Thermal Diffusivity in Supercritical Fluids Measured by Thermal Lensing

Thermal diffusivities of supercritical CO2 and C2H6 were determined over a wide density range with a photothermal technique. The thermal lens, formed by the degradation of the absorbed light energy as heat by the sample, allows the employment of a nonequilibrium method in the critical region. Controlling the refractive-index gradient, i.e., a density gradient, perturbations can be maintained at levels where convection is negligible. An easy-to-operate setup allowed us to measure thermal diffusivities in the density ranges 5 to 20mol.dm- 3 for CO2 at 308 and 313 K and 2 to 12mol.dm-3 for C2H6 at 308 K with a standard precision of 15%.