Spreading characteristics and microscale evaporative heat transfer in an ultrathin film containing a binary mixture

Using image-analyzing interferometry. the thickness profile, in the range of delta(0) (adsorbed thickness) < delta < 3 mu m, at the leading edge of a moving ultrathin film with phase change, was measured for a mixture of pentane-octane and compared to that of pure pentane. An improved data-analysis procedure was used to enhance the use of the measured thickness profile. There were significant differences between these two systems, demonstrating the presence of large Marangoni interfacial shear stresses with the mixture. A control volume model was developed to evaluate the differences between the pure fluid and the mixture. The disjoining pressure at the leading edge was found to control fluid flow in the evaporating pure system. However due to Marangoni stresses, the effect of disjoining pressure on the mixture was found to be small at steady state for the fluxes studied. With an upstream bulk mixture of 2% octane and 98% pentane, a shear stress due to the gradient of the liquid-vapor interfacial surface tension resulting from distillation controlled fluid flow in the contact line region. The average curvature of the evaporating pseudo-steady state pure system was significantly larger (smaller length and larger apparent contact angle at delta=0.1 mu m) than the isothermal value, whereas the reverse occurred for the mixture. Using a continuum model, a comparison of numerically obtained Marangoni stresses and local evaporative heat flux profiles between the two, systems was also made.