Improvement of Interference Fringe Analysis for Soret Coefficient Measurement in Soret-Facet Mission

Abstract

In the Soret-Facet performed on the International Space Station, the Soret coefficient ST for salol/tert-butyl alcohol was measured by using a two-wavelength Mach-Zehnder interferometer. The temperature difference between the sides of the solution was set to 10C so that its mean temperature was 45C. The refractive index changes in a narrow observation field were measured by using a charge-coupled device camera. We improved the interference fringe analysis used to determine the refractive index changes by determining the interference fringe shifts in a wide area of the solution rather than in the narrow observation field. The interference fringe shifts outside the observation field were measured by moving the field of view and comparing the interference fringe positions. The fringes were found to shift linearly in the wide area. Then, the values of ST in the observation field and the wide area, STnarrow and STwide, respectively, were determined based on the interference fringe shifts. The measurement error δ(ST) was caused by the standard deviation of the slopes of the fit lines, and values of δ(ST)narrow = plus or minus 0.34 K(exp -1) and δ(ST)wide = plus or minus 0.024 K(exp -1) were obtained for the observation field and the wide area, respectively. Based on the fit lines, which satisfied two constraints, STnarrow and STwide were determined to be -0.17 K(exp -1) and -0.06 K(exp -1), respectively, for tert-butyl alcohol in salol. Consequently, δ(ST)narrow/STnarrow = 190% and δ(ST)wide/STwide = 40% were obtained. Thus, the error δ(ST)/ST decreased from 190% to 40% when the interference fringe shifts were measured in the wide area in 0.25mm intervals rather than in the narrow observation field.