Thermodiffusion

Thermodiffusion, also known as the Soret effect, is the phenomenon in which a temperature gradient induces a redistribution of the components of a multicomponent system, creating a nonuniform concentration. It is observed in gases, liquids, and solids, and is most readily studied in binary mixtures where one component becomes enriched on the hot side or the cold side depending on the system.

The effect arises from the coupling between heat flow and mass transport: molecules experience different thermal agitation and interparticle interactions across a temperature gradient, leading to a net flux of species in response to the gradient of temperature. In transport descriptions, the diffusion flux of a component contains a term proportional to the temperature gradient, characterized by a thermal diffusion coefficient. In binary mixtures, the Soret coefficient describes the steady-state relation between the induced concentration gradient and the applied temperature gradient; its sign and magnitude depend on molecular properties, interactions, and composition. The phenomenon can be quantified by the Soret coefficient, S_T, which relates the concentration gradient to the temperature gradient under steady conditions.

The magnitude of thermodiffusion is generally small compared with ordinary diffusion and is highly sensitive to concentration, temperature, and the molecular nature of the components. It is more prominent in liquids than in gases and can be significant in complex fluids such as polymer solutions and electrolyte solutions. The Soret effect has historical importance for isotope separation and continues to be relevant in microfluidics, materials science, and geochemical processes, where temperature gradients coexist with compositional variation.