl-red doped nematic liquid crystalline films

We describe the nonlinear optical limiting behaviors of the azo dye methyl-red-doped nematic liquid crystalline (NLC) films and report the phenomena of the formation of an interesting dark spot and a dynamic self-action diffraction ring pattern in the far field beyond the sample cell. We present the method of measuring the optical limiting characteristics of the methyl-red-dye-doped NLC films with typical test results obtained in the experiments. We theoretically analyze the optical limiting mechanisms of the methyl-red-dye-doped NLC films and discuss the advantages and restrictions of the methyl-red dye doped liquid crystal limiters, and propose several possible solutions to overcome these restrictions.

Surface Effects on the Dynamics of Liquid Crystalline Thin Films Confined in Nanoscale Cavities

The dynamics of 4-n-pentyl-4‘-cyanobiphenyl (5CB) nematic liquid crystalline thin films have been studied in real time using step-scan Fourier transform infrared spectroscopy (FTIR). In these studies, the liquid crystal was confined in a nanocavity defined and bounded by an interdigitated gold electrode array. The gold microstructures were microfabricated on a zinc selenide (IR-transparent) window. The 5CB interactions with the ZnSe substrate result in surface-induced ordering of the ultrathin layers (on the order of 40 nm). As the films increase in thickness, the nanoscale organization induced by the surface layer becomes a less significant contributor to the overall bulk structure of the sample. Time-resolved FTIR studies have enabled the measurement of rate constants for the orientation and relaxation of the thin films under an applied electric field as a direct function of confinement dimensions. Cell thicknesses ranging from 40 to 300 nm were studied. The measured rate behaviors demonstrate the stron...

On the origin of repulsive forces of nematic liquid-crystalline films in the surface forces apparatus

Starting from the Frank-Oseen free energy, the elastic forces of a thin nematic liquid-crystalline film between a cylinder and a planar substrate are calculated. The boundary conditions are assumed to be homeotropic. The repulsive forces obtained are found to be too small to be responsible for the elastic background frequently measured in surface forces apparatus experiments. Results from grand-canonical-ensemble Monte Carlo simulations which do not take into consideration a global deformation of the director field are sufficient for describing the experimental data. Repulsive forces appear to be a consequence of reorientations within intermediate strata between the fully developed molecular layers in the microscopic structure of the liquid-crystalline film.

Solvation forces in thin films confined between macroscopically curved substrates

The microscopic structure of molecularly thin fluid films confined between solid substrates with macroscopically curved surfaces is investigated by means of grand canonical ensemble Monte Carlo (GCEMC) simulations, in which the thermodynamic state of the film is determined by its chemical potential μ and temperature T. This situation is akin to experiments involving the surface forces apparatus (SFA) in which the film is confined between two crossed cylinders of macroscopic radius R. The key quantity measured directly in SFA experiments is the “force per radius R,” F(h)/R, exerted by the film on the curved surfaces. This “solvation force” can be related to the local stress Tzz(h) normal to the locally planar surfaces, where h is the shortest distance between them. Because Tzz(h) and the microscopic structure of the confined film can be computed by GCEMC, the relation between Tzz and the macroscopically defined quantity F/R can be employed to interpret the dependence of the latter in terms of variations of the film’s local microscopic structure with h. For a liquid-crystalline film it is shown that reorientational effects are distinctly manifest in Tzz(h) but are reflected only weakly in F(h)/R and are therefore likely to be missed if the interpretation of SFA experiments is based solely upon the latter. Pseudo-experimental F(h)/R curves generated by GCEMC for a nematic liquid-crystalline film are in qualitative agreement with recent SFA data [M. Ruths, S. Steinberg, and J. N. Israelachvili, Langmuir 12, 6637 (1996)], which suggests that one may gain deeper insight into the microscopic structure of confined films through a decomposition of experimentally determined solvation forces F(h)/R into Tzz(h) by inverting the integral relation linking the two.

Circularly polarized fluorescence from chiral nematic liquid crystalline films: theory and experiment

A theory was formulated for the description of circularly polarized fluorescence (CPF) from a chiral nematic film in the spectral region outside the selective reflection band. The CPF theory incorporates: (1) the ability of a chiral nematic film to accomplish both circular dichroism and circular polarization and (2) linearly polarized fluorescence from chromophores unaxially aligned in the nematic sublayers comprising the film. Chiral nematic fluid films consisting of a nematic fluid, BDH 18523, and a chiral dopant, cholesteryl oleate, were prepared for hosting 1,6-diphenylhexatriene as a fluorescent dye. The experimentally determined dissymmetry factor using both left- and right-handed circularly polarized excitations was found to be in good agreement with the theoretical prediction with all the system parameters determined a priori. The theory was also employed to furnish insight into the effects of the concentration of the fluorescent dye and chiral nematic film thickness on the dissymmetry factor.