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′ 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.