Kink-antikink interaction in a linear defect of the electroconvective structure of a nematic

Features of the interaction of two dislocations in a new type of a singular defect appearing in a one-dimensional domain structure at electroconvection in $\pi/2$-twisted nematic liquid crystals have been studied experimentally and theoretically. This type of defects is characterized by a quite extended strain field or the degree of “dissociation”. In this case, domains above the core of the defect are shifted with respect to domains below the core of the defect by half the spatial period. Hydrodynamic flows in domains of twisted nematics have not only the tangential velocity component but also the axial component whose directions in neighboring rolls are opposite. The axial component is responsible for specific features in the oscillation dynamics of various types of defects in twisted nematics. Above a certain critical applied voltage, the defect begins to oscillate so that domains with codirected axial components are closed. As a result, so-called “zigzag” oscillations appear. Simultaneously, a pair of dislocations with opposite topological charges $S = \pm1$ is generated at the edges of the oscillating defect; these dislocations move toward each other. Using the demodulation technique of initial images of domain structures, it has been shown that the collision of dislocations in the extended defect is well described by a two-kink solution of the sine-Gordon equation.