An Interferometric Method for Determining the Coexistence Curve and Diffusion Coefficient of a Binary Fluid

An interferometric method is described for determining the coexistence curve of a binary fluid. A thin sample containing a binary fluid at a temperature below the critical temperature such that two phases coexist is heated to a temperature greater than T c . As the two phases diffuse together, the concentration and refractive index attain sigmoid distributions. The Fraunhofer diffraction pattern produced by a collimated laser beam traversing the sample is analyzed to obtain the concentration difference as a function of temperature. The change of the Fraunhofer pattern with time yields information about the temperature and concentration dependence of the diffusion coefficient. Results for the binary fluid aniline–cyclohexane are presented.