Ultrasonic Studies of Some Nematic Liquid Crystals

The ultrasonic velocity and absorption of three nematic liquid crystals 1. N-(p-hexyloxybenzalidene)- p-toluidine (HT), 2. N-(p-octyloxybenzalidene)-p-toluidine, and 3. (p-ethoxybenzalidine)- p-n-butylaniline were measured both in isotropic liquid and liquid crystalline ranges. The velocity was measured by the interferometric technique and absorption was measured by the pulse technique. An anomalous increase in absorption and decrease in sound velocity was observed in the vicinity of the isotropic-nematic transition. In OBT the smectic-nematic transition was marked only by a change of slope in velocity. The results are qualitatively discussed in the light of present theories. A short review of the present theories of ultrasonic velocity and absorption is also given.