C.J. Gerritsma

Relaxation of the dielectric constant and electrohydro-dynamic instabilities in a liquid crystal

Abstract Electrohydrodynamic instabilities are studied in a liquid crystal in which the dielectric anisotropy δϵ changes sign because of a relaxation of ϵ ∥ . Above the frequency where δϵ = 0 a new conduction regime is found.

Periodic perturbations in the cholesteric plane texture

Abstract In a cholesteric plane texture identical two-dimensional periodic perturbations can be obtained using an electric field, thermal treatment or mechanical deformation. Results are reported for the period of the pattern as a function of sample thickness and pitch length.

Transient effects in the electrically controlled light transmission of a twisted nematic layer

Abstract An oscillatory after-effect in the light transmission of a twisted nematic layer is often observed when the driving field is switched off. The effect is explained as being caused by fluid motion.

Distortion of a twisted nematic liquid crystal by a magnetic field

Abstract The critical magnetic field H c for distortion of a twisted nematic structure is measured as a function of the sample thickness d . Except for values of d H c · d is constant, in agreement with the theoretical predictions by Leslie.

Electrohydrodynamic Instabilities in Some Nematic Azoxy Compounds with Dielectric Anisotropies of Different Sign

Electrohydrodynamic instabilities are observed in planar slabs of nematic azoxy compounds with values for Δ e = e∥ − e⊥ between −0.2 and +0.2. Above a threshold voltage for negative Δe the well‐known cellular domain pattern is found. For positive Δe a reorientation only is observed in the nematic phase. However, for dc fields at the same voltage, cellular flow is present in the isotropic phase. Moreover, in this case domains can be introduced by a magnetic field H ⊥ E. These different effects cannot be explained by considering stability criteria only. A discussion is given in which the orientating torques of the field on the nematic determine whether an instability leads to domains or not.

Instabilities in electric fields of nematic liquid crytals with positive dielectric anisotropy: domains, loop domains and reorientation

Abstract Instabilities in electric fields are reported for p,p′-di- n -butyl azoxybenzene (DIBAB), a new room temperature nematic with a positive dielectric anisotropy. The threshold voltage is found to be almost independent of the frequency up to 100 kHz. However, unlike in nematics with negative dielectric anisotropy usually no normal domains are observed. The instabilities take the form of a reorientation, while loop domains are observed as a transiet effect. These loop domains indicate that the instability is not purely dielectric in origin.

Magnetic-field-induced motion of disclinations in a twisted nematic layer

Abstract In a twisted nematic layer oppositely twisted regions can exist, separated by a disclination line, as demonstrated by the orientation of Williams domains. The disclination line can move under the influence of a magnetic field. Measurements of the velocity as a function of magnetic field strength and sample thickness lead to an estimate of the size of the disclination core.

Electrohydrodynamic instabilities in nematic liquid crystals

Abstract Contrary to earlier reports nematic liquid crystals with a positive dielectric anisotropy can form domains on application of an electric field. Results are reported for N-p-ethoxybenzylidene-p-amino-benzonitrile (PEBAB).

Changes of twisted nematic liquid-crystal layers by frequency switching of applied electric fields

Abstract Electric fields of low and high frequency were applied successively to a twisted nematic layer consisting of a material with a dielectric anisotropy whose sign depends on the frequency. Six different areas have been observed which are found to have twist angles of ± π /2, ±3 π /2 and ±5 π /2. The formation process of these regions is described in some detail.

The dependence of the electric-field-induced cholesteric-nematic transition on the dielectric anisotropy

Abstract Untwisting of the helical structure by an electric field is studied in a cholesteric liquid crystal for which Δ e decreases at increasing frequency due to relaxation of e ∥ . The threshold field for the cholesteric-nematic transition is found to be proportional to Δ e −0.5 , in agreement with theoretical predictions.