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Abstract:
A new experiment has been designed to investigate the effects of a mechanical perturbation applied parallel to the plane of the smectic layers of a free-standing film. Runs have been performed on films of different thicknesses in liquid, hexatic and crystal smectic phases present in the thermotropic liquid crystal compound (7o.7). We show that the thermodynamic phase of the meniscus is a crucial parameter because it determines the perturbation regime experienced by the film. In light of a theoretical framework developed in this paper, two perturbation regimes were found: a flow regime and a pure-elastic regime. Data recorded in the latter case enable us to extract the elastic constant of the smectic B and the smectic G crystal phases. We observe an unexpected discrepancy in the compressibility coefficient between the two crystal-like phases indicating that the molecular tilt angle provides the film with one additional macroscopic variable to respond to the mechanical modulation.
