(154f) Reflectivity Based Analysis of Contact Line Behavior and Microscale Heat Transfer in Binary Ultrathin Films

An image analyzing technique based on reflectivity measurements was developed for the estimation of the thickness, contact angle and curvature profiles of ultrathin films, drops, and curved menisci with high spatial resolution (horizontal resolution of 0.177 μm). Using the “reflectivity technique”, we studied fluid flow and heat transfer in a wickless, miniature heat pipe, a device which will be a very effective passive heat exchanger in a microgravity environment. The broad objective of this work was to increase the efficiency of the miniature heat pipe by enhancing the liquid flow towards the hotter region. This was achieved by understanding and manipulating the wetting and spreading characteristics of the liquid on the solid surface. By using a binary mixture (98 % pentane and 2 % octane by vol.) instead of either pure pentane or octane, we were able to achieve a significant increase in the microscale phase change heat transfer.

The experimental work was supported by numerical studies to understand the physics of the system at microscopic scale. The continuum model includes slip at the solid-liquid interface and curvature effects in the transition region. The experimental profiles obtained for the binary mixture of pentane and octane and pure pentane were used as input to the theoretical model for the estimation of interfacial parameters i.e., liquid-vapor interface temperature, concentration, Marangoni stress, mass flow rate and evaporative heat flux profiles at microscale. The results show that the slip at the solid-liquid interface helps in spreading of pure pentane and binary mixture menisci. However, in the case of mixtures, Marangoni stresses (due to concentration and temperature gradients) at the liquid-vapor interface assists the fluid flow, whereas for the pure pentane, the Marangoni stress (due to temperature gradient alone) hinders the fluid flow in the transition region. Also a comparison of heat flux profiles of the binary mixture for varying concentrations of pentane in the adsorbed film region was made.

In addition, using the reflectivity technique with various fluids, we enhanced our understanding of interfacial phenomena in the three-phase contact line region. Experiments included flow instabilities in an HFE-7000 meniscus on quartz, the spreading of a pentane meniscus, the evaporation of 99+% pure octane meniscus, and fluid flow of a “nanofluid” (octane containing Pt particles of size ~ 50nm) on quartz due to phase change. The significance of the disjoining pressure (or the intermolecular interactions) at microscopic scale in the above systems will be emphasized.