Bulk Liquid Crystal Research Articles

High-frequency dielectric relaxation of liquid crystals: THz time-domain spectroscopy of liquid crystal colloids

Terahertz time-domain spectroscopy has been used to study the dielectric relaxation of pure 4'-n-pentyl-4-cyanobiphenyl (5CB) liquid crystal (LC) and its mixtures with 10 mum SiO2 particles in the frequency range 0.2-2 THz. For the pure sample, we find that spatial inhomogeneities consisting of oriented domains, comparable in size to our probe area (~1 mm(2)), cause a large scatter in the measured dielectric function, due to varying contributions from the ordinary and extraordinary components. In the LC/particle mixtures, ordering of the LC at the surface of the SiO2 particles results in a break-up of these domains, giving rise to a spatially much more homogeneous dielectric response. The inferred dielectric function can be interpreted using effective medium theory and the Debye relaxation model. We observe this stabilizing effect for interparticle distances < ~30 mum, setting a lower limit for the size of oriented domains in the bulk LC.

2D 129Xe EXSY of xenon atoms in a thermotropic liquid crystal confined to a controlled-pore glass

Two-dimensional (129)Xe exchange spectroscopy (EXSY) NMR measurements are presented for xenon atoms dissolved in a thermotropic nematic Liquid Crystal (LC), Merck Phase 4, confined to a mesoporous Controlled-Pore Glass (CPG) material with an average pore diameter of 81 A. Experiments were carried out as a function of mixing time at two different temperatures in which Phase 4 appears in nematic and isotropic phases. The exchange rate constants of xenon atoms between two different sites were determined utilizing the intensities of diagonal and off-diagonal signals measured in the EXSY spectra. In the studied system, the sites are: (a) xenon dissolved in the bulk LC between the CPG particles; and (b) xenon in the LC confined inside the pores. The diffusion rate of xenon atoms between the various sites was observed to be very slow.

Tunable Command Layers for Liquid Crystal Alignment

A simple method for the construction of a stable, tunable, self-assembled command layer for liquid crystal display purposes is described. A pyridine-functionalized oligosiloxane spontaneously forms an anisotropic, grooved surface on indium-tin-oxide, enabling it to align liquid crystalline molecules. The pyridine functions act as seeds for the epitaxial growth of stacks of highly ordered zinc phthalocyanines, the height of which can be controlled. These stacks increase the interaction between the surface and the liquid crystalline matrix by amplifying the surface ordering into the liquid crystal bulk. By varying the height of the stacks, direct control over the properties of the liquid crystal domains is achieved. These properties can be further tuned by adding to the liquid crystal, micro- and nanomolar concentrations of nitrogen-containing compounds, which are capable of interacting with and dissolving the stacks. The procedures we describe offer possibilities to use such tunable systems in LCD-based sensor devices as well as in solar-cell applications.

Determination of liquid-crystal polar anchoring energy by electrical measurements

We propose a simple method for the determination of liquid-crystal (LC) polar anchoring energy by electrical measurements. The basic idea of this method is a two-channel scheme for capacitance measurements. The first channel uses one cell with a planar LC cell, while the second a LC cell with vertical alignment. One of the LC cells can have a high pretilt angle. The proposed method allows investigating anchoring properties of both planar and vertical aligned LC materials. Simultaneous measurements of the two cells compensate all volume effects in LC bulk and provide a good opportunity to study directly the LC-surface interaction. The method can be applied for LC cells, which do not have uniform azimuthal orientation. We used this method to investigate the polar anchoring properties of photoaligning material before and after illumination and for LC structures with a high pretilt angle.

Photothermal calorimetry and microcalorimetry in thermally inhomogeneous liquid samples

The implementation of a high resolution photopyroelectric calorimeter for the study of thermally inhomogeneous liquids is reported. The set-up allows the application of electric fields to the sample and a spatially resolved (approx 100 μm) simultaneous measurement of the specific heat, the thermal conductivity and the thermal diffusivity. The thermal parameters of bulk liquid crystal samples and colloidal dispersions of nanoparticles in liquid crystalline compounds have been investigated.

