Cholesteric-nematic Transition Research Articles

The Contrast of Cholesteric-Nematic Transition in Indused Cholesterics

The contrast characteristics of cholesteric-nematic transition (CNT) in nematic-cholestric mixtures with a low (up to 2weight %) optical active dopant content were analysed.

The effect of a modified surface anchoring potential on the cholesteric–nematic transition

As it is known, the Rapini-Papoular potential is suitable for description of anchoring energy between the liquid crystal and bounding surfaces in the case of small deviations of the director from the easy orientation axis. We use modified surface potential that can describe the anchoring energy at large deviations of the director more adequately. The interaction effect of a cholesteric liquid crystal with a bounding surfaces on the nature and features of cholesteric-nematic phase transitions induced by an external magnetic field is studied. We determine the conditions for existence of three different phases in a cholesteric liquid crystal layer with a finite homeotropic anchoring at the boundaries. The dependencies of the threshold fields of orientational transitions between the phases of a liquid crystal on the material parameters of the cell are calculated. Landau expansions of the total free energy of cholesteric near the transition points are constructed. We determine the tricritical values of the anisotropy parameter of the surface anchoring, which characterize the nature of transitions between the phases of a cholesteric liquid crystal.

Liquid Crystal Auto-Induction and Amplification Function for Circularly Polarized Luminescence (CPL) with High gem-Value, and Dynamically Controllable CPL Devices

Optically active phenylenevinylene derivatives with fluorescence are employed for preparation of a cholesteric liquid crystal based circularly polarized light (CPL) emission device. The device shows intense CPL with a quite large value of the gem-factor (= 0.6) and quick CPL light switching driven by cholesteric-nematic transition with a homeotropic alignment. This research develops the auto-induction of fluorescent chiral inducers in host liquid crystal for formation of a helical structure with CPL amplification. The present research applies a classical LC light-scattering system using chiral technology as a new method for obtaining CPL dynamic control with an intense gem-factor.

Cholesteric–nematic transition induced by a rotating magnetic field

We study theoretically the unwinding of helical orientational structure of a cholesteric liquid crystal under rotating magnetic field action. We analyze the non-stationary rotation regime of the orientational structure at the initial stages of the cholesteric helix unwinding in weak magnetic fields. For small deformations of the orientational structure we analytically obtain the dependence of the pitch of the cholesteric helix on the strength and rotation velocity of a magnetic field. For the stationary rotation regime in a magnetic field, the orientational phase diagram of the cholesteric–nematic transition is constructed. We study numerically and analytically the dependences of the cholesteric helical pitch on the strength and rotation velocity of the magnetic field. We reveal that in the narrow range of rotating magnetic field strength, the reentrant cholesteric–nematic–cholesteric orientational transitions take place. We show that in the stationary rotation regime, an increase in the rotation velocity of the field leads to a decrease in the cholesteric–nematic transition field. In the absence of rotation, our results are reduced to the well-known for static magnetic fields.

Cholesteric–nematic transitions induced by a shear flow and a magnetic field

The untwisting of the helical structure of a cholesteric liquid crystal under the action of a magnetic field and a shear flow has been studied theoretically. Both factors can induce the cholesteric–nematic transition independently; however, the difference in the orienting actions of the magnetic field and the shear flow leads to competition between magnetic and hydrodynamic mechanisms of influence on the cholesteric liquid crystal. We have analyzed different orientations of the magnetic field relative to the direction of the flow in the shear plane. In a number of limiting cases, the analytic dependences are obtained for the pitch of the cholesteric helix deformed by the shear flow. The phase diagrams of the cholesteric–nematic transitions and the pitch of the cholesteric helix are calculated for different values of the magnetic field strength and the angle of orientation, the flow velocity gradient, and the reactive parameter. It is shown that the magnetic field stabilizes the orientation of the director in the shear flow and expands the boundaries of orientability of cholesterics. It has been established that the shear flow shifts the critical magnetic field strength of the transition. It is shown that a sequence of reentrant orientational cholesteric–nematic–cholesteric transitions can be induced by rotating the magnetic field in certain intervals of its strength and shear flow velocity gradients.

