In the nineteenth century, Ludwig 1 and Soret 2 observed independently from each other that a temperature gradient in an isotropic mixture can induce a concentration gradient (Figure ). This coupling phenomenon between heat and mass flow is known as thermal diffusion, thermodiffusion, Ludwig-Soret effect, or simply Soret effect. Coupled heat and mass transfer occurs in many industrially relevant processes such as absorption, distillation, extraction, drying, crystallization, or condensation. As a second-order effect, the mass flux induced by the Soret effect is usually relatively small, but it can play an important role in many natural 4;5 and technical processes. Thermal diffusion can be employed to separate mixtures that are hard to handle by conventional methods, for example, the fractionation of isomers, isotopes, 8 polymers, and colloids. Further applications include surface coating, geological carbon dioxide storage, modeling of hydrocarbon reservoirs, and hydrogen enriched flames. Chemical systems that undergo thermodiffusion in practical settings are frequently binary systems in the liquid state. In such cases, the separation of the two components can be quantified by the Soret coefficient S T , which is a preferred quantity in the study of liquids. In case of multi-component mixtures, the Soret coefficient becomes a vectorial quantity. The Soret coefficient relates the thermal diffusion coefficient D T to the Fick diffusion coefficient D