Short pitch cholesteric electro-optical device stabilized by nonuniform polymer network

We have developed a method for stabilization of the uniform lying helix (ULH) texture of short pitch cholesterics in an electro-optical device, based on the flexoelectro-optic effect in such a texture. By using a small concentration of photoreactive liquid crystal monomer (less than 5wt.%) and selecting the illumination conditions, we were able to create a nonuniform polymeric network in the liquid crystal bulk (localized essentially at both substrate surfaces) which stabilized efficiently the amplitude and the phase modulation modes of the device. Most importantly, the effect of the residual birefringence of the polymeric network in the field-unwound state of the device was eliminated resulting thus in a substantial improvement of device performance.

Interplay between nonlocality and nonlinearity in nematic liquid crystals

Spatial nonlocality has a crucial role in the optical response of bulk liquid crystals. With specific reference to one-dimensional modulation instability, we demonstrate the tuning of transverse nonlocality in the nonlinear response of nematics.

Ordered droplet structures at the liquid crystal surface and elastic-capillary colloidal interactions.

We demonstrate a variety of ordered patterns, including hexagonal structures and chains, formed by colloidal particles (droplets) at the free surface of a nematic liquid crystal (LC). The surface placement introduces a new type of particle interaction as compared to particles entirely in the LC bulk. Namely, director deformations caused by the particles lead to distortions of the interface and thus to capillary attraction. The elastic-capillary coupling is strong enough to remain relevant even at the micron-scale when its buoyancy-capillary counterpart becomes irrelevant.

Order parameters of the liquid crystal interface layer at a rubbed polymer surface

In this paper, the liquid crystal (LC) order parameters of the interface layers at rubbed polymer surfaces were studied. The LC films in this study were made with either polyvinyl alcohol or polyimide and the test LCs were filled into wedge-shaped cells for various measurements. The real distribution of order parameters from LC bulk to the interface was obtained by measuring the anisotropic infrared absorbance of sample films. It was found that the order parameters start to decrease where the LC layer thickness is smaller than 10nm, and the order parameter of LC monolayer at the rubbed polymer surface is only 1∕3–1∕2 of that of the LC bulk even in a strong rubbing condition. When the temperature was increased to the transition point, the LC interface layer (excluding the adsorption monolayer) completed the phase transition while the bulk layer remained in LC phase. This was a further evidence that the order parameter of the interface layer is lower than that of the bulk.

Dynamics of the surface layer in cyanobiphenyl–aerosil nanocomposites with a high silica density

Composites were prepared from an aerosil and 4-n-alkyl-4′cyanobiphenyls with five to eight carbon atoms in the alkyl chain. Their high silica density of ∼7 g aerosil in 1 cm3 of liquid crystal (LC) allows the observation of the behaviour of a thin cyanobiphenyl layer (having nearly a monolayer structure) on the silica particles. The systems are investigated by dielectric spectroscopy (10−2–109 Hz) in a large temperature range (220–370 K). All the composites show a (main) relaxation process at frequencies much lower than the processes observed for the bulk LC that was assigned to the dynamics of the molecules in the surface layer. The temperature dependence of its characteristic frequencies obeys the Vogel–Fulcher–Tammann law, which is found to be typical for glass-forming liquids. The quasi two-dimensional character of the glass transition in the surface layer is discussed for the first time. At the nematic-to-isotropic transition temperature of the bulk, the composites show a continuous decrease of the characteristic frequencies as a function of the alkyl chain length, while the bulk LCs show the well known odd–even behaviour. The magnitude and temperature dependence of the slow relaxation process in the composites (molecules on an outer surface) agree with those of the same molecules confined to the nanopores of molecular sieves (internal surface).

Coherent Response Signals of Dipolar-coupled Spin Systems

Recently, it has been demonstrated that long pulses of a weak radio-frequency field can generate long-lived coherent NMR signals in bulk liquid crystals, which are systems of dipolar-coupled spins with unresolved conventional spectra. Here we describe this phenomenon in more detail and present results of new experimental investigations and computer simulations. It is shown that such response signals can also be excited when the initial spin state of a system corresponds to dipolar ordering. In addition, results of the application of weak pulses on liquid crystalline systems with heteronuclear dipolar couplings are presented, and the role of overlapping peaks is explored.

Bistable nematic and smectic anchoring in the liquid crystal octylcyanobiphenyl (8CB) adsorbed on a MoS2 single crystal.

