CHAPTER 2:Optical Methods

The Chapter presents a number of the most significant developments in the use of optical techniques for the measurement of the transport properties of liquids over the last twenty years. First the chapter examines the application of light scattering under three different circumstances. In Section 2.2 the use of the natural oscillations of a liquid surface caused by random thermal molecular motions are used as a probe of viscosity through surface light scattering, applicable most easily to low viscosity fluids of moderately high surface tension. For systems with rather higher viscosities and lower surface tensions the advantages of creating a grating in the surface of a liquid with a laser to create more readily observable effects of optical diffraction in laser- induced capillary wave scattering are described in Section 2.3. Applications of both of these techniques to industrially important fluids are illustrated. Section 2.4 describes how dynamic light scattering has been exploited to study diffusion in experiments near critical points. The final two sections then present work that has explored the phenomenon of thermal diffusion in liquids scarcely touched by optical techniques two decades ago. First the situation with respect to binary mixtures of liquids is summarized with examples of the successes by a variety of methods and then the more difficult challenge of ternary systems is explained. The current state of the art in measurement is set out as well as the remaining difficulties.