The DCMIX project: Measurement of thermodiffusion processes in ternary mixtures on ground and in space

Ternary mixtures play an important role as model systems for the understanding of non-isothermal transport phenomena in liquids. The DCMIX project of ESA aims to establish reliable reference data for the Soret effect in ternary liquids and serves as a nucleus for ground based work. Up to now, three DCMIX campaigns have been carried out aboard the International Space Station ISS. The investigated ternary mixtures are dodecane/isobutylbenzene/tetralin (DCMIX1), methanol/toluene/cyclohexane (DCMIX2) and water/ethanol/triethylene glycol (DCMIX3). DCMIX4 is scheduled for 2018. The microgravity experiments are performed by means of the SODI instrument, a digital Mach-Zehnder interferometer. In order to fully characterize a ternary mixture, it is necessary to extract six independent parameters from the measured data, which is particularly difficult in case of similar diffusion eigenvalues. A detailed analysis shows that, nevertheless, stable values for the thermodiffusion and the Soret coefficients can be obtained. Additional errors arise from the inversion of the contrast factor matrix, which is frequently ill-conditioned. Besides first data from the DCMIX3 microgravity experiments, which have been coordinated by our group at the University of Bayreuth, we review some important results from the DCMIX1 campaign. These include a benchmark with the participation of a number of different groups and the measurement of a complete data set for the DCMIX1 system in the laboratory. The latter results can be rationalized on the basis of the so-called thermophobicity concept previously introduced for binary mixtures. •The DCMIX project aims at convection-free thermodiffusion data for ternary mixtures•Experiments aboard the ISS by means of a two-color digital interferometer (SODI)•Very good agreement between microgravity and ground experiments•First results underline the critical importance of precise optical contrast factors