Defect Dynamics In Roll Structures Of Twisted Nematics

At application of ac voltage exceeding some critical value U >= U-c to the twisted at pi/2 nematic liquid crystal (NLC) sandwiched between transparent electrodes the electroconvective instability is occurred. So that, a periodic roll system is formed, with the rolls axes oriented normally to the initial molecule orientation (so-called director n) in the middle of NLC layer. The distinctive feature of this roll system is a presence of an helical hydrodynamic flows with the opposite direction in the neighboring rolls in addition to the convective (tangential) component of the velocity of the hydrodynamic flow. An arising electroconvective domain structure is usually accompanied by the generation of very diverse defects, which is stationary near the threshold voltage. Among them there exist ordinary singular dislocations as well as nonsingular defects. Of particular interest are the defects with the nonsingular core, which are not observed in the NLCs with the uniform molecule orientation, as well defects with core dissociated in line (so-called "extended core"). Above threshold voltage all defects in domain structure begin to oscillate. Such oscillations are periodic closures and openings of co-aligned axial components of the velocity of the hydrodynamic flow in rolls located on both sides of the defect core (or "zig-zag" oscillation). The specific features of oscillation dynamics of singular and nonsingular defects above threshold voltage have been revealed. It has been found that the average frequency of the "zig-zag" oscillation depends on the defect core size and applied voltage. It slightly decreases with defect core growth and almost linearly increases with voltage. For both singular and nonsingular defects the marked asymmetry in their space-temporal dynamics has been observed. The origin of similar asymmetry has established to be connected with clockwise or counterclockwise direction of director n turn at twisted NLC cell preparation stage.