Patterned states in the nematic phase of flexible-core phenyl benzoate dimers
This study deals with both spontaneously-formed and electrically-induced structures observed in the nematic phase of two dielectrically negative twist-bend nematogens. In planar cells, the nematic layers are inhomogeneous, existing in a quasiperiodic ground state that involves essentially azimuthal director deviations. This phenomenon is understood using a simple model based on the relative flexoelectric and elastic contributions to free-energy. In an external electric field, a variety of instabilities are obtained. In an increasing static field, for example, the initial periodic surface electroconvective instability is followed by the volume periodic flexoelectric instability. Uncommonly, the growth of the latter takes place via nucleation and front propagation. Due to the low bend elastic deformation cost, flexoelectric bands progressively distort as they narrow (mediated by edge dislocations) under an increasing field to eventually form fanlike objects. In the megahertz region, in each half cycle following 0 V, the electroconvective and flexoelectric instabilities appear transiently with the latter setting in at a higher voltage. Above a few hertz, flexoelectric instability ceases; the sequence of patterned states then is oblique roll to bimodal-grid to normal-roll to chevron. Above 100 kHz, periodic wide bands oriented normal to the rubbing axis are obtained at a threshold voltage that decreases with increasing frequency. Our measurement of threshold voltage corresponding to different frequencies (or electrical conductivity and permittivity values) in this regime agrees well with the earlier theoretical predictions relating to the inertial conduction instability.