A. Light Scattering from a Liquid-liquid Interface

Our studies of thermally excited surface waves on liquids by means of light scattering, as described in earlier reports,l'2 have been extended to the problem of surface waves on the interface between two liquids. As a specific example, we have considered the methanol-hexane interface. A laser beam is scattered off the interface, and the light scattered in a certain direction is analyzed by means of heterodyne spectroscopy, by using the specularly reflected light beam as a "local oscillator." Examples of recorded beat spectra (actually the square root of the beam-power spectra) obtained at three different scattering angles are shown in Fig. XVI-1. It is interesting to note that as the scattering angle (and the wave number) increases, the scattered spectrum changes markedly in character from a Brillouin doublet to a single line. At low scattering angles corresponding to long wavelengths there is a distinct Brillouin doublet, but as the scattering angle increases (wavelength decreases), the doublet gradually turns into a single line. The same thing occurs at a given wavelength when the temperature is increased. Measurements of this sort were carried out at different temperatures ranging from 25 0 C to 350C, and at each temperature several scattering angles corresponding to different ripple wavelengths were considered. To interpret these experimental results, we again used the classical hydrodynamic theory of surface waves adapted to the present problem, assuming no slippage between the two liquids at the interface. The resulting dispersion relation is found to be of the form