Two-dimensional assemblies of banana-shaped liquid crystal molecules on HOPG surface

Two-dimensional assemblies of a series of banana-shaped liquid crystal molecules, 1,3-phenylene bis[4-(4-n-alkylphenyliminomethyl) benzoates] (P-n-PIMB, n = 12, 14, 18), are investigated on a highly oriented pyrolytic graphite (HOPG) surface by using scanning tunneling microscopy (STM) at a submolecular level in an ambient environment. Two types of molecular arrangements are observed in these self-assemblies with different stripe widths that resulted from the organization of the alkyl chains. The polarization of the P-n-PIMB monolayers is exactly compensated by the adjacent lamellae with opposite bent direction. Furthermore, a bilayer structure is found in the assembly of P-18-PIMB molecules. In comparison with a gemini surfactant molecule, 4,4'-di[4-(octadecylmethylamino)styryl]-1,1'-pentamethylenebispyridinium dibromide (OPD), P-n-PIMB molecules appear in more complicated packing arrangements. The results in this paper demonstrate the effects of the molecular structures on the adlayer arrangement and will be helpful in fabricating two-dimensional molecular structures on solid surfaces.     

Dynamics of surface anchoring breaking in a nematic liquid crystal

Abstract

We present a simple dynamical model which describes the time dependence of the threshold electric field for breaking the surface anchoring of nematics. This model includes a surface friction, resulting from volume hydrodynamic dissipation. Experimental data with AC and DC pulsed fields are explained by this model. For DC, the threshold difference versus the field polarity is well-described by the flexo-electric effects in a uniform field for short times and in a non-uniform field for long times.     

Dynamics of surface anchoring breaking in a nematic liquid crystal

We present a simple dynamical model which describes the time dependence of the threshold electric field for breaking the surface anchoring of nematics. This model includes a surface friction, resulting from volume hydrodynamic dissipation. Experimental data with AC and DC pulsed fields are explained by this model. For DC, the threshold difference versus the field polarity is well-described by the flexo-electric effects in a uniform field for short times and in a non-uniform field for long times.     

Transportation of a microdroplet on an oriented liquid crystal surface

Wetting phenomena play important roles in several technological applications and in many physical and biological thin‐film phenomena, such as wetting, adhesion and friction. One of key issues of these studies is to control the surface energy (or wettability) dynamically for liquid transportation. We have developed a liquid crystal (LC) surface for use as a transport substrate since we expected that the surface energy of an LC surface can be controlled rapidly using an electric field. The rapid control of the polarisability (or wettability) of a liquid crystalline surface by an electric field has been demonstrated, together with the transportation of a liquid microdroplet.     

Are short-pitch bistable ferroelectric liquid crystal cells surface stabilized?

Recent papers have described the short-pitch bistable ferroelectric liquid crystal effect (SBF) attributing its bistability to its stripe layer texture [1]. We have studied the bistability of this SBF effect for the short-pitch ferroelectric liquid crystal (FLC) mixture ZhKS-76 using thin planar aligned cells. Both the SBF texture (the stripe texture) and a uniform texture (uniform tilt layer structure with the extinction direction along the layer normal) were observed in different regions of a given cell when the cell was cooled down slowly from the isotropic phase to the chiral smectic C phase. Upon applying external fields, both regions are characterized by the formation of helix unwinding lines. The stripe area showed zig-zag unwinding lines and the uniform area exhibited straight unwinding lines, both running parallel to the layers. The bistability study shows a similar hysteresis curve and threshold behaviour on switching for both the SBF texture area and the uniform area, although the uniform area gcve better contrast. These facts strongly indicate that as in the long pitch FLCs, the surfaces rather than the layer stripe texture hinder the formation of the helix in the cell, and this produces dynamic bistable switching.     

Thin films of nematic liquid crystal on liquid substrate. Bulk and surface structures,surface and line tensions

Summary The structure of a liquid crystal (L.C) thin film on water has been studied optically and the molecular orientation at its interfaces has been deduced. The surface tension shift corresponding to the orientations of the molecules either normal or parallel to the free surface has been deduced. From the study of the formation and of the stability of holes in the thin film we deduce the order of magnitude of the line tensionx. This tension originates in the elastic deformation of LC in the neighbourhood of the hole.     

