INTEC   05402
INSTITUTO DE DESARROLLO TECNOLOGICO PARA LA INDUSTRIA QUIMICA
Unidad Ejecutora - UE
artículos
Título:
Numerical analysis of the Rayleigh instability in capillary tubes. The influence of surfactant solubility
Autor/es:
DIEGO MARTIN CAMPANA; FERNANDO ADOLFO SAITA
Revista:
PHYSICS OF FLUIDS
Editorial:
American Institute of Physics
Referencias:
Año: 2006 vol. 18 p. 1 - 16
ISSN:
1070-6631
Resumen:
A 2-D free surface flow model already used to study the Rayleigh instability of thin films lining the interior of capillary tubes under the presence of insoluble surfactants, [D. M. Campana, J. Di Paolo, and F. A. Saita, “A 2-D model of Rayleigh instability in capillary tubes. Surfactant effects,” Int. J. Multiphase Flow 30, 431 (2004)] is here extended to deal with soluble solutes. This new version that accounts for the mass transfer of surfactant in the bulk phase, as well as for its interfacial adsorption/desorption, is employed in this work to assess the influence of surfactant solubility on the unstable evolution. We confirm previously reported findings that surfactants do not affect the system stability but the growth rate of the instability, [D. R. Otis, M. Johnson, T. J. Pedley, and R. D. Kamm, “The role of pulmonary surfactant in airway closure,” J. Appl. Physiol. 59, 1323 (1993)] and that they do not change the successive shapes adopted by the liquid film throughout the process. [ S. Kwak and C. Pozrikidis, “Effects of surfactants on the instability of a liquid thread or annular layer. Part I: Quiescent fluids,” Int. J. Multiphase Flow 27, 1 (2001)] This last characteristic is advantageously employed to compare the evolution of systems having surfactants with different solubility values; in particular, the time needed to complete the formation of liquid lenses that disconnect the gas phase —the closure time— is used as yardstick. Insoluble surfactants delay the instability process and their closure times are four/five times larger than those of pure liquids; however, this retardation effect is considerably reduced when the surfactant considered is just slightly soluble. For a typical system adopted as a reference case, detailed computed predictions are shown; among them, curves of closure time versus adsorption number for solubility values ranging from non soluble to totally soluble conditions, are given. In addition, the evolution of the four mass transport terms appearing in the interfacial mass balance equation —normal and tangential convection, diffusion and adsorption/ desorption— is scrutinized to uncover the mechanisms by which surfactant solubility affects the growth rate of the instability.