Nonsingular absorption of ultrasound near the critical mixing point of a binary liquid

The absorption and velocity of ultrasound and the critical opalescence have been measured for a critical-composition mixture of nitrobenzene and 2,2,4-trimethylpentane along the critical isochore in the homogeneous phase of the system. The measurements cover the relatively wide range of temperature interval of 30 K to 7 mK from the critical value. The nearness of approach to the critical point was affirmed by the critical-opalescence measurements. The measurements of acoustic absorption covered the range of 1 to 91 MHz, and the strong frequency dependence observed can be satisfactorily described by the Fixman-Kawasaki theory with the adjustment of three parameters. Singular behavior for the sound absorption, consistent with critical slowing down, was observed when the mixture was more than about 3 K away from the critical point. When within about 0.3 K of the critical temperature the sound absorption was observed to become constant. The absence of any effects on the critical sound absorption, while the mixture was under a large electric field, is also reported.