Optical Measurement of the Soret Coefficient of Ethanol-Water Mixtures

A key parameter in convection experiments in binary fluid mixtures is the separation ratio Psi which is proportional to the Soret coefficient S Tau. We have used a laser-beam-deflection technique to measure S Tau in ethanol-water mixtures in the temperature range 10-40 degrees C, for ethanol mass fractions c ranging from 2% to 40%. For small c, the precision of the technique is plus or minus 1%. The mass diffusion coefficient D is also determined. These measurements reveal that the oscillation period measured in convection experiments is consistent with the prediction of linear stability theory for -0.56 Psi -0.05. The Rayleigh number at onset, however, is not consistent with the theory at small Psi.