Optical properties of suspensions of organic and inorganic red pigments.

Optical characterization of three red pigments, two organic and one inorganic, was studied in order to provide important and reliable parameters for research and applications. The pigments were C.I. Pigment Red 176, C.I. Pigment Red 57:1, and C.I. Pigment Red 101. All three pigments were used in their transparent form, that is, their radius of gyration was smaller than 250 nm. The particles were suspended in an apolar solvent and absorption, reflection, and scattering of the suspensions were measured by means of a single spectrophotometer and the effective complex refractive index was determined.

Director distortions and singularities in inhomogeneous fields.

The effect of the electric-field inhomogeneity has been experimentally studied by polarizing microscopy in a homeotropic nematic liquid crystal in confined electrode geometries which may be relevant in display applications. Defects related to tilt inversion have been detected by monitoring the transmitted intensity profile as a function of the applied voltage. The position of the defects could be controlled by an additional magnetic field breaking the symmetry of the original arrangement. The phenomenon has been interpreted via numerical calculation of the director distribution using the continuum theory of nematics. The influence of oblique light incidence and of weak anchoring has also been analyzed. Simulations have provided good qualitative agreement with the observations. The method has turned out to be a sensitive tool to detect small misalignment angles between the magnetic field and the cell plane.

Convection-roll instability in spite of a large stabilizing torque.

Two electroconvection (EC) pattern morphologies--a cellular and a subsequent roll pattern--have been detected in the same frequency range in a nematic with positive permittivity and conductivity anisotropies. The frequency dependences of the onset voltages and critical wave numbers have been determined both for homeotropic and planar alignments. It has been proven that both pattern morphologies have a dielectric time symmetry. We also discuss possible sources for the pattern formation in the frame of both the isotropic (Felici-Benard) mechanism, as well as the standard model of EC.

Time reversal of the excitation wave form in a dissipative pattern-forming system.

We discuss the consequences of time reversal of the excitation parameters on the stability of a periodically driven dynamic system. The example chosen, electrohydrodynamic convection of nematics, is a well-investigated dissipative pattern-forming system, usually driven with time-periodic external electric fields E(t) . The dynamic model that describes linear stability at onset can be represented by a set of two differential equations for the amplitudes of the dynamic variables. Floquet analysis reveals an astonishing dynamic property: Under time reversal of any periodic excitation function E(t) , the dynamic equations reproduce the same stability thresholds, pattern types, and critical pattern wavelengths, even when the dynamics of all involved system variables are essentially different for the excitations E(t) and E(-t) . Experimental confirmation of these mathematical predictions is provided by the measurement of threshold curves and the analysis of the time-resolved optical characteristics of the convection roll patterns.

Longitudinal and normal electroconvection rolls in a nematic liquid crystal with positive dielectric and negative conductivity anisotropy.

We study electroconvection patterns that appear above the splay Fréedericksz transition in a bent-core nematic liquid crystal with positive dielectric and negative conductivity anisotropy. In contrast to most of the previously observed convection rolls in nematics, they are not shadowgraph patterns at onset. Unusual effects are observed like a transition between longitudinal and normal rolls, controlled by the excitation frequency, as well as a voltage dependent reorientation of the convection rolls in a metastable domain. We present experimentally determined pattern state diagrams for planar alignment and characterize the patterns optically.

Influence of excitation wave forms and frequencies on the fundamental time symmetry of the system dynamics, studied in nematic electroconvection.

The dynamics of periodically driven nematic electroconvection, a classical dissipative pattern forming system, is studied experimentally and theoretically. We demonstrate that for certain excitation wave forms, the system's dynamic response can be periodic with the excitation or subharmonic, depending on the periodicity of the excitation as control parameter, while for some classes of wave forms, a subharmonic response seems to be principally excluded. In particular, we describe influences of frequency and time symmetry of triangular excitation wave forms. Two intrinsically different routes for the transition to subharmonic dynamics are observed. The time characteristics of the system variables are determined by numerical solution of appropriate model equations and a Floquet analysis. Experimental data are compared to calculations of the model system of two coupled linear differential equations. Results of experiment and model are in excellent agreement.

Fundamental relations between the symmetry of excitation and the existence of spatiotemporal subharmonic structures in a pattern-forming dynamic system.

In electrohydrodynamic convection of nematics, excitation with sine or square waves of period T leads to convection structures in which the charge density, director, and velocity fields perform T-periodic oscillations. Nonconventional waveforms such as sawtooth excitation can lead to patterns with T-periodic as well as T-antiperiodic (subharmonic) dynamics. We consider different classes of excitation fields E(t): such with antisymmetry in the two half periods E(t)=-E(t+T/2), such with time inversion symmetry E(t)=E(-t), and dichotomous waveforms (two alternating values E1, E2) and discuss their influence on the pattern dynamics. From the analysis of linear differential equations that describe the system near threshold, we show analytically that each of these conditions inhibits subharmonic dynamics at onset. Numerical and experimental support of these predictions is provided.

Optical properties of electrohydrodynamic convection patterns: rigorous and approximate methods.

We analyze the optical behavior of two-dimensionally periodic structures that occur in electrohydrodynamic convection (EHC) patterns in nematic sandwich cells. These structures are anisotropic, locally uniaxial, and periodic on the scale of micrometers. For the first time, the optics of these structures is investigated with a rigorous method. The method used for the description of the electromagnetic waves interacting with EHC director patterns is a numerical approach that discretizes directly the Maxwell equations. It works as a space-grid-time-domain method and computes electric and magnetic fields in time steps. This so-called finite-difference-time-domain (FDTD) method is able to generate the fields with arbitrary accuracy. We compare this rigorous method with earlier attempts based on ray-tracing and analytical approximations. Results of optical studies of EHC structures made earlier based on ray-tracing methods are confirmed for thin cells, when the spatial periods of the pattern are sufficiently large. For the treatment of small-scale convection structures, the FDTD method is without alternatives.