Unusual Optical Techniques in Diffusivity Measurements

In studying hydrodynamic instabilities between two miscible fluid mixtures one often faces the problem of assigning a reliable value to diffusion coefficients. As for diffusion between two binary mixtures such as water and glycerin, water and salt, etc., no complete data are available in the literature. This shortage of data justifies the search for simple and accurate experimental techniques. Optical techniques are now recognized as powerful tools to investigate fluid flow phenomena in transparent media. In particular, in this paper we discuss three different, recently proposed, optical techniques for diffusivity measurements, namely speckle decorrelation, common path shearing interferometry and digital moire´. Although all these techniques depend on variation of the index of refraction in a transparent fluid, each method involves the use of specific instrumentation, with experimental uncertainties and ranges of applicability which differ from case to case. A comparison of the techniques is presented as well as a discussion of their possible integration.