Suppression of spatially periodic patterns by dc voltage

TL;DR

This study investigates how superimposed dc and ac voltages influence spatially periodic instabilities in nematic liquid crystals, revealing that combined voltages generally suppress pattern formation and significantly alter threshold behaviors.

Contribution

It provides new insights into the suppression mechanisms of spatially periodic patterns by combined dc and ac voltages in nematic liquid crystals, including experimental and theoretical analysis.

Findings

Superposition of dc and ac voltages inhibits pattern formation.

Threshold voltages for instabilities increase with combined voltages.

Dc bias affects electrical conductivity and transition behaviors.

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 behavior of spatially periodic instabilities was obtained.