Interferometric studies of light scattering in viscous liquids and glasses
In an interferometric study of light scattering by highly viscous liquids, sharp and well-defined Brillouin components have been recorded for glycerine and castor oil. The intensity of these components in glycerine shows a progressive increase as the temperature is raised from 0° to 110° C, the central undisplaced component indicating a corresponding diminution in intensity. The Brillouin components appear with great intensity in cyclohexanol and glycol which are relatively less viscous. On the other hand, no displaced components are observed in glucose and in styrol and optical glasses. While the velocity of the thermal sound waves calculated from the Brillouin displacement agrees well with the supersonic velocity in cyclohexanol and glycol, it is considerably higher than the latter in glycerine and castor oil. The divergence between the ‘hypersonic’ and the ‘ultrasonic’ velocities diminishes as the temperature of glycerine is raised to 110° C. The appearance of well-defined Brillouin components in these highly viscous liquids indicates that the ordinary laws of hydrodynamics are not valid for the propagation of these high frequency waves in the medium. The results are also shown to be in contradiction to the predictions made by the theory of light scattering in liquids put forward by Leontowitsch. From the ultrasonic and hypersonic velocities in glycerine and castor oil, the bulk modulus,k, and the rigidity modulus,n, have been determined. The time of relaxation τ calculated using the Maxwell’s formula η =nτ where η is the viscosity, is in both cases greater than the time-period of the thermal sound waves, thereby furnishing an experimental proof of the thesis that the liquid behaves essentially as an amorphous solid at sufficiently high frequencies. These conclusions are also supported by observations on the dependence of the intensity of the rotational ‘wing’ on the viscosity of the liquid.