Interferometric measurement of diffusion coefficients through a scanning laser beam

A new interferometric setup for the determination of the concentration distribution in a diffusion experiment is proposed. The method is applied to the determination of the diffusion coefficient D in a binary saline solution. A thin laser beam scans the diffusion cell along the concentration gradient, which is assumed to be one-dimensional. This beam interferes with the reference beam in a Mach-Zehnder interferometer, and the variation of intensity is recorded by means of photomultiplier tube, digitized, and stored in a computer for further analysis. The phase of the intensity function, module 2 pi, is determined by applying the Fourier-transform method. Assuming linear dependence of the refraction index on the concentration, the unwrapped phase gives a curve similar to the theoretical concentration distribution function. The value of the diffusion coefficient D is obtained by means of an iterative process, comparing experimental and theoretical curves until the best fit is achieved between them, by adjusting the parameter D. (C) 2000 Society of Photo-Optical Instrumentation Engineers. [S0091-3286(00)02403-X].