Orientational Transition and Defect Formation in the Electroconvective State of MBBA

Abstract

We have studied the electroconvective behaviour of MBBA, with initial homeotropic boundary conditions, employing a.c. fields acting transversely to the line of sight. Two major types of convective structure occur. They may be described as the distorted-homeotropic (V _D ) and distorted-planar (H _D ) states. The first bifurcation is to the V _D state and takes place at a threshold voltage which varies parabolically with the electrode-separation. The second bifurcation corresponds to V _D -H _D transformation occurring close to turbulence. The H _D structure, once nucleated, propagates in a highly organized and coherently winding manner so that an array of + 1 linear defects form at the axes of circulation. The growth speed of H _D front is strongly voltage dependent. Field induced undulatory structures develop along the V-H interface below the threshold of electroconvection.

Key Words:

• Electroconvection
• orientational transition
• nematic MBBA
• defect formation