On the origin of electrohydrodynamic instabilities in smectic C liquid crystals

Abstract

2014 A l'aide d'observation en lumière polarisée, on montre que les instabilités électrohydrodynamiques des smectiques C résultent d'une déformation de courbure (flexion ou flexion et torsion) de l'orientation moléculaire, couplée à un écoulement périodique confinée dans les plans smectiques. La tension de seuil et la période spatiale des instabilités suivent la même dépendance en fréquence que dans les nématiques. Les instabilités « initiales » et « fondamentales » des smectiques C correspondent aux deux regimes de « conduction » et « diélectrique » des instabilités électrohydrodynamiques des nématiques, provoqués par le même mécanisme de Carr-Helfrich. Abstract. 2014 Using polarized light observation, the electrohydrodynamic instabilities (EHDI) of smectic C liquid crystals are shown to result from a nematic like periodic bend-twist or bend of the molecular director, coupled to a periodic hydrodynamic flow restricted inside the smectic layers. The threshold voltage and spatial period of the instabilities follow the same frequency dependence as for the EHDI in nematics. We propose to explain the « initial » and « fundamental » EHDI of smectic C with the Carr-Helfrich mechanism responsible for the « conduction » and « dielectric » EHDI in nematic liquid crystals.

New variants of polymorphism in banana­shaped mesogens with cyano­substituted central core

“…The domain pattern shows the characteristic feature of the electrohydrodynamic ''fundamental'' domains which obviously correspond to the dielectric regime of the SmC phase. 11 In some respects compound C 12 O behaves quite similarly to compound C 14 because the transition SmCAB 2 is first order and the transition SmC-SmA is second order. The electrooptical behaviour is similar to that of compound C 14 which could be followed up to 30 ‡C whereby also large homogeneous domains were obtained.…”

Inhomogeneous Freedericksz effect in a quarter turn twisted achiral smectic C liquid crystal

“…Adding to all these, electroconvection is also observed coexisting with the FS at low-frequencies at which ionic effects are prominent. We may recall here that in planar untwisted SmC samples of several compounds (including HOAB), which are stable against dielectric reorientation, electroconvection has been observed [19,20]. The so-called low-frequency initial and high-frequency fundamental modes of this instability, which appear with different wave vectors, are considered analogues of conduction and dielectric modes in nematics.…”