Suppression of spatially periodic patterns by dc voltage

Abstract

The effect of superposed dc and ac applied voltages on two types of spatially periodic instabilities in nematic liquid crystals, flexoelectric domains (FD) and electroconvection (EC), was studied. The onset characteristics, threshold voltages and critical wave vectors, were determined. We found that in general the superposition of driving with different time symmetries inhibits the pattern forming mechanisms for FD and EC as well. As a consequence the onset extends to much higher voltages than the individual dc or ac thresholds. A dc bias induced reduction of the crossover frequency from the conductive to the dielectric EC regimes and a peculiar transition between two types of flexodomains with different wavelengths were detected. Direct measurements of the change of the electrical conductivity and its anisotropy, induced by the applied dc voltage component, showed that the dc bias substantially affects both parameters. Taking into account the experimentally detected variations of the conductivity in the linear stability analysis of the underlying nemato-hydrodynamic equations, a qualitative agreement with the experimental findings on the onset behaviour of spatially periodic instabilities was obtained.

Electric-field-induced patterns in a hockey-stick nematic

“…conductivity related) effects. We note here that recent studies on standard EC of calamitic nematics at combined (ac + dc) driving pointed out that adding a dc voltage may lead to unexpected increase of U c , including a suppression of pattern formation [38,39], due to dc-voltage-induced reduction of the conductivity and its anisotropy.…”

Influence of DC voltage on the dielectric properties of nematics

“…Some qualitative features of the pattern formation at superposed ac and dc voltages can, however, be estimated by testing the effect of varying σ ⊥ and δσ on the pattern onset voltages. Recent simulations have shown that reduction of δσ, that occurs upon increasing the dc bias voltage, results in the increase of the threshold voltage of electroconvection, independently of the frequency of the ac voltage component [2]. This offers an explanation for the experimental observations and thus resolves the mismatch with theory.…”

“…The results shown above clearly demonstrate that this assumption fails and should be given up if the applied voltage has a dc component. Therefore, if it is not done so, a mismatch between theoretical predictions and experimental observations about the onset characteristics (threshold voltage, critical wave number) of electroconvection patterns is practically unavoidable and actually has been reported for patterns induced by superposed ac and dc voltages [1][2]. A precise description of all ionic processes mentioned above and thus the understanding of the dc-voltage-induced conductivity changes would be a huge challenge for the future development of the theory of electric-field-induced pattern formation.…”

“…1. It is a material used for studying pattern formation in electric field [2][3]. Precise impedance measurements were carried out with the dielectric analyzer Novocontrol Alpha equipped with a ZG4 test interface.…”

“…Experimental studies of the electrooptical properties of nematics, as well as most display applications of them utilize ac driving voltage; investigations under dc voltage are rather scarce. Recently, electroconvection and flexoelectric patterns occurring under superposed action of ac and dc voltages have been studied and, unexpectedly, an inhibition of the pattern formation at high voltages (an increase of the thresholds) was found [1][2]. The fact that conductivity is an important factor determining the threshold voltage, motivated the present testing of the effect of dc bias on the electrical conductivity.…”