Giant surface electroclinic effect in a chiral smectic A liquid crystal

We report the observation of a very large surface electroclinic effect in the smectic A* phase of a chiral liquid crystal. In planar-aligned cells of enantiomerically pure W415, the smectic A* phase grows in from the isotropic state with the layer normal rotated ψ = - 24° from the rubbing direction, a consequence of the surface electroclinic tilt θ_s of the director. The sign of θ_s depends on the molecular handedness, with θ_s ≡ 0 in the racemate, and increasing linearly with moderate enantiomeric excess before saturating as ee → 1. A uniform layer structure can be achieved using cross-rubbed alignment layers, in which case thin cells of W415 in the smectic C* phase display V-shaped (analogue) electro-optic switching.     

The effect of surface polarity of glass on liquid crystal alignment

The effects of the surface polarity of a glass substrate on the orientation of nematic liquid crystals (LCs) were studied using the polarised optical microscope and Fourier-transform infrared spectroscopy. On the surface of oxygen plasma treated glass, a homeotropic alignment of LCs was induced for LCs with negative dielectric anisotropy. This suggests that vertical orientation of LCs could be induced on a polar glass substrate without using an LC alignment layer. Upon cooling towards the isotropic–nematic transition, E7 with positive dielectric anisotropy changes its LC arrangement to isotropic, homeotropic, planar orientations in order. The nematic LC anchoring transition of E7 was interpreted by considering the competition between van der Waals forces and dipole interactions that control the alignment of LC molecules on a polar glass surface.     

Photoelectrochemical reaction of chlorophyll immobilized with liquid crystal on metal surface

Chlorophyll was immobilized with liquid crystal on a platinum surface to prepare a photoexcitable electrode. Liquid crystals such as N-(p-methoxybenzylidene)-p-butylanil-ine were found to be effective on the photoexcitation of the immobilized chlorophyll. Such a chlorophyll-liquid crystal electrode produced photocurrent when it was coupled with a solution of nicotinamide adenine dinucleotide and exposed to light. The electron transfer accompanied by the photoelectrochemical reaction is discussed.     

Optical diode effect of nematic liquid crystal induced by plasma-treated surface

We reported an optical diode effect of a nematic liquid crystal (LC) sample induced by a plasma-treated surface. The LC sample whose one of the substrates was plasma treated transmitted or blocked the incident light depending on the sign of the applied electric field. On the other hand, the LC samples whose both substrates were plasma treated or none of the substrates were plasma treated showed the same transmittance regardless of the sign of the electric field. The optical diode effect was shown when the nematic LC materials contain strong dipole moments, while the same effect was not observed when the weak polar LC was used. A thin polar LC layer formed near the plasma-treated surface makes the net field in the positive and the negative frame to be different, resulting in the optical diode effect.     

Optical rotation at the free surface of a nematic liquid crystal

When a small droplet of a nematic liquid crystal is placed on a horizontal glass plate in the presence of a magnetic field, the plane of polarization of light transmitted upward through the liquid crystal can be rotated. A defect usually forms preferring a direction perpendicular to the magnetic field and forming a diagonal of the droplet. This defect divides the free surface into regions giving rise to optical rotations that are clockwise and counterclockwise. It is suggested that the defect may be similar to a Helfrich splay-bend wall and the optical rotation in the regions near the defect may be explained by surface effects at the free surface of a liquid crystal.     

Field-induced reorientation dynamics of a surface stabilized ferroelectric liquid crystal mixture

Time-resolved measurements of electro-optical response and Fourier-transform infrared absorption have been employed to study the submolecular motion of a surface stabilized ferroelectric liquid crystal (SSFLC) mixture during a field-induced reorientation process. All observed submolecular groups were found to rotate in unison about the layer normal in a steady d.c. field. In a transient situation, the FLC director can respond rapidly to a pulsed driving field. However, the orientation distribution was found to spread at first and then slowly converge to the new direction of the FLC director. When the field is switched off, all the core groups relax to a steady-state direction with varying relaxation times. The results reveal that submolecular fragments of different molecular species in the FLC mixture move correlatively in a steady d.c. field and during a field-induced reorientation process.     

Volume generation towards dynamic surface morphing in liquid crystal polymer networks

ABSTRACT

Thin coatings based on liquid crystal networks (LCNs) modified with azobenzene moieties are able to create dynamic surface topographies in the micrometre range by exposure with UV light. The surface corrugations can be erased and restored by switching ‘off’ and ‘on’ the UV illumination. Various configurations were presented. The formation of the protrusions was proven to be induced mainly by excessive volume formation when the order in the LCNs is reduced. It is suggested that this extra volume formation can be further enhanced by stimulating the oscillatory dynamics of trans-cis and cis-trans isomerisation. Therefore, dual-wavelength exposure not only exciting the trans state of azobenzene by 365 nm UV light but simultaneously also the cis state by 455 nm blue light was shown to enhance the effect.     

