Determination of diffusivities in binary fluid mixtures with various Lewis numbers as well as positive and negative Soret coefficients by the shadowgraph method

• Published in: The Journal of chemical physics 2023-08, Vol.159 (7)
• Main Authors: Wu, W., Schmidt, P. S., Piszko, M., Giraudet, C., Rausch, M. H., Fröba, A. P.
• Format: Article
• Language: English
• Subjects: Advection; Binary fluids; Binary mixtures; Carbon dioxide; Diffusion coefficient; Diphenyl methane; Hexanes; Lewis numbers; Phosphates; Photon correlation spectroscopy; Soret coefficient; Thermal diffusivity
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/5.0159773

The present work demonstrates the accessibility of the Fick diffusion coefficient D11 and/or the thermal diffusivity a of the binary mixtures dicyclohexylmethane/diphenylmethane, n-hexane/carbon dioxide, 1-hexanol/carbon dioxide, and methane/propane by the analysis of the dynamics of non-equilibrium fluctuations using the shadowgraph method. It is evidenced that D11 and a can be simultaneously determined for binary mixtures with Lewis numbers Le = a/D11 ranging over two orders of magnitude down to Le ≈ 5 or in the presence of minor advection for binary mixtures possessing a negative Soret coefficient in the investigated temperature and pressure ranges from (298.15 to 473.15) K and from about (0.5 to 25) MPa. The determined diffusivities are compared with those measured by heterodyne dynamic light scattering or obtained from the literature, with a focus on achievable uncertainties. By this comparison, it is shown that the determination of a by the shadowgraph method was hindered by mode-coupling effects for Le ≈ 5, whereas a determination of D11 was always possible for mixtures with Le ≥ 5. Furthermore, it is demonstrated that even in the presence of solutal advection, the description of the purely diffusive behavior of non-equilibrium fluctuations in concentration remains valid.