Soret effect

In an isothermal mixture, ordinary molecular diffusion tends to smooth concentration differences. When a temperature gradient is imposed, components can also migrate at different rates toward the hotter or colder region. This temperature-driven transport is called thermodiffusion; in mixtures it is commonly known as the Soret effect. At steady state, thermodiffusion and ordinary diffusion can balance, leaving a concentration gradient even when the net diffusive flux is zero. The effect is quantified using thermodiffusion and Soret coefficients. Their values, and sometimes their signs, depend on the mixture, composition and temperature; there is no universal rule that every component moves toward the same end.

Researchers measure the effect by establishing a known temperature difference in a mixture and observing the resulting concentration change or optical signal. Methods include Soret cells, optical beam deflection and interferometry, laser-induced thermal gratings, and thermogravitational columns. Experiments must distinguish thermodiffusion from convection and other transport processes. Gravity can complicate measurements by driving fluid motion, but the influence depends on experimental design; ground-based methods can be carefully controlled, while microgravity investigations help reduce buoyancy effects and examine regimes that are difficult to isolate on Earth.

Thermodiffusion contributes to component distributions in settings where temperature gradients persist. Research has considered its role in geothermal and hydrocarbon systems, solar ponds, crystal growth, and transport in mixtures. It is also relevant to separation processes and microfluidic systems. In practice, predicting the effect requires mixture-specific measurements: coefficients can vary with composition, and measurements in multicomponent mixtures are more complicated than in binary mixtures because several coupled concentration gradients must be resolved.

The phenomenon is conventionally named for Swiss chemist Charles Soret, whose late-nineteenth-century experiments examined salt distributions in solutions held at different temperatures. Earlier observations were also reported by Carl Ludwig. Modern work has refined experimental methods and coefficient measurements, while continuing to investigate how thermal diffusion behaves in more complex mixtures and under microgravity conditions.