On the Wiener optical method to study molecular diffusion in liquids

摘要

In 1893, Otto Wiener developed the first optical method for measuring the rate at which two liquids mix by interdiffusion. His elegant experiment is often reproduced at University to illustrate the phenomenon of molecular diffusion. It is based on the property that an interface between two liquids deflects light by refraction because of the composition gradient formed by interdiffusion. In this study, we refine the analysis that is generally made of Wiener's experiment by performing both simulations of the interdiffusion and ray tracing in the tank that contains the two liquids. By considering first the water/glycerol model system, we determine the time interval during which it is possible to simply extract the value of the interdiffusion coefficient. Then, we extend this study to alcohol/water systems for which unfamiliar experimental patterns are obtained. These images are explained qualitatively by modeling the two main ingredients responsible for their formation, namely, how the interdiffusion coefficient and the refractive index depend on the composition.