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°-83° from the normal and for frequencies from 10-100 kHz. Near the smectic-$A$—to—nematic phase transition the smectic elastic constant can be described by $B=(6.03\mathrm{}\pm 0.05)\times {10}^7$ ${(T_{\mathrm{NA}}-T)}^{0.32\pm 0.015}$ dyn/${\mathrm{cm}}^2$.