Contact Angle between Smectic Film and Its Meniscus

Abstract

We apply de Gennes model (Langmuir, 1990, 6, 1448−1450) for presmectic ordering to a freely suspended smectic film coupled to the meniscus. We show that the quartic term in the expansion of the free energy in power of the smectic order parameter cannot be neglected in the model in order to explain the full behavior of the contact angle, θm, between the film and the meniscus. As shown previously (Phys. Rev. E 2001, 63, 021705-1−021705-9), this contact angle is nonzero due to the disjoining pressure caused by the enhanced smectic ordering at the film surfaces. The temperature dependence of θm with a maximum above the bulk temperature of the nematic-smectic phase transition, TNA, is qualitatively reproduced by the model for all film thicknesses h = Nd, where N is the number of layers and d the smectic period, although some discrepancy between theory and experiment for the temperature at the maximum still remains. The apparent change of slope of the contact angle at TNA observed experimentally in thin films cannot be reproduced by the mean-field treatment of the model and presumably critical fluctuations in the meniscus and their influence on the behavior of the surface tension close to TNA have to be taken into account.