Measurement of the Diffusion Coefficient of Miscible Fluids Using Both Interferometry

Measurements of the diffusion coefficient of two miscible liquids are reported. The liquids are various combinations of pure silicone oils and those to which small amounts of solvents are added to control the difference in density between the fluids. The liquids were placed in a quartz cell such that the interface is initially horizontal. As the fluids diffuse, the profile of the index of refraction near the interface is time dependent and is related to the local concentration of the diffusing fluids. The concentration gradient profile was measured by a shear-ing interferometer incorporating a Wollaston prism, as well as Wiener’s method. In the latter technique, a 45 % light sheet was passed through the test cell, and the local deflection of the light beam was measured. The average diffusion coef-ficient was obtained by analysis of the measured concentration gradient profile, assuming that the diffusion process is one-dimensional and is characterized by a constant value of the diffusion coefficient. KEY WORDS: diffusion coefficient; interferometry; miscible interfaces; Wiener’s method