Experimental Evaluation of Marangoni Shear in the Contact Line Region of an Evaporating 99+ % Pure Octane Meniscus

Image analyzing interferometry was used to study the spreading characteristics of an evaporating meniscus containing octane (purity: 99+ %) on a quartz surface. The thickness and curvature profiles in the contact line region of the meniscus were obtained using a microscopic data analysis procedure. The results obtained for the octane were compared with that of 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 pentane demonstrate that the disjoining pressure (the intermolecular interactions) in the thin film region controls the fluid flow. Also, an imbalance between the disjoining pressure in the thin film region and the capillary pressure in the thicker meniscus region resulted in a creeping evaporating pentane meniscus, which spread over the solid (quartz) surface. On the contrary, for less pure octane, the intermolecular interactions between octane and quartz had a significantly different contribution for fluid flow and hence the octane meniscus of lower purity did not creep over the quartz surface. As a result, we had a stationary, evaporating octane meniscus. Using the experimental data and a simple model for velocity distribution, we evaluated the Marangoni shear in a portion of the stationary, evaporating octane meniscus. An extremely small change in the concentration due to distillation had a significant effect on fluid flow and microscale heat transfer. Also, it was found that non-idealities in small interfacial systems, i.e. the presence of impurities in the working fluid, can have a significant effect on the thickness of the adsorbed film and therefore the spreading characteristics of an almost pure octane meniscus.