Ludwig-Soret effect

Equations (4.16) and (4.17) are examples of the so-called cross effects whereby a force Ac can induce fluxes j) despite that X = 0. Another example of a cross effect is thermotransport in which temperature gradients (fluxes of heat) induce fluxes of atomic species, j,. An application of this concept is the steady state demixing of a (closed solid) solution system, which has been exposed to a temperature gradient (heat flux). This is the Ludwig-Soret effect originally observed with fluid systems. 

Demixing in Temperature Gradients (Ludwig-Soret Effect)... 

Thermal diffusion, also known as the Ludwig-Soret effect [1, 2], is the occurrence of mass transport driven by a temperature gradient in a multicomponent system. While the effect has been known since the last century, the investigation of the Ludwig-Soret effect in polymeric systems dates back to only the middle of this century, where Debye and Bueche employed a Clusius-Dickel thermogravi-tational column for polymer fractionation [3]. Langhammer [4] and recently Ecenarro [5, 6] utilized the same experimental technique, in which separation results from the interplay between thermal diffusion and convection. This results in a rather complicated experimental situation, which has been analyzed in detail by Tyrrell [7]. 

Most data about the Ludwig-Soret effect of polymers in solution have been obtained from thermal field-flow fractionation (TFFF), developed by Giddings and coworkers [17,18]. TFFF is one member of the family of field-flow fractionation techniques, which are all characterized by a laminar flow of the polymer solution or colloidal suspension within a relatively narrow channel. An external field, which may be gravitation, cross-flow, or temperature as in TFFF, is applied... 

Concentration grating Due to the Ludwig-Soret effect, the temperature grating is the driving force for a secondary concentration grating, which starts to build up and is superimposed upon the thermal one. Its temporal and spatial evolution is obtained from the one-dimensional form of the extended diffusion equation... 

We have outlined how TDFRS not only provides a useful tool for the study of the Ludwig-Soret effect in multicomponent liquids, but can also contribute valuable pieces of information towards solving the puzzles encountered in polymer analysis. Though TDFRS is conceptually simple, real experiments can be rather elaborate because of the relatively low diffraction efficiencies, which require repetitive exposures and a reliable homodyne/heterodyne signal separation. As an optical scattering technique it has much in common with PCS, and the diffusion coefficients obtained in the hydrodynamic limit (q —> 0) for monodisperse solutions are indeed identical. 

Ludwig-Soret effect. The difference in concentration of one of the components of a mixed crystal due to temperature. 

Ludwig-Soret effect Soret effect Thermal diffusion Thermodiffusion... 

Thermomigration, also known as the Soret or Ludwig-Soret Effect, is mass transport in response to a driving force resulting from a temperature gradient. It is one of the driving forces noted in Eq. (5), usually represented as,... 

The first effect, thermal diffusion, is exemplified by the two experiments shown in Fig. 21.5-1. In the first, a tall column of salt solution is heated at the top and cooled at the bottom. The salt s concentration is initially uniform, but later becomes more concentrated near the bottom of the tube. This experiment was originally made in 1856 by Tick s mentor, Carl Ludwig more complete experiments were later made by Charles Soret, after whom this effect is named. A similar experiment, shown schematically in Fig. 

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