Liquid diffusion coefficients by digital moiré

In studying hydrodynamic instabilities between two miscible fluid mixtures one often faces the problem of assigning a reliable value to very small diffusion coefficients. As for diffusion between two binary mixtures such as water and glycerin, water and salt, etc., no complete data are available in the literature. To measure isothermal diffusivity of binary liquid mixtures, we propose an improved version of digital projection moiré. The system uses a simple and flexible fringe generator realized with a diffractive optical element (DOE). The fringe patterns are projected on the bottom of a ground glass plate. The phase object (diffusion cell) is placed in front of the ground glass (in other words, in front of the fringe pattern), which is imaged by a digital video camera. Grating patterns, during the evolution of diffusion phenomena, are captured by a charge-coupled device (CCD) camera and stored in a computer at different times. With the aid of Fast Fourier Transform (FFT) and signal demodulating techniques, the images are processed to obtain the diffusion coefficients. The theoretical basis of the device is presented. We also report experiments conducted for validating the system.