Ultrasonic attenuation anisotropy in a nematic liquid crystal

We have studied the temperature dependence of the anisotropy in the ultrasonic attenuation $\Delta \alpha$ in the nematic phase of MBBA at 1, 2, 4, and 10 MHz. $\Delta \alpha$ is analyzed in terms of a critical, a noncritical, and a nonrelaxing part. For the relaxation time of the noncritical component we find a reverse-barrier activation enthalpy $\Delta H_2=2.25\mathrm{}\pm 10\%$ kcal ${\mathrm{mole}}^{-1\mathrm{}}$. This value is consistent with rotational isomerism in the butyl end group of the molecule. The relaxation time of the critical component in the nematic phase is found to have a critical exponent $\gamma =1.06\mathrm{}\pm 10\%$. It is tentatively suggested that the critical part of $\Delta \alpha$ may be due to relaxation of fluctuations of the order parameter and not to relaxation of the order parameter itself.