A Robust Data Evaluation Method for the DCMIX Microgravity Experiments

Abstract

Measurements of non-isothermal diffusion in ternary mixtures under microgravity conditions are performed in the framework of the DCMIX (Diffusion and thermodiffusion Coefficient Measurements in ternary mIXtures) project aboard the ISS by means of the SODI instrument. Its digital Mach-Zehnder interferometer permits the time resolved determination of the 2d-refractive index profile within a Soret cell at two different detection wavelengths of 670 and 935 nm. The operation of the interferometer is based on temporal phase stepping, where five consecutive images with a phase shift of π/2 are combined to an image stack. We present a robust method for the evaluation of these interferometric data by mapping the time evolution of the phase gradient in the center of the cell to an equivalent two-color optical beam deflection (2-OBD) experiment. It allows to extract transients of the central phase gradient even in occasional situations where temporal phase stepping breaks down due to laser instabilities. For this purpose, only a single image from every image stack, instead of five images with well controlled π/2-phase stepping, is required. Our results demonstrate the sound design and the robustness of the SODI instrument, which still yields fully valid data even in the event of unexpected laser phase instabilities.