Chevron Layer Structure Research Articles

Dynamic Response of Surface-Stabilized Ferroelectric Liquid Crystals with Chevron Layer Structure

The dynamic response of surface-stabilized ferroelectric liquid crystals with chevron layer structure to a triangular wave voltage is studied based on the continuum theory. From systematic numerical calculations, the shape of polarization reversal current is found to be crucially affected by the surface-anchoring condition, and the frequency and the amplitude of applied voltages. The theoretical results are confirmed through comparison with the experimental results.

Study of molecular orientational states of ferroelectric liquid crystals in a surface stabilized geometry

Abstract The molecular orientational states of homogeneously aligned, helix unwinding, chiral smectic C liquid crystals placed in a thin cell (surface-stabilized ferroelectric liquid crystals [SSFLC]) were studied. They were classified by the optical viewing conditions and the relationship between the directions of the chevron layer structure and the surface pretilt. The molecular orientational models of the states were considered and illustrated with regard to the experimental results. The models of molecular orientation give us a total understanding of the orientational states which appear in SSFLCs with parallel rubbing. Furthermore, the effect of the surface pretilt angle on the orientational and optical properties of SSFLCs is discussed.

Elastic Distortions of Compressible SmC*

In this paper, the elastic distortions of compressible smectics are investigated near the boundary of a pair of opposite chevron layer structures on the basis of the elastic theory. The Lagrange multipliers are explicitly taken into account to assure the orthogonality of the wave vector a and the c -director as well as the normalization of c . In addition an external DC electric field effect is also investigated. From numerical computations, it is concluded that there exists a parallelogram region with a flat layer and a uniform c -director configuration between the two opposite chevrons. In addition the external electric field is found to break up the symmetry of the layer structure and result in the layer undulation along the electric field.

On the dynamics of ferro- and antiferro-electric liquid crystals

Abstract In this work, a set of Landau–Ginzburg equations to investigate the dynamic properties of ferro- and antiferro-electric smectic phases is formulated on the basis of the elastic continuum theory of compressible smectics. In the present framework, the polarization electric field is consistently taken into account through the Poisson equation as seen in our previous work. As a practical application, a few numerical results are presented for the surface-stabilized geometry with inclined and chevron layer structures. An asymmetric bistable switching is found to be achieved in the chevron layer structure under an alternating external field. In an inclined layer structure, however, a symmetric switching is found to be possible. In addition, it is first presented from a theoretical standpoint that the compressible smectic layer structure may be drastically deformed in the chevron and inclined layer structures with a sufficiently large external field.

Calculation of chevron profiles in ferroelectric liquid crystal cells

Abstract The energetic aspects of layer deformation in ferroelectric liquid crystal cells are discussed and the actual chevron shape is calculated in some situations. We emphasize two, in our view, essential energy contributions. One term considers the layer curvature. The other one refers to the variations in the distance between layers and the consequent changes of the smectic cone angle. In some simple cases we can determine optimal shapes of the chevron layer structure by analytical solutions, based on these two energy terms. In more complicated situations other contributions have to be considered and the chevron profiles are simulated numerically. The influence of the applied voltage and the choice of parameter values are studied.

On the Layer Dynamics in Smectic Phases. I

In this work a set of dynamic equations to study the elastic properties of the smectic phases is formulated explicitly on the basis of the elastic continuum theory of compressible smectics previously formulated. In this framework, the polarization electric field is consistently taken into account through the Poisson's equation with a polarization electric charge. Then a few numerical results are presented for the surface-stabilised geometry with a bent and a chevron layer structures. It is explored that a bistable switching can be achieved even in the chevron layer structure while it is accompanied with a certain distortion under an alternative high external field. In addition it is first presented in a theoretical aspect that the compressible smectic layer structure may be drastically deformed under such a relatively high electric field and with a sufficiently small molecular tilt angle.

Anchoring Effects on the Layer Structures in Compressible Smectics

In this work, let us investigate anchoring effects on the layer structure in compressible smectic phases. The layer anchoring force is taken into account in an analysis of the chevron layer structure in the smectic phase. Our numerical results show that the anchoring strength comparable to (elastic coefficient)/(penetration length) is enough to orient the molecules at the boundary to such a preferred direction as the SiO columns evaporated on the bounding plates. In a weak anchoring limit, the present result is reduced to the previous one corresponding to the chevron layer structure. In a strong anchoring limit with a small pretilt angle at the boundaries, however, a quasi bookshelf layer structure is found to be realised.

