A method is developed for measuring diffusion constants in transparent mixtures by means of double-exposure holographic interferometry. The diffusion constant is deduced by a simple and direct measurement of the interferograms. The method was checked with sugar solution and water and gives reasonable agreement with other methods.

We describe a new optical interferometric technique which we have applied to 3He-4He liquid mixtures in the phase separation region. The formation and slow propagation of a concentration front is observed when the phase separation equilibrium is disturbed by sudden heating. The phenomenon is analyzed in terms of a concentration-dependent diffusion constant, and the results indicate critical behavior of the diffusion constant on the normal sideof the lambda line. The application of the optical method to studies of interphase surface tension and crystallization is also indicated.