We have studied the anchoring directions imposed on 4-n-octyl-4(')-cyanobiphenyl (8CB) smectic-A and nematic phases by a single crystal of molybdenum disulfide (MoS2). Combining optical microscopy and x-ray diffraction under grazing incidence we have demonstrated the occurrence of a bistable planar anchoring. A previous study of the two-dimensional (2D) network of adsorbed 8CB molecules under the liquid crystal film allows a direct connection to be made between the interface structure and the anchoring directions, demonstrating that bistability is induced by the presence of two dipolar groups in the skeleton of the 2D network. It is demonstrated that the Landau-de Gennes theory cannot account for the observed anchoring in the nematic phase. The Landau-de Gennes free energy has to be associated with a coupling with both the surface order and the MoS2 substrate to explain the experimental observations. The hypothesis of a nematic layer under the liquid crystal bulk is postulated in the smectic phase.

Raman investigation of stable and metastable states of4−octyl−4′-cyanobiphenyl confined in porous silica matrices

We study the polymorphism of solid phases of 4-octyl-4(')-cyanobiphenyl (8CB) by Raman spectroscopy. For bulk 8CB, the Raman spectrum of the CN stretch is featured by a single peak, which shifts abruptly at the smectic-A-crystal transition. In confinement, the CN peak splits both at high and low temperatures. In the isotropic and liquid crystal phases, the signature of the liquid crystal bulk (LC) coexists with another peak that is assigned to LC molecules interacting with the matrice interface. We find correlations between the volumic fractions of interfacial liquid and the texture of the matrices. At low temperatures, we assign the splitting of the CN peak to the coexistence of different metastable solid phases. For strong confinements, the temperature dependence of the CN stretching frequency extends to that of the liquid, which suggests the existence of frozen-in smecticlike solid phases. We discuss the structure of these metastable solid phases in the light of neutron diffraction measurements. We also report on the peculiar analogy between the effect of quenched disorder due to the porous matrices and the effect of thermal quenching.

Extremely sensitive light-induced reorientation in nondoped nematic liquid crystal cells due to photoelectric activation of the interface

We report an investigation of the extremely sensitive molecular reorientation in pure nematic liquid crystal film induced by the combined application of low dc electric field (less than 0.1 V/μm) and very low intensity optical irradiation (few mW/cm2). The effect is observed in planar cells of well-known commercial nematic mixture (E7) aligned with rubbed polyvinyl alcohol layers, which exhibit photorefractive-like effect. We analyze the dependence of the photoinduced changes in birefringence upon the applied dc voltage and the light intensity. According to our results we believe that the effect is due to photoinduced recombination of the opposite charged carriers accumulated near the interface. In the low dc voltage regime (a few volts) the voltage mainly drops on the electric double layers at the interfaces as a consequence of dc field collected charge carriers from liquid crystalline and polymeric films to the border surfaces. Irradiation with appropriate wavelength reduces the interfacial charges density, because of photoinduced carrier injection and recombination processes, and consequently, induces a relocation of the electric field from the surface to the liquid crystal bulk. The light-induced additional electric field component in the nematic film results in a lowering of the Fréedericksz threshold or an enhanced molecular reorientation.

Nematostatics of triple lines.

The Landau-de Gennes model for nematic liquid crystal bulk and interfaces has been extended to nematic triple lines involving the intersection of two isotropic fluids and one nematic liquid crystalline phase. A complete set of bulk, interface, and triple line force and torque balance equations has been formulated. The triple line force and torque balance equations have linear, interfacial, and bulk contributions. The bulk contributions appear as junction integrals, the surface contributions as junctions sums, and the line contributions as gradients of stresses. Reduction of dimensionality from three to one dimensional creates the following effects: (a) bulk terms enter interfacial balances as surface jumps and line balances as junction integrals, and (b) surface terms enter linear balances as junction sums. Line stress and torque equations are derived using classical liquid crystal models. The correspondence between line stress and line torque and their surface and bulk analogs is established. The triple line force and torque balance equations are use to analyze the contact angle in a nematic lens lying at the interface between two isotropic fluids, when the preferred surface orientation is tangential. The effect of anisotropy and long range elasticity on triple line phases is established. Under weak anchoring the contact angle is shown to be a function of the anchoring energy at the nematic-isotropic interface, while under strong anchoring conditions the contact angle is a function of the Peach-Koehler force that originates from bulk long range elasticity and acts on the triple line. The use of the complete set of balance equations removes the classical inconsistency in force balances at a contact line by properly taking into account long range (bulk gradient elasticity) and anisotropic (interfacial anchoring elasticity) effects.