Layer Tilt Angle of Chevron Structure in Surface-Stabilized Ferroelectric Liquid Crystals

Smectic layer structures of some surface-stabilized ferroelectric liquid crystal cells were studied using high-resolution X-ray analysis. The cells were constructed of two glass plates coated with a polymer aligning film rubbed in parallel and exhibited a typical chevron layer structure. The correlation between the layer tilt angle and the molecular tilt angle was investigated in detail, taking into account the differences between the molecular tilt and apparent tilt angles. It was confirmed that the layer tilt angle correlated with the molecular tilt angle though it was slightly smaller, and the ratio of the layer tilt angle to the molecular tilt angle gradually decreased as the molecular tilt angle increased.

Molecular director and layer response of chevron surface stabilized ferroelectric liquid crystals to low electric field

Abstract Recent papers on chevron surface stabilized ferroelectric liquid crystal cells claim that the chevron layer structure can be reversibly uprighted by application of the low to moderate electric fields typically employed to produce director reorientation. In this paper we show, using optical microscopy and X-ray scattering, that there is no significant change in the smectic layer thickness or chevron layer structure of our chevron surface stabilized ferroelectric liquid crystal cells under typical director switching conditions. Furthermore, we present arguments, based on the known elastic properties of smectics, that there is not likely to be a significant elastic layer response to these levels of applied electric field in any surface stabilized ferroelectric liquid crystal cell with anchored layers. Both the switching and observed continuous optical response to applied field can be understood on the basis of electric field induced reorientation of a non-uniform molecular director distribution. We ...

Smectic layer orientation during switching of surface stabilized cells filled with fluorinated ferroelectric liquid crystals

X-ray diffraction study of smectic-C* layer orientation in surface stabilized cells filled with fluorinated ferroelectric liquid crystals show chevron layer structure. The chevrons remain unperturbed up to a field of 12.5 × 106 V/m while optical observations suggest uniform switching at low fields, and nucleation and growth mediated switching at high-field strengths.

Director structures of a chiral Smectic I liquid crystal in the surface stabilized geometry

Abstract The director structures of the Smectic I* (SmI*) liquid crystal 8SI, confined in the surface stabilized ferroelectric liquid crystal cell geometry, were studied using the polarized light microscope. Upon cooling from smectic C* (SmC*) to SmI* phase, the zigzag defects of the C* phase were unchanged indicating that the chevron layer structure remains in the SmI* phase. This phase transition is first order with Sml* domains growing from the SmC* background. Eight stable states were observed in the Sml* phase. These states were divided into two groups of four, separated by the chevron interface transition, revealed by its characteristic boat shaped switching domains. Additionally there were three surface transitions at each bounding glass plate with stripe shaped domains. The polarization P-director n configurations for all eight states are presented.

The field induced stripe texture in surface stabilized ferroelectric liquid crystal cells

Abstract We investigate the texture which appears as parallel stripes perpendicular to the smectic layers in SSFLC cells. When the temperature approaches the smectic A to smectic C* transition from above in the smectic A phase, a DC field induces the chevron structure due to the electroclinic effect and then the stripe texture is formed. We have identified the stripes to be parallel zig-zag walls. We show that a reasonably high DC voltage changes the structure of the cell from chevron to book-shelf in the smectic C* phase and we show how this happens. We have experimentally found that the width of the stripes is approximately equal to the thickness of the cell and we present a theoretical explanation of the equivalence. We have measured the layer tilt angles in the plane of the cell as a function of temperature and have found them to be the same as the chevron tilt angles determined by X-ray scattering. The result confirms the chevron layer structure and stripe texture models and also gives a simple way t...

Optical studies of thin layers of smectic-C materials

The optical behaviour of surface-stabilized ferroelectric liquid crystal samples cannot be described using a uniform director profile. The simplest model based on the chevron layer structure and the surface alignment is a triangular director profile. Analytical expressions for the resulting transmitted light were calculated using the Jones matrix formalism and fitted to experimental data results of a smectic-C host made using an optical multichannel analyser (OMA). This simple model gives an excellent fit, with results that are consistent with X-ray measurements of the chevron structure.

Chevron layer structures in surface stabilized ferroelectric liquid crystal (SSFLC) cells filled with a material which exhibits the chiral nematic to smectic C*phase transition

Abstract Single component liquid crystals which exhibit the chiral nematic (N*) to smectic C* (SC*) phase transition exhibit a large director tilt with respect to the local layer normal (typically 30 degrees or greater) in the SC* phase which appears immediately upon cooling through the N* - SC* transition. In the SSFLC geometry and in the presence of an applied AC field these materials have been observed to align with their molecular long axes oriented along the buffing direction (for a buffed polymer alignment treatment) and with two domains of the smectic layers symmetrically disposed at a large angle (approximately the director tilt angle) with respect to the buffing direction[1]. Furthermore, the layer domains are selectable with the application of a DC field applied normal to the bounding plates. We demonstrate, for the large tilt mixture SCE-10, that the layer domains in all cases adopt the chevron local layer structure[2] immediately upon cooling through the transition. With decreasing temperature...

