Influence of Soret effect on the pre-solidification state in the neopentylglycol-(D)camphor system during microgravity experiments

Abstract

We present results from a combined experimental and numerical approach to estimate the impact of thermodiffusion in the liquid phase of the binary system neopentylglycol-(D)camphor (NPG-DC) during the pre-solidification holding period of experiments on the International Space Station (ISS) in reduced gravity conditions. The Soret coefficient ST, diffusion coefficient D and thermodiffusion coefficient DT have been measured by means of the optical beam deflection technique using mixtures close to the eutectic composition of the alloy (0.547 wt frac. NPG) in the composition range of 0.522–0.581 wt frac. NPG and at different temperatures above their respective liquidus temperatures. Numerically, we implemented a one-dimensional time-dependent thermodiffusion equation to describe the composition evolution during the holding pre-solidification phase in the NPG-DC alloy in a given temperature gradient. The validated model was then applied to the more complex case of spatially changing thermal gradients, which are expected in the experimental facility used onboard the ISS. Numerical simulations estimating a maximum effect of thermodiffusion showed a small but clear effect. Using the previously determined transport coefficients and a holding phase of 10 hours, a change of the NPG concentration of less than 7% of the initial composition was obtained for the given set-up. Even if the real thermodiffusion effect is expected to be smaller, this lateral inhomogeneity in concentration has to be taken into consideration in the microgravity experiments.