Electrohydrodynamic Instabilities in Some Nematic Azoxy Compounds with Dielectric Anisotropies of Different Sign

Electrohydrodynamic instabilities are observed in planar slabs of nematic azoxy compounds with values for Δ ε = ε∥ − ε⊥ between −0.2 and +0.2. Above a threshold voltage for negative Δε the well-known cellular domain pattern is found. For positive Δε a reorientation only is observed in the nematic phase. However, for dc fields at the same voltage, cellular flow is present in the isotropic phase. Moreover, in this case domains can be introduced by a magnetic field H ⊥ E. These different effects cannot be explained by considering stability criteria only. A discussion is given in which the orientating torques of the field on the nematic determine whether an instability leads to domains or not.