P. Schiller

Bistability and domain wall motion in smectic C phases induced by strong electric fields

Abstract The director configuration in smectic C phases is bistable in strong electric fields for suitable field directions with respect to the smectic layer planes. Therefore different domains separated by walls are possible. Bistability can be accompanied by director reorientation which causes domain wall motion and pronounced hysteresis effects. Transitions of this type have been investigated both experimentally and theoretically.

Freedericksz transition of planar oriented smectic C phases

Abstract The Freedericksz transition of planar oriented smectic C phases has been investigated experimentally. The optical picture of the effect observed under a polarizing microscope is described in detail. For a number of selected materials the threshold voltage and the switching times were measured. The experimental data were used to estimate the elastic constant and the rotational viscosity of smectic C phases. The experimental results are discussed and interpreted theoretically.

Perturbation theory for planar nematic twisted layers

Abstract As is well known, planar nematic layers exhibit a Freedericksz transition in electric fields. This effect can be strongly influenced by both geometrical and material parameters. Based on a perturbation method an analytical mathematical description is proposed, which is valid for small and intermediate deformation angles.

Phase diagrams of cholesteric films in electric fields

Abstract Planar films of cholesteric liquid crystals exhibit several types of instabilities in electric fields. A periodic equilibrium structure can appear when the field is parallel to the helix axis. Conditions for the occurrence of the modulated phase have been derived by an analytical theory, which describes long wavelength distortions in the vicinity of Lifshitz points.

Stochastic model for the reorientation of molecules around their long axes in a smectic C phase

Abstract In smectic phases molecules can reorientate around their long and short axes. The motion of a molecule around its long axis is non-cooperative and influenced by both stochastic and deterministic forces exerted by neighbouring molecules. Solving the Smoluchovsky equation for such a reorientation process, two-time correlation functions are calculated which are related to frequency-dependent susceptibilities. The results are used for investigating how deterministic forces in higher ordered smectic phases have an effect on microwave spectra. It can be concluded that the quadrupolar ordering produces a splitting of the relaxation frequency. This splitting, which should occur at the transition from the smectic A to the smectic C phase, is only weakly influenced by an additional dipolar (ferroelectric) ordering.

Analytical theory for field induced periodic equilibrium structures in nematic and cholesteric films

Abstract Planar films of cholesteric liquid crystals exhibit an instability in electric or magnetic fields. Depending on a special choice of the elastic constants and the helix pitch either a striped texture appears at a threshold field or an ordinary Freedericksz transition is observed. The type of field-induced transition can be predicted by a simple criterion.

Bifurcation and domain walls in thin films of liquid crystals

Abstract A perturbation method is proposed to determine the bifurcation diagram for instabilities in thin liquid-crystalline layers which are subject to an external magnetic or electric field. Different types of continuous and discontinuous field-induced director reorientations can be classified by using elementary bifurcation theory. When two differently oriented director positions are stable, domains may appear. For weakly distorted samples the structure and velocity of walls between stable differently oriented domains is described by a solitary wavy solution of the non-linear director equation. The results of the perturbation method are applied to nematic and smectic C films.

FREEDERICKSZ TRANSITION IN POLYMERIC HEXATIC LIQUID CRYSTALS

The threshold voltage for the Freedericksz transition of a polymer hexatic liquid crystal depends on the film thickness. This behaviour is attributed to the coupling of the director and the bonds defined by the hexatic order. We fit the results of a simple elastic continuum model to threshold data. The fit is suitable for the estimation of several material constants.