Crossover between flexoelectric stripe patterns and electroconvection in hybrid aligned nematics

We report experimental and theoretical results on the flexoelectric instabi lity and crossover between flexoelectric domains and electroconvection in a hybrid aligned nematic MBBA under d.c. voltage. At threshold a spatially p eriodic flexoelectric deformation in the form of longitudinal domains (alon g the planar director) was observed. With increasing voltage the director d eviation out of the initial plane becomes clearly detectable. Observations of tracers show that hydrodynamic flow develops inside the flexoelectric st ripe pattern in the form of circular orbits perpendicular to the domain str ipes. With further increasing d.c. voltage electroconvection sets in in the form of drifting oblique rolls inside the flexoelectric domains.