Repulsive forces in thin smectic-C* films on substrates

X-ray reflectivity studies of spin-coated ultrathin films of smectic-C* liquid crystals show dramatic variation of the smectic layer spacing L as a function of the number of smectic layers n. Results for three different liquid crystals suggest a long-range force between the interfaces that decays algebraically like 1/nγ where γ=3.0±0.3. This decay is consistent with a van der Waals type of force, although its magnitude cannot be explained by the existing mechanisms. Interactions between the electric dipoles and their images as well as the formation of ferroelectric domains may provide a mechanism to explain the magnitude of the force.