Magnetic-field-induced stepwise director reorientation and untwisting of a planar cholesteric structure with finite anchoring energy.

Within the continuum approach we study the equilibrium configurations of a cholesteric liquid crystal confined between two parallel plates, when a magnetic field is applied perpendicularly to the plates. We analyze the role of soft anchoring boundary conditions on magnetic-field-induced cholesteric-nematic transitions in a finite thickness cholesteric cell, treated to induce soft planar alignment. We study the stepwise behavior of cholesteric pitch as a function of the anchoring energy, the thickness of a layer, and the field strength. We analyze some kinds of soft anchoring potentials, including the case of degeneration of the easy axes. We show that the variation of the thickness or intrinsic pitch induces the the stepwise behavior of a pitch of the planar cholesteric structure, and the stepwise variations of the average tensor of diamagnetic susceptibility. The values of these jumps are determined by the anchoring energy. We find the values of critical parameters for the transitions between planar and confocal cholesteric states, and homeotropic nematic state. We show that the variation of the anchoring energy leads to change of the phase transition character; the conditions for hysteresis behavior are obtained. We show that for rather soft anchoring the confocal state is metastable, and the increase of a magnetic field leads to the direct transition between the planar cholesteric and homeotropic nematic phases. We also give a detailed derivation of the threshold and saturation properties of planar cholesteric to homeotropic nematic transition.

Cholesteric-nematic transition in an asymmetric strong-weak anchoring cell

We discuss the cholesteric-nematic transition induced by an external field in a cell in the shape of a slab of finite thickness whose anchoring energy is infinitely strong on one surface and negligible on the other. The case where the surface orientations of the cholesteric director are parallel in the ground state is considered. In a sample of thickness ℓ, multiple of the natural periodicity of the system Λ, the number of complete twists in the cholesteric liquid crystal is reduced by increasing the strength of the external distorting field. The sequence of the field values for expelling complete twists accumulate as the strength of the field approaches a critical value. Beyond this critical value the system assumes an unwinding configuration going toward a uniform structure, corresponding to nematic phase. Generalization of the results to the weak-weak anchoring cell case is also discussed. In our analysis the anisotropic part of the surface energy is assumed or very large or very small with respect to the total energy per unit surface due to the bulk distortion induced by the magnetic field.

Cholesteric pitch transitions induced by mechanical strain

We investigate thickness and surface anchoring strength influence on pitch transitions in a planar cholesteric liquid crystal layer. The cholesteric-nematic transition is also investigated. We assume planar boundary conditions, with strong anchoring strength at one interface and weak anchoring strength at the other. The surface anchoring energy we consider to describe the deviation of the surface twist angle from the easy axis induced by a bulk deformation is a parabolic potential or Rapini and Papoular periodic potential, respectively. We show that under strain, all pitch transitions take place at a critical thickness that is equal to the quarter of the natural cholesteric pitch. The latter result does not depend on the anchoring strength, the particular surface potential, or material properties. The twist angle on the limiting surface characterized by weak anchoring varies with strain either by slipping and or in a discontinuous manner according to the thickness of the sample. The position of the bifurcation point depends only on the ratio of the extrapolation length over the layer thickness, but its value is model dependent. Multistability and multiplicity of the transition are discussed.

Cholesteric pitch-transitions induced by a magnetic field in a sample containing incomplete number of pitches

AbstractWe investigate the pitch transitions induced by an external bulk field in a Cholesteric Liquid Crystal slab of finite thickness ℓ that contains an incomplete number of π-twists. The analysis is performed for a magnetic field that is (i) perpendicular to the helical axis, and (ii) tilted with respect to one of the easy directions imposed by planar and rigid boundary conditions. For finite ℓ we obtain a cascade of transitions, where the bulk expels a half-pitch at a time with increasing field to avoid divergences in the elastic energy. The dependence of the threshold magnetic field inducing the expulsion on the easy axes twist angle δ is investigated for all the cascade of pitch transitions and in particular for the final one, corresponding to the Cholesteric-Nematic transition. In the ℓ → ∞ limit this dependence disappears and we reobtain the results of de Gennes for an infinite sample.