Low-birefringence liquid crystal mixtures for photonic liquid crystal fibres application

ABSTRACT

Series of low-birefringence liquid crystal mixtures composed of alkyl alkylbicyclohexyl carbonates and modified mixtures with other compounds have been formulated, their refractive indices and electric permittivity measured upon temperature. They exhibit the ordinary refraction index n_o lower than the refractive index of silica glass in a different range of temperature. This enables to observe in photonic liquid crystal fibres (PLCFs) a change in the light propagation mechanism from photonic band gap guiding to modified total internal reflection at different temperatures. Selected applications of PLCFs are also discussed.     

Relationship between alignment layer polymer surface structures and liquid crystal display parameters

The polar alignment layer (AL) surface provided relatively small liquid crystal (LC) pretilt angles while polyimides with long alkyl side chains gave relatively large LC pretilt angles. The results suggest that LC pretilt angles, in addition to an anchoring effect, are greatly affected by both electronic and steric interactions between LC molecules and a polyimide alignment layer surface. Rubbing with a cotton cloth induces functional groups, side chains, and repeat units at the surface of a liquid crystal polyimide AL to re-orient. It was discovered that rubbing induced polar functional groups and repeat units to re-orient out-of-the-plane of the surface, and it made non-polar aliphatic side chains partially re-orient inwards, toward the bulk of the film.     

Numerical analysis of nematic liquid crystal alignment on asymmetric surface grating structures

The influence of an asymmetric periodic grooved cell surface on the 2D static director configuration of a nematic liquid crystal has been investigated. The minimum in the Frank-Oseen free energy was solved numerically with the Rapini-Papoular form of the surface anchoring energy at the nematic-grating interface. Results are presented for the variation of pretilt angle in the tilted bulk director field as a function of the surface groove depth, pitch and asymmetry and the bulk parameters. The simulations demonstrate the existence of two energetically degenerate high and low pretilted bulk alignment configurations. The pretilt values in these two regimes and also for the low tilt regime with finite surface anchoring are consistent with experimental results. An effective increase in the resolution of the model is obtained by using an irregular grid to describe the surface profile.     

Numerical analysis of nematic liquid crystal alignment on asymmetric surface grating structures

The influence of an asymmetric periodic grooved cell surface on the 2D static director configuration of a nematic liquid crystal has been investigated. The minimum in the Frank-Oseen free energy was solved numerically with the Rapini-Papoular form of the surface anchoring energy at the nematic-grating interface. Results are presented for the variation of pretilt angle in the tilted bulk director field as a function of the surface groove depth, pitch and asymmetry and the bulk parameters. The simulations demonstrate the existence of two energetically degenerate high and low pretilted bulk alignment configurations. The pretilt values in these two regimes and also for the low tilt regime with finite surface anchoring are consistent with experimental results. An effective increase in the resolution of the model is obtained by using an irregular grid to describe the surface profile.     

Structure of the B4 liquid crystal phase near a glass surface

The B4 liquid crystal phase of bent-core molecules, a smectic phase of helical nanofilaments, is one of the most complex hierarchical self-assemblies in soft materials. We describe the layer topology of the B4 phase of mesogens in the P-n-OPIMB homologous series near the liquid crystal/glass interface. Freeze-fracture transmission electron microscopy reveals that the twisted layer structure of the bulk is suppressed, the layers instead forming a structure with periodic layer undulations, with the topography depending on the distance from the glass. The surface layer structure is modeled as parabolic focal conic arrays generated by equidistant parabolas whose foci are defect lines along the glass surface. Nucleation and growth of toric focal conics near the glass substrate is also observed. Although the growth of twisted nanofilaments, the usual manifestation of structural chirality in the B4 phase, is suppressed near the surface, the smectic layers are intrinsically chiral, and the helical filaments that form on top of them grow with specific handedness.     

An addressing effective computer model for surface stabilized ferroelectric liquid crystal cells

A simple uniform director model is discussed which provides, for the first time, an agreement with experimental data for the dynamic optical behaviour of SSFLC cells subject to matrix addressing waveforms. Its use in fast running, interactive software has enabled us to design new, high speed matrix addressing schemes which have been experimentally tested.