Birefringence induced by Acoustic Waves in Polystyrene Solutions

Abstract

WHEN an ultrasonic wave passes through certain liquids and solutions they exhibit double refraction and behave as uniaxial crystals with their optic axes lying along the direction of propagation of the acoustic beam. This effect, which may be called acoustic birefringence, was first observed by Lucas^1,2 and may be considered to be the result of a velocity gradient caused by the wave. Acoustic birefringence is related to flow birefringence in that both effects are caused by a similar mechanism. However, it appears that in liquids and solutions of small or flexible particles the distribution function defining the number of particles in a definite state of equilibrium during the passage of the acoustic wave is a function of frequency, whereas in flow the function is constant. Thus acoustic birefringence is oscillating and usually a root mean square value is measured. Also the oscillating birefringence is not in phase with the acoustic wave, the difference being dependent on the viscosity of the liquid and some molecular parameter. Studies of the effect are being made in this laboratory both on pure liquids and solutions.