Perspective of research on diffusion: From microgravity to space exploration

Liquid mixtures are fundamental model systems for the investigation of non-equilibrium processes because they exhibit stable or unstable configurations due to their density stratification, determined by the interplay of their temperature and concentration profiles. The microgravity conditions on platforms like the International Space Station (ISS) provide a unique environment to research multicomponent mixtures under stable conditions, minimizing convective motions. Projects like IVIDIL, SCCO, and GRADFLEX have advanced the understanding of diffusion and thermal diffusion in binary mixtures, triggering the development of specialized research facilities needed for the investigation of multi-component mixtures. The DCMIX project was pivotal in this area because the investigation of thermodiffusion in ternary mixtures demonstrated the importance of microgravity for a precise determination of the transport coefficients. Based on DCMIX's success, the Giant Fluctuations project (also called NEUF-DIX) will investigate diffusion processes in complex fluids at the mesoscopic scale, including polymers, colloids, and proteins. A significant revelation from these space projects is the emergence of the role of phenomena like cross-diffusion and diffusiophoresis in ternary mixtures. In fact, these phenomena are especially pertinent to space exploration, where understanding the stability of liquid mixtures is crucial for the endurance of pharmaceuticals, food ingredients, and construction materials during missions to the Moon and Mars. Control over the stability of these mixtures is essential to prevent phase separation and aggregation in complex fluids and to process these substances in space under defined conditions.