Ultrasonic Shear Mechanical Impedance of Smectic-A Liquid Crystals

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Abstract
The ultrasonic shear mechanical impedance of octyloxy cyanobiphenyl and cyanobenzylidene octyloxyaniline was measured by the torsional quartz crystal method. The measured impedance suggested a finite shear elasticity in the smectic-A phase, which was interpreted as an evidence of the existence of the finite smectic layer compression modulus giving rise to a propagating “second sound mode”. From the impedance the smectic layer compression modulus was evaluated as ∼107 cgs for both compounds and it showed a pretransitional decrease as the nematic phase was approached.