Dynamic properties near the nematic-smectic-A transition by ultrasonic absorption

Measurements of ultrasonic attenuation at frequencies from 0.6 to 25 MHz in magnetically aligned CBOOA are reported. On the nematic side of the A-N transition the ultrasonic absorption presents a peak in the MHz range which is more and more apparent as the frequency is decreased. This attenuation peak has been explained as being due to two contributions. One is the critical viscosity, the other reflects the behaviour of the critical heat capacity. This latter is the dominant contribution. In the smectic-A phase there is evidence of an additional relaxation mechanism which can be attributed to a Landau-Khalatnikov process. The relaxation frequency of the smectic order parameter is deduced from these measurements