The electrohydrodynamic instability in twisted nematic liquid crystals

(Received 5 July 1993, received in final form 21 October 1993, accepted 9 November 1993)

Abstract
We present some experiments on electrohydrodynamic convection (EHC) in nematic liquid crystals with a planar, but twisted configuration (MBBA). The threshold voltage is shown to depend only weakly on the twist angle. A theoretical analysis of the onset behaviour agrees well with the experiments. More sensitive is a secondary transition to modulation structures, which are not easily observed in untwisted geometries. A rough estimate of the secondary instability based on the behaviour of the most rapidly growing modulation mode seems to account well for the observations. Additional theoretical investigations for materials with positive dielectric anisotropy concerning the competition between the Fréedericksz transition and EHC are included. We predict the existence of a crossover codimension-2 point, which could easily be tested by variation of the twist angle.