Absolute measurement of the critical behavior of the smectic elastic constant of bilayer and monolayer smectic- A liquid crystals on approaching the transition to the nematic phase

A new technique which provides an absolute measurement of the smectic elastic constant B is presented. This technique measures second sound on samples whose top surface is an air---liquid-crystal interface. The free surface of the sample is electrically driven and the resulting surface displacements are measured with an optical heterodyne technique. The smectic elastic constant is obtained from the spectrum of the scattered light which is determined by the bulk second-sound modes. Two bilayer and two monolayer smectic-A liquid crystals were studied. Near the smectic-A---to---nematic phase transition the data can be described by a simple power law B=B₁(T-TNA)^φ. However, the resulting critical exponents φ are not universal. In addition, the B₁ values for the bilayer smectics are considerably smaller than the values for the monolayer smectics.