Behavior of Superparamagnetic Microbeads in Smectic Films*

We investigate the behaviour and magnetic response of superparamagnetic, silica-coated iron oxide microbeads embedded in freely suspended films of the smectic A liquid crystal 8CB. The microbeads, which are 5 mm in diameter, are mixed into the liquid crystal material at the edge of the film holder before the films of a few smectic layers are spread. Small aggregates and short chains of beads are observed in the film a few minutes after spreading, with longer chains forming along layer steps and at the heavily dislocated meniscus. The microbeads appear to affect the line tension of the layer steps, resulting in an increased tendency for thicker material to sweep in from the meniscus onto the initially thin film and causing islands to shrink much more rapidly than in films of neat 8CB. Moderate applied magnetic fields can be used to translate the beads on the film and to change the rate of island shrinkage.