Interferometric measurement of diffusion coefficients through a scanning laser beam

A new interferometric setup for the determination of the con- centration distribution in a diffusion experiment is proposed. The method is applied to the determination of the diffusion coefficientD in a binary saline solution. A thin laser beam scans the diffusion cell along the con- centration 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, modulo 2p, 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 coefficientD is obtained by means of an iterative process, com- paring experimental and theoretical curves until the best fit is achieved between them, by adjusting the parameter D.