P. Van Zanten

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.

Electric-Field-Induced Texture Transformation and Pitch Contraction in a Cholesteric Liquid Crystal

Abstract D.C. electric-field-induced pitch contraction and a simultaneous transition from the cholesteric plane texture to the focal conic texture were studied microsopically in a mixture of 20% cholesteryl chloride and 80% cholesteryl oleyl carbonate. The texture change is preceded by the formation of a periodic pattern in the plane texture. The decrease in helix pitch p is very small for applied fields less than 25 kV/cm: above this threshold field the decrease if greater: dp/dv = 0.33nm/V. In the latter region the increase in reciprocal pitch length is proportional to the applied field, in aggrement with Meyers theory. However, the experimental slope is much less than predicted.

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.

An explanation of the observed field-induced blue shift of an imperfectly aligned planar cholesteric texture

Abstract The imperfectly aligned planar cholesteric texture is found to consist of four different planar regions with differing pitches and threshold fields for static distortions. The electric-field-induced blue shift of the selective light reflections is shown to result from a successive distortion of the planar regions.

Short Communication: Scanning Limitations of Twisted Nematic Display Devices

Alt and Pleshko1 have shown that the maximum number of scanned lines Nmax, in the case of multiplexing rms-responding liquid crystal devices, is directly dependent on the optical response vs. voltage curve of the device. In their analysis the rms-value of the on- and off-voltage is: