Electrohydrodynamic convection in a homeotropically aligned nematic sample.

Abstract

Different features of electroconvection patterns at threshold in a homeotropically aligned nematic sample are studied as a function of the driving frequency and the conductivity of the sample. The primary instability is the bend Fr\'eedericksz transition that spontaneously breaks the rotational symmetry. The director orientation is optically determined in the Fr\'eedericksz distorted state. Stationary normal and oblique convection rolls appear as a secondary instability. A complex time dependence of the pattern at low frequencies involving a reorientation of the rolls together with the underlying director configuration is observed as predicted by theory. For low conductivity samples a transition to traveling waves is detected and the Hopf frequency is measured as a function of the driving frequency for samples with different conductivities.