Behavior of a surface phase transition in freely suspended liquid-crystal films

The smectic-A–smectic-C transition at the free surface of freely suspended liquid-crystal films with thicknesses between three and several hundred smectic layers has been studied by ellipsometry. The results indicate that the temperature dependence of the tilt angle near the surface transition is influenced by the film thickness even in the range up to ≊100 layers and that not only the first smectic layer at the surface is involved in the transition. In films of medium thickness (between 30 and 100 layers) a splitting of the transition into two steps is observed which probably does not correspond to the layer-by-layer behavior observed frequently for other liquid-crystal phase transitions