Freely-suspended films of smectic liquid crystals can be regarded as membranes consisting of a stack of an integral number of molecular (smectic) layers with the layer planes being parallel to the two free surfaces. Because of their variable thickness (between thousands and only two layers) and the large variety of phase transitions between different smectic phases, freely-suspended films are excellent systems to study the influence of the dimensional cross-over from three to two dimensions on phase transitions. Further, because the free surface of a liquid crystal has a strong ordering effect (contrary to solids which exhibit generally surface-induced disorder), freely-suspended films are well-suited for the study of the effect of enhanced surface order on phase transitions. A review of the corresponding experimental work is given.
