Dynamic evolution of topological defects around drops and bubbles rising in a nematic liquid crystal.

We report observations of topological defects around drops and bubbles that rise through a vertically aligned nematic liquid crystal. We provide direct evidence for downstream convection of the Saturn-ring defect and its transformation to a hyperbolic point defect. The point defect is convected further in the wake of the drop or bubble as the rising velocity increases. In equilibrium, both defect configurations may persist for long times in the narrow cell. But the point defect sometimes spontaneously opens into a Saturn ring, indicating the latter as the globally stable configuration in the presence of tight wall confinement.