(355c) Spreading Characteristics and Microscale Evaporative Heat Transfer in a Moving Meniscus Containing a Binary Mixture

The interfacial phenomena in the contact line region of a curved liquid-vapor interface in contact with a solid surface are important to many equilibrium and non-equilibrium processes. The liquid phase in these processes can be pure liquids or mixtures. The transport processes and microscale evaporative heat transfer between a mixture and a solid surface are different compared to a pure liquid in contact with a solid surface. This difference, which can be constructive or destructive, is insufficiently explained in the literature due to the limitations in the resolution of the optical technique used. The present work is directed at alleviating this deficiency by taking the data for a mixture at the microscopic level with high spatial resolution. An improved data analysis procedure was used to compare the measured film thickness profiles during spreading and evaporation of a pure liquid (pentane) and a binary mixture (pentane and octane) on a quartz surface for thicknesses, δ < 2 μm.

Image analyzing interferometry was used to study the spreading characteristics of a liquid-vapor interface containing a binary mixture (initial concentration of 98 % pentane and 2 % octane by vol.) on a quartz surface. The thickness and curvature profiles in the contact line region of the meniscus were obtained using an improved data analysis procedure. The results obtained for the mixture were compared with those obtained for pure pentane under similar operating conditions. Isothermal experimental conditions of the meniscus were used for the in-situ estimation of the retarded dispersion constant. The experimental results for the pure fluid demonstrate that the disjoining pressure or the intermolecular interactions in the thin film region control the fluid flow within an evaporating completely wetting meniscus. Also, an imbalance between the disjoining pressure in the thin film region and the capillary pressure in the thicker meniscus region caused the evaporating pentane meniscus to spread over the solid (quartz) surface. Details of the procedure concerning the use with a pure system are presented in Ref. [1].

For the mixture, the Marangoni flow (due to concentration driven stresses at the liquid-vapor interface) dominates over the disjoining pressure (in the thin film region) in determining the spreading characteristics of the binary mixture meniscus. Here, the meniscus spreads over the quartz surface due to Marangoni flow towards the thin film region. Also, a comparison of the interfacial heat flux for pure pentane and the mixture was made, and it was found that composition in small interfacial systems, e.g. higher boiling impurities in the working fluid, can have a significant effect on the microscale heat transfer process.

Reference

[1] S. S. Panchamgam, S. J. Gokhale, S. DasGupta, J. L. Plawsky, and P. C. Wayner Jr., ?Experimental Determination of the Effect of Disjoining Pressure on Shear in the Contact Line Region of a Moving Evaporating Thin Film,? J. Heat Transfer, 127 (3), 231 (2005).