Microgravity measurement of the Soret effect in a molten salts mixture

We have performed the first measurement of the Soret effect in a molten salt mixture under microgravity conditions during two different flights (D1 and D2) of the spacelab. After justifying our selection of the molten salt used (AgI/KI) and a description of the microgravity facility, we give the results of a three-dimensional simulation of combined convection phenomena and thermal diffusion effects. This simulation verifies the feasibility of the experiment and aids selection of the various geometric and physical parameters of the experiment. The Soret coefficient deduced from these measurements is positive, corresponding to a heavy component migrating towards the cold end of the cell. Its value deduced from the thermopower is slightly different for the two flights (1.0 x 10(-3) K-1 for D1 and 0.7-0.9 x 10(-3) K-1 for D2) while the salt analysis after return to Earth gives the value 1.4 x 10(-3) K-1 but for an experiment duration corresponding to about one time constant. Its is concluded that useful measurements of thermodiffusion are really possible in good conditions in space, but blind measurements must necessarily be confirmed by optical measurements in order to demonstrate the possible convection of the liquid and to know the real level of separation attained, compared with the perfectly nonconvective situation.