Cholesteric-nematic transition induced by a magnetic field in the strong-anchoring model

We investigate the cholesteric-nematic transition induced by an external bulk field in a sample of finite thickness ℓ. The analysis is performed by considering a tilted magnetic field with respect to the easy direction imposed by rigid boundary conditions inducing planar orientation. In the case of parallel orientation between the magnetic field and of the easy direction, in the limit of ℓ→∞ we reobtain the results of de Gennes where the effective pitch of the cholesteric is a continuous function of the magnetic field diverging at the critical field related to the cholesteric-nematic transition. For finite ℓ we obtain a cascade of transitions, where the bulk expels a half-pitch at a time to avoid divergences in the elastic energy, in a similar manner as solids expel defects in the presence of strong deformation. In the case of oblique orientation between the magnetic field and the easy direction, only the completely untwisted state depends on the tilt angle. Therefore, only the cholesteric-nematic transition depends on the tilt angle while all the other magnetic transition values are unchanged.

New Method of the Threshold Voltages Determination of a Cholesteric-Nematic Transition

We develop the method of determining the threshold voltages of a cholesteric-nematic transition which is based on the analysis of conoscopic images. To improve the measurement accuracy of the threshold voltages and to eliminate the effect of a radiation wavelength, we propose a new method which also could be efficiently used to study the influence of surface conditions on the liquid crystal texture and to obtain the additional information about the liquid crystal texture in the process of cholesteric-nematic transition.

The Cholesteric-Nematic Transition in Thin Layers of Nematic-Cholesteric Mixtures

In this article the investigation of the cholesteric-nematic transition in thin layers of nematic-cholesteric mixtures is carried out. Nematic-cholesteric mixtures based on a nematic matrix MLC-1 with a low concentration (up to 2%) of an optically active dopant were the object of this investigation. The dependencies of threshold voltages of focal-conic deformation and forward cholesteric-nematic transition for different thicknesses of a liquid-crystal layer are obtained. It was demonstrated that the twist effect in induced cholesterics can be considered as a simple case of a cholesteric-nematic effect in thin liquid-crystal layers.

Study of Elastic Constants of Cholesteric–Nematic Mixtures

We propose a technique for determining of elastic constants K22 and K33 of induced cholesterics, which is based on studies of the electro-optic characteristics of cholesteric–nematic transition, dielectric properties, and pitch of induced cholesterics. Instead of the linear approximation of experimental dependencies of the threshold fields, we propose to use the hyperbolic and parabolic approximations of experimental data to determine constants K22 and K33 with increased accuracy.

Influence of the Surfaces on the Nucleation Process During the Cholesteric-Nematic Transition

The nucleation phenomenon in short pitch cholesteric liquid crystals (LC), taking place during the relaxation from the electric field-induced cholesteric-nematic (CN) transition, has been investigated for various boundary conditions. The dynamic hysteresis properties have been studied by direct optical methods. For the first time it is shown the strong evidence that the nucleation is not exceptionally a bulk phenomenon and the supporting solid surfaces may act as nucleation sites. It is confirmed also that the nucleation velocity is not a function of the type of molecular alignment at the liquid crystal/solid interface rather than it depends on the uniformity of the surface interactions with liquid crystal molecules. This result is of importance for time scale optimization in modern fast driving schemes for cholesteric LCDs.

P‐85: Simple Drive Scheme for Bistable Cholesteric LCDs

AbstractIn this paper we discuss a new dynamic drive scheme for bistable reflective cholesteric liquid crystal displays with high multiplexing level. Only two voltage levels (U and 0) are used for the addressing in this drive scheme. Addressing speed is about the time of the fast transition of a cholesteric liquid crystal from the homeotropic state to the transient planar state per line. The maximum value of using voltage is slightly overtop the critical voltage of a cholesteric‐nematic transition. These facts make this drive scheme very perspective in application for such kind of displays.

