Structural force in a presmectic liquid

The structural interaction in a presmectic film is analyzed theoretically u sing the Cartesian rather than the polar representation of the smectic orde r parameter. This representation diagonalizes the phenomenological Hamilton ian of the ordering and leads to a unified description of the mean-held int eraction-first studied by de Gennes [Langmuir 6, 1448 (1990)]-and the fluct uation-induced interaction, The fluctuation-induced interaction turns out t o be short-range, attractive, and unaffected by the apparent bidomain struc ture of the film, which controls the oscillatory mean-held interaction.