Pattern Formation in a Nematic Liquid Crystal Mixture with Negative Anisotropy of the Electric Conductivity—A Long-Known System with “Inverse” Light Scattering Revisited

Abstract

Pattern formation in binary calamitic liquid crystal mixtures with positive dielectric anisotropy and negative conductivity anisotropy, which attracted attention owing to field-induced light scattering under unusual conditions many years ago, is reinvestigated in the conductive regime. Homeotropic cells with these mixtures exhibit a direct transition to isotropic electroconvection, while planar cells show a Fréedericksz transition to the quasi-homeotropic state and subsequent electroconvection at higher voltages. A temperature-induced change from normal and oblique convection rolls to longitudinal rolls reveals a transition from standard electroconvection to nonstandard electroconvection, which can be attributed to a sign inversion of the conductivity anisotropy. In summary, this system shows an unusually large variety of patterns and effects, which were observed and theoretically considered more recently in other systems of quite different types.

Variable pitch hydrodynamic electro-optic gratings utilising bent liquid crystal dimers

“…TBATPB was chosen as it (and other similar salts) has been shown previously to enhance conduction of LCs. 31,56,100,101 Increase of T NI in mixtures were observed when measured over several months and application of high electric fields, potentially due to the breakdown of one of the lower clearing point components in the mixture caused by the addition of the ions. Hence, comparisons are made at the same reduced temperature (T NI À T) where T NI was checked at the time of each measurement.…”

Electroconvection in Zwitterion‐Doped Nematic Liquid Crystals and Application as Smart Windows

“…[ 10 ] It is well known as one of the first LCD technologies, and has been extensively studied as a model system for the formation of regular nonequilibrium structures, such as rolls, chevrons, squares, fingerprint, bimodal varicose, and cellular or wavy patterns. [ 1,10–12 ] The key parameters determining the formation of EC patterns are the symmetry of the system (parallel or perpendicular boundary conditions), the dielectric permittivity, and the conductivity as well as the anisotropy of the LC, the driving conditions (i.e., frequency and voltage) of the electric field. Depending on these parameters, EC can be divided into standard and nonstandard types which demonstrate clearly different patterns.…”

“…The Carr–Helfrich feedback mechanism is commonly used to theoretically analyze the standard EC phenomenon, [ 13,14 ] while the origin for the nonstandard EC is still subject to an ongoing scientific discussion. [ 10–12,15–17 ]…”

“…For example, tetrabutylammonium tetraphenylborate was added to a nematic LC mixture and found to enlarge the threshold voltage in the nonstandard EC regime. [ 12 ] A similar electrolyte, tetrabutylammonium benzoate, can lead to the formation and stabilization of a large number of umbilics in a square arrangement when doped into a nematic LC. [ 18,19 ] The weak electrolyte model (WEM) has been developed to include the additional ionic dissociation and recombination effect, therefore, explaining a number of unique EC phenomena.…”

Cholesteric Flakes in Motion Driven by the Elastic Force from Nematic Liquid Crystals

“…15−18 Also, electro-hydrodynamic instabilities were explored for smart windows in which scattering stems from the chaotic turbulence created in LCs when charge carriers oscillate under an applied ac electric field. 19−21 A common disadvantage of all these approaches is that in their scattering state they block only a small part of the incoming light as most of the light scatters in the forwarding direction. This provides privacy but cannot exclude light and heat from entering the interior.…”