8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Tue 611:23-14:00P9.11A novel optical approach to the Ludwig-Soret effect:validating data close to the critical point of a binarymixtureFabrizio Croccolo 1 and Frank Scheffold 11 Université de Fribourg, Ch. du Musée 3, 1700, Fribourg, SwitzerlandThe Ludwig-Soret effect is the counter-diffusion of particles in a fluid mixture due to temperaturegradients. The Soret coefficient is then proportional to the ratio between the concentration and theapplied thermal gradient. Even though this has been known for more than 150 years, it is still notcompletely understood from the theoretical point of view. A deeper understanding of the Soreteffect has important consequences in a lot of applications among which we can cite the analysisof oil fields and the utilization of thermophoresis for screening of biomolecules. Mostly opticalmethods have been utilized to measure the Soret coefficient, such as beam deflection, thermallensing and forced Rayleigh scattering. In this work we present a full optical method in which thefluid is observed parallel to the temperature gradient and attention is focused on the concentrationof non-equilibrium fluctuations (NEFs) that take place within the fluids as a consequence of thethermal and concentration gradients. Through the analysis of the static and dynamic spectra ofNEFs and their interaction with gravitational forces we are able to obtain detailed informationabout the generated concentration gradient and infer the Soret coefficient. A set of validatingmeasurements are performed to check the critical exponent of the Soret coefficient in the vicinityof the critical point for an aniline/cyclo-hexane mixture.11