On the Chevron Structure in Compressible SmC*Phases

Some numerical solutions corresponding to the chevron layer structure in compressible SmC * phases are obtained in terms of the torque balance equations on the basis of a phenomenological elastic free energy including the layer compression energy as well as the c -director deformation energy. Similar to the previously derived analytical solution (M. Nakagawa: Mol. Cryst. & Liq. Cryst. 174 (1989) 65), a soliton-like solution is found to exist as an equilibrium state in SmC * phase. In addition the electric field effect on the layer structure and the c -director configuration in the chevrons is presented taking account of the polarization electric field induced by the polarization charge.

Director reorientation dynamics in chevron ferroelectric liquid crystal cells

Abstract We present sample numerical solutions of the equation of motion that governs the dynamics of molecular orientation in ferroelectric liquid crystal cells with chevron layer structure. We show that the chevron structure significantly influences the director field, the chevron interface providing surface stabilization on a plane interior to the FLC layer. Assuming non-polar nematic-like elasticity in the vicinity of the chevron interface, we have modelled the effects of applied field on cells with purely non-polar cell boundary interactions that have uniform director orientation at zero field, and on cells in which the cell walls are strongly polar and the zero-field states are splayed. The simulations with strongly polar surfaces give bistable operation with the two states having fixed orientations at the FLC-solid surfaces, different orientation of P at the chevron interface, and P splayed in either the upper or lower portion of the cell. A monostable state can arise when the chevron interface is ...

A Method for Computing the Optical Properties of a Smectic C∗ Liquid Crystal Cell with a Chevron Layer Structure

Abstract In this paper a model developed previously for computing the director orientation in a cell containing chiral smectic C liquid crystal (SmC∗ LC) is extended to include a chevron layer structure in the relaxed (zero voltage) state. Results of the director orientation calculations are presented and used in a formalism developed elsewhere to compute the transmittance of a 7 μm thick cell for different azimuthal orientations of the cell between crossed polarisers. It is shown that the theoretical and measured orientations which give minimum transmittance are in good agreement over the entire voltage range considered (0–20 V). The computed average director tilt angle is found to be consistent with conoscopic measurements performed with a polarising microscope. Finally, on the basis of the computed director orientations, an explanation is given of why the zero-voltage minimum transmission orientation angle is so much smaller in thick (∼7 μm) SmC∗ LC cells than it is in thin (∼2 μm) cells.

Theoretical study of the chevron layer structure in smectic phases

In this work, a simple soliton model is proposed to explain the chevron layer structure in smectic phases. It is shown that a compression energy related to the molecular tilt results in the chevron structure in the SmC or SmC ∗ phases, and that a coupling energy between the layer compression and the density change via the SmAN phase transition plays an essential role for that in the SmA phase. The temperature dependence of the layer tilt angle and the transition layer thickness is discussed to examine the validity of the present theoretical results. In addition, the external field effect on the layer tilt angle is investigated within a low field approximation.

Chevron Layer Structure and Parabolic Focal Conics in Smectic A Liquid Crystals

The smectic A layer structures of homogeneously aligned 8CB and 4O.8 liquid crystals have been studied by X-ray diffraction analysis. It has been confirmed that the homogeneously aligned SmA cell is frequently characterized by the chevron layer structure with an array of elongated parabolic focal conics or by the tilted layer structure with the undulation pattern texture. This behavior is caused by the layer thinning and is generally affected by the cell thickness. The layer tilt angle of the chevron structure often depends on a phase sequence and a thermal history; for example, when a homogeneously aligned 4O.8 cell has cooled down to a SmB phase from a SmA phase, the layer tilt angle in the SmA phase in the heating process becomes two times larger than that in the cooling process.

A Soliton Model for the Chevron Layer Structure in SmC* Liquid Crystals

Abstract In this work, a simple explanation is given for the chevron layer structure which has been observed in SmC* phase. A phenomenological elastic free energy concerned with layer distortion, c−director deformation, and their coupling is proposed to discuss the layer structure of them. It will be shown that there exists a soliton solution corresponding to the chevron structure. The transition layer thickness of the chevron is theoretically estimated as the penetration length (∼molecular length)/molecular tilt angle. In similar to the kinked layer structure, the c-director rotation angle is also found to show a soliton-like behaviour within the transition region. In addition, the existence of the selective pre-tilt at the bounding surface is explained in the relation with the chevron structure.