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

Abstract Fresnel diffraction from a phase step for measuring diffusion coefficient in transparent liquids is investigated. When a transparent glass plate immersed vertically in a cell containing two diffusing liquids is illuminated by a parallel beam of light, the diffraction pattern of the plate edge forms on a screen perpendicular to the beam direction and varies by diffusion. The gradient of the refractive index in the liquids is then obtained by analyzing the diffraction patterns at different times after the beginning of the diffusion process, from which the diffusion coefficient is determined. Using this method, we study the diffusion process of the sucrose-water solution and report the diffusion coefficient with a reliable accuracy.