Electrohydrodynamic Instabilities Above a Nematic to Smectic a (Or C) Transition

Abstract

R6sumB.-Des observations d'instabilites 6lectrohydrodynamiques dans des materiaux pour lesquels le paramktre de Helfrich 5 2 depend trks fortement de la temperature sont prksentees et discutees. Quand 5 2 tend vers un par valeurs supkrieures la tension seuil du regime de conduction diverge, ainsi que le champ seuil du rkgime dielectrique. Les deux types d'instabilite disparaissent pour 5 2 = 1. Pour 5 2 < 1 il existe encore des instabilites qui sont interpretdes comme provenant de l'amplification de fluctuations de torsion du directeur. Cette amplification peut se produire si le coefficient de friction a s est positif, et si le rapport des conductivitks electriques parallele et perpendiculaire aux molecules all/q est plus petit que un.

Nonstandard electroconvection and flexoelectricity in nematic liquid crystals

“…We mention here that apart from the 4-n-alkyloxyphenyl-4-n'-alkyloxy-benzoate homologous series referred to in this paper, patterns now called as ns-EC have also been seen on a few other substances with σ a < 0 and a < 0 [24,25]. Several ideas had been suggested as possible explanations like, destabilization of twist fluctuations [24], an isotropic mechanism [25,26] as well as the flexoelectric effect [20], but no detailed theoretical analysis has been given at that time, especially not in line with the experiments.…”

Convective Patterns in Liquid Crystals Driven by Electric Field

“…Standard EC (based on the CH mechanism) cannot occur for the material parameter combination a < 0, σ a < 0 [2] except the "α induced" pattern type. Nevertheless, convection associated with roll formation has been observed in ac electric field in the homologous series of N-(p-n-alkoxybenzylidene)-n-alkylanilines, di-n-4-4'-alkyloxyazoxybenzenes and 4-n-alkyloxy-phenyl-4-n'alkyloxy-benzoates [52][53][54]. The characteristics of the patterns: the orientation of the rolls, contrast, frequency dependence of the wavevector and the threshold, director variation in space and time etc.…”

“…As possible explanations, several ideas have been proposed: a hand-waving argument based on "destabilization of twist fluctuations" [52], a possibility of an isotropic mechanism based on the non-uniform space charge distribution along the field [53] and the flexoelectric effect [55][56][57].…”

Electroconvection in nematic mixtures of bent-core and calamitic molecules

“…Electroconvection could experimentally be detected in calamitic nematics even with ε a < 0, σ a < 0 [19][20][21]. Since the SM of EC predicts no instability for the case when the dielectric and the conductivity anisotropies are of the same sign, the observed patterns were classified as nonstandard EC [7,21,22].…”