Order parameters of liquid crystal on the rubbing surfaces of alignment layers

Liquid crystal (LC) alignment is most important in LC devices. In this paper, we quantitatively analyze the LC scalar order parameters on the rubbed surface of an alignment layer. Careful measurement of dichroic infrared absorbance is performed. The result gives the evidence that the order parameter of LC just on the rubbed alignment film is only 1/3-1/2 that in the LC bulk.

Brownian ratchets and the photoalignment of liquid crystals

Molecular motors play key roles in areas ranging from biological transport to emerging nanotechnology. They produce current as a result of transfer of energy but not of momentum from a source; many molecular motor scenarios are based on the translational Brownian ratchet mechanism. Here we consider the mechanism of photoalignment of liquid crystals both in the bulk and at the surface by a photosensitive alignment layer. We show that the photoalignment is due to an orientational ratchet mechanism, where the azo-dye molecules, functionalized into a polymer alignment layer, when irradiated by polarized light act as the rotors of Brownian motors which reorient the bulk liquid crystal against an elastic restoring torque. Results of this photoalignment experiment can be obtained directly from a remote experiment set up at the Liquid Crystal Institute, via the WWW. In addition to experimental results, we present a detailed Fokker-Planck description of this system. We discuss the implementation and the results of numerical simulations, and compare these with the experimentally observed dynamics.

Dynamics of a Light Driven Molecular Motor

Molecular motors play key roles in areas ranging from biological transport to emerging nanotechnology. They produce current as a result of transfer of energy but not of momentum from a source; many molecular motor scenarios are based on the translational Brownian ratchet mechanism. Here we consider the mechanism of photoalignment of liquid crystals by a photosensitive alignment layer. We show that the photoalignment is due to an orientational ratchet mechanism, where the azo-dye molecules, functionalized into a polymer alignment layer, when irradiated by polarized light act as the rotors of Brownian motors which reorient the bulk liquid crystal against an elastic restoring torque. Results of this photoalignment experiment can be obtained directly from a remote experiment set up at the Liquid Crystal Institute, via the WWW. In addition to experimental results, we present a detailed Fokker-Planck description of this system. We discuss the implementation and the results of numerical simulations, and compare these with the experimentally observed dynamics.

Filling initiated ion transport processesin liquid crystal cell

The ion adsorption/desorption processes initiated by LC filling in liquid crystal (LC) cells are studied. The LC we used was 5CB and the aligning substrates were UV irradiated polyvinylcinnamate (PVCN) films providing planar alignment of LC. It is shown that intensity of adsorption/desorption processes is strongly determined by purity of LC and irradiation associated parameters, such as exposure dose \ au_{\\exp} and time period between irradiation and LC filling \ au_{\\rm f-i} . To fill the cells we used both 5CB kept in air and 5CB vacuumed at 10 −2 Torr over 1 h. The ion concentration in first portion of LC was estimated to be higher by factor 2 compared with the second one. The cell filling with LC kept in atmosphere is associated with intensive ion adsorption from LC bulk to aligning substrates (when exposure time is zero or when \ au_{\\rm f-I}1\\,h ) or with both adsorption and desorption of ions (in case when \ au_{\\rm f-i} \\ll 1\\,h ). In contrast, cell filling with vacuumed LC mainly initiates ions desorption from the polymer substrates. The latter effect is observed for non-irradiated cells too. This shows that even non-irradiated substrate charges, more likely by adsorption of air ions. The following UV irradiation just increases charging of aligning substrates.

Orientation of liquid crystal monolayers on polyimide alignment layers: A molecular dynamics simulation study

Detailed atomistic molecular dynamics simulations were performed on monolayers of 4′-n-octyl-4-cyanobiphenyl (8CB) adsorbed onto a surface of poly-m-alkanpyromellitimide (poly-m-APM), where m is the number of CH2 units between the imide moieties. Poly-3-APM and poly-4-APM surfaces served as model surfaces to investigate the influence of microscopic grooves, polar carbonyl groups exposed to the surface, and an anisotropic van der Waals interaction between the liquid crystal (LC) molecules and the polymer chains. A Lennard-Jones fluid was chosen as the bulk phase in order to mimic the bulk LC phase. The fluid lubricates the motion of the LC molecules and increases the molecular tilt angle. While microgrooves dominate the alignment of isolated molecules, an anisotropic van der Waals interaction with the main chain is stronger in the case of entire monolayers.