Visualization Measurement of Diffusion Coefficient and Soret Coefficient by using Laser Holography

We propose an original system to measure two transport properties of aqueous solutions, diffusion coefficient and Soret coefficient, in the same optical system by using laser holography real-time interferometry. First, the validity and reliability of this measurement method were examined. Diffusion coefficients were calculated for NaCl and KCl solutions by using the time variation of the thickness of the interference fringes, since the analytical solution of the diffusion equation follows a normal distribution, and the results were in good agreement with the literature value. The Soret coefficients were calculated for NaCl solutions from the position of the interference fringes at a certain time, the analytical solution of the diffusion equation including the thermal diffusion term, and the diffusion coefficient calculated above, and were in good agreement with the measured values in previous studies. After confirming the validity of the results, both transport properties were measured in a Na₂CO₃ solution of 10 wt%, for which there are few literature values. In addition, the error factors for this measurement were clarified, and the uncertainty ranges of the measurement value were estimated. The present method utilizes the relationship between the refractive index of the solution, temperature, and concentration to analyze the interference fringes that coincide with the isorefractive index line, which simplifies the calculation of the diffusion coefficient and Soret coefficient compared to existing methods.