Ultrasonic study of liquid crystals

The ultrasonic velocity and attenuation in magnetically aligned liquid crystals has been measured. All well-known liquid-crystalline symmetries (nematic, cholesteric, and smectic $A$, $B$, and $C$) were studied. Sound anisotropy measurements in the smectic-$B$ and -$C$ and cholesteric materials are reported here for the first time; the measurements in the smectic-$C$ and cholesteric liquid crystals are still in a preliminary stage. We unambiguously attributed the sound velocity anisotropy to the existence of a repeated structure (broken translational symmetry) in the hydrodynamic (low-frequency) limit by measuring the anisotropy across the nematic-smectic-$A$ transition in $N-p$-cyanobenzylidene-$p$-octyloxyaniline (CBOOA); a sudden increase of the anisotropy extrapolated to zero frequency was observed at the transition despite a large dispersion. The smectic-$A$— smectic-$B$ transition in ethyl-$p$-[($p$-methoxybenzylidene)amino] cinnamate shows a two-dimensional liquid-to-solid transition. A distinct minimum was found at the nematic-smectic-$C$ transition in $p-p^{\prime }$ heptyloxyazoxybenzene which was not accompanied by an attenuation peak. The velocity anisotropy in a cholesteric mixture of cholesteryl chloride and cholesteryl myristate was tentatively assigned to the dispersion.