Annihilation of point defect pairs in freely suspended liquid-crystal films

Abstract

We study the annihilation of topological defect pairs in the quasi-twodimensional (2D) geometry of freely suspended smectic films. This elementary process is at the basis of all models describing the statistics of complex defect patterns. We prepare pairs with opposite topological charges and retrieve the interaction mechanisms from their trajectories. The square-root dependence of the defect separation on the time until annihilation and the asymmetry in propagation velocities of the opponents predicted by theory are confirmed. The importance of defect orientations is demonstrated. Trajectories are in general curved, depending on the mutual orientations (phase mismatch) of the defects and on the orientation of the pair respective to the far, undisturbed director. The experiments provide the basis for an adaption of the theoretical models to the real complexity of the annihilation.

Observation of Backflow during the Anihilation of Topologocal Defects in Freely Suspended Smectic Films

“…The annihilation dynamics strongly depends on the mutual orientation of the defects [25,26], and on the alignment of the pair respective to the far, undistorted director field [14,26,34]. In the experiment, this orientation cannot be controlled easily.…”

“…When the orientations of the two defects do not match, i.e., when the director field along the straight line connecting the cores is not uniform, the trajectories are curved [25,26,34]. In some cases, very large mismatch angles have been observed (Figure 5).…”

“…In this study, we focus on a particular feature of such topological defects, the annihilation of point disclinations of opposite topological charges in two dimensions. This scenario has been extensively investigated in analytical studies (e.g., Reference [20][21][22][23]), numerical simulations (e.g., Reference [24][25][26][27][28]), and in experiments (e.g., Reference [29][30][31][32][33][34]).…”

“…This approach is described in the 1994 PhD thesis of Chris Muzny, which was directed by Noel Clark [35]. The elementary process of this coarsening, the defect pair coalescence, has been studied experimentally by Missaoui et al [34]. An alternative approach to create multiple defect patterns has been described by Stannarius and Harth [36].…”

“…Our previous study described the annihilation of defect pairs with the topological strengths of s = ±1 [34]. It was demonstrated that the mutual orientation of the defect pairs and their orientation with respect to the surrounding director field plays a crucial role in their dynamics.…”