Absolute Measurement of the Critical Properties of Second Sound at the Smectic-A—to—Nematic Phase Transition in 8OCB

Light scattered from the driven free surface of the smectic-$A$ phase of octyloxy cyanobiphenyl (8OCB) provides a low-frequency measurement of second sound. This yields an absolute measurement of the smectic elastic constant, $B$. There is no dispersion in either the angular or frequency dependence of $B$ for angles 35\ifmmode^\circ\else\textdegree\fi{}-83\ifmmode^\circ\else\textdegree\fi{} from the normal and for frequencies from 10-100 kHz. Near the smectic-$A$\char22{}to\char22{}nematic phase transition the smectic elastic constant can be described by $B=(6.03\ifmmode\pm\else\textpm\fi{}0.05)\ifmmode\times\else\texttimes\fi{}{10}^{7}$ ${({T}_{\mathrm{NA}}\ensuremath{-}T)}^{0.32\ifmmode\pm\else\textpm\fi{}0.015}$ dyn/${\mathrm{cm}}^{2}$.