Measurement of the diffusion coefficient in liquids using Fresnel diffraction from a phase step

In this paper, we use Fresnel diffraction from a phase step in transmission to measure the diffusion coefficient in a transparent liquid. A transparent glass plate immersed vertically in a cell containing two diffusing liquids makes a phase step in transmission. When it is illuminated by a parallel beam of light, the diffraction pattern is formed on a screen perpendicular to the direction of the light beam. The diffusion of liquids inside the cell leads to the variation of the diffraction pattern along the edge of the glass plate. The refractive index gradient in the liquids is then obtained by analyzing the diffraction patterns at different times after the beginning of the diffusion process. We then get the diffusion coefficient from the measured gradient. Using this method, we study the diffusion process of the sucrose-water solution and report the diffusion coefficient with a reliable accuracy.