Dynamics of defects in electroconvection patterns

Abstract

In homeotropically aligned nematics with negative dielectric anisotropy the electrohydrodynamic instability occurs above a bend Fréedericksz transition. In the presence of a magnetic field parallel to the liquid crystal slab, ordered roll patterns with a well-defined uniform wave vector id appear above the onset of convection. By rotating the cell around an axis perpendicular to the slab by a small angle α, one can manipulate the system into a state with wave vector = id + Δ, where Δ is roughly perpendicular to id. We have studied experimentally the motion of defects, which then move essentially perpendicular to the rolls. The direction as well as the magnitude of the velocity as a function of Δ agrees with predictions of the weakly nonlinear theory. In particular, we obtain evidence for the nonanalyticity for Δ → 0.