Surface-Modified Hysteresis of Short Pitch Cholesteric-Nematic Mixtures under Externally Applied Fields

Dynamic hysteresis behaviors are subjected to testing in short pitch cholesteric-nematic liquid crystal layers at the various boundary conditions. Modification of hysteresis properties near the field induced cholesteric-nematic transition was observed for the differently treated boundary layers and inspected by direct optical methods. It was found the interaction energy between liquid crystal (LC) and boundary layers plays rather essential role in nucleation process than director configuration at the interface and if this interaction energy is essential for tangential constrains, the nucleation occurs mainly at the surface interface and then permeate into the bulk gradually. The experimental results show an influence of surface factor even for a big ratio d/Po .

Static and dynamic properties of cholesteric fingers in electric field

In a cholesteric liquid crystal, the rod-like molecules present a helical arrangement in a single direction. The pitch of the helix at equilibrium can be continuously changed by dissolving a chiral molecule in a nematic phase. This helix can also be distorted, and even completely unwound, by confining the liquid crystal between two electrodes treated for homeotropic anchoring. Indeed, this surface treatment imposes a topological frustration which tends to unwind the helical structure and to favor a homeotropic nematic phase. Similarly, applying a voltage between the two electrodes tend to unwind the helix provided that the dielectric anisotropy of the liquid crystal is positive. It is therefore possible to study the cholesteric–nematic transition by changing either the voltage V or the confinement ratio defined as the ratio of the thickness to the pitch ( C= d/ p). It turns out that this transition is usually first order. In this case, we can define a coexistence line V 2( C) between the two phases in the parameter plane (V, C) . The experiment shows that near V 2( C), the cholesteric phase forms localized structures called cholesteric fingers. Three types of fingers exist (at least) and may nucleate, depending on experimental conditions. In a segment of a finger of the first kind (CF-1) the director field is continuous; by contrast, a segment of a finger of the second kind (CF-2) possesses a point defect near each end and a cholesteric finger of the third kind (CF-3) is discontinuous along two twist disclination lines parallel to its axis. Our purpose is to describe the static and dynamical properties of these three kinds of fingers in an AC electric field. In particular, we shall see that due to their symmetry, only the CF-2's can drift perpendicularly to their axes and form spirals or growing loops in an AC electric field. We shall also describe the dynamical properties of the CF-1's and their morphological transitions observed while the voltage is decreased in a free growth experiment. Cholesteric bubbles, that are other metastable configurations, will also be described in close connection with the fingers.

Effect of various external factors and pretransitional phenomena on structural transformations in cholesteric liquid crystals

The publications on low-molecular thermotropic liquid crystals with cholesteric structures have been reviewed. The effect of an applied electric field on cholesteric structures is studied. Bistability of the cholesteric-nematic transition, electric field-induced color textures of cholesteric mixtures, and electro-optics of amorphous cholesteric structures are considered as well as pretransitional (blue and TGB) phases, discotic cholesterics, PDLC films, flexoelectric electro-optics, and photostimulated switching in cholesterics.

The cholesteric-nematic transition in droplets subjected to electric fields

We have observed the behaviour of chiral nematic droplets with positive dielectric anisotropy subjected to electric fields. With increasing field, the Frank-Pryce spherulitic director configuration is distorted progressively into a quasi-cylindrical spiral twist wall. As the field increases further this wall progressively unwinds, resulting in a confined cholesteric-nematic transition. Measurements of the pitch versus electric field show that, despite the confinement, the pitch is still described by the theory for the unconfined cholesteric-nematic transition.

Determination of Elastic Constants of Nematic-Cholesteric Mixtures

The method of elasticity constants K 22 and K 33 determining for nematic-cholesteric mixtures of liquid crystals from the data of electrooptical measurements are described. The theoretical model and experimental results shows that it is necessary to take into consideration the influence of surface forces on threshold voltages of cholesteric-nematic transition.