Measurement of mass diffusion coefficients by digital moiré

In order to measure small diffusion coefficients of miscible fluids, in this paper we propose an improved version of digital projection moiré. The system uses a simple and flexible fringe generator realized by means of a video projector with a modification of projection optic. 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 CCD camera and stored in a computer at different times. With the aid of signal demodulating techniques, the images are processed to obtain the diffusion coefficients. The theoretical aspects of the method are presented and the relationship between the fringe shift and the diffusion coefficient is derived. Furthermore, we report some experiments conducted for demonstrating the usefulness of the system.