Measurement of the birefringence induced in liquids by ultrasonic waves : application to the study of the isotropic phase of PAA near the transition point

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Abstract
A technique has been developed for ultrasound-induced birefringence experiments. Measurements made in the isotropic phase of PAA (p-azoxyanisole) show that it is possible to obtain reliable results quickly and in a very simple way through straightforward improvements of the traditional technique. In addition to the induced birefringence measurements our technique also provides measurements of the absorption coefficient of the ultrasonic wave. The cell has a low sample volume and is particularly suited to studies on rare liquids and/or highly attenuating liquids. The results on PAA show that the quantity Δn/√P, where Δn is the induced birefringence and P the acoustic intensity, diverges according to a power law. As one approaches Tc the system crosses over from the ωτ ⪡ 1 regime to the ωτ ∼ 1 regime. The results are interpreted on the basis of the de Gennes model. The orientation relaxation time is deduced from these measurements