Effect of a Nanoporous Surface on Wettability of an Evaporating Meniscus
Manas Ojha, Joel L. Plawsky, and Peter C. Wayner Jr. The Isermann Department of Chemical and Biological Engineering, Rensselaer Polytechnic Institute, Ricketts Building, 110, 8th Street, Troy, NY 12180
Controlling the interfacial force gradients that dominate contact line dynamics is critical to the success of technologies such as heat pipes, boiling, isothermal and non-isothermal spreading and wetting, fuel cells, evaporation induced self-assembly, fluid distribution systems in labs-on-a-chip, and ink-jet printed rapid prototyping, etc. The future development and optimization of such technologies requires an understanding of the processes that occur at the contact line. The transport processes in the three phase contact region depend on the solid-liquid system, the temperature, and the interface shape, which is a measure of the intermolecular force (pressure) field. Alteration of the surface properties such as surface morphology can lead to significant changes in the intermolecular pressure field. However, the majority of the research presented so far involves transport processes occurring at smooth solid surfaces. There is a lack of theory and experimental data at the microscale concerning effect of irregular surfaces on contact line dynamics. In this work, the solid surface structure was modified to alter the intermolecular force field and thereby enhance the transport rate in the contact line region of a sidewall meniscus. Interfacial phenomena occurring during the microscale phase change processes on a nanoporous coating is presented. A reflectivity/interferometry based interfacial analysis of a liquid-nanoporous silica (NPS) system will be presented. The reflectivity/interferometry technique was developed specifically for the interference patterns produced on a 3-layered structure. Octane is a wetting fluid which completely imbibes the NPS, formed as a thin film coating (< 1Ám). NPS has an interconnected pore structure with an average pore size of 4-10 nm. Comparison of the thickness profile, slope angle profile and liquid pressure profile of an octane meniscus on NPS and on smooth glass under isothermal and non isothermal conditions was performed. The presence of surface roughness and porosity produces a jump in the curvature profile of the liquid film. The surface morphology parameters were obtained by a height-height correlation function derived from AFM based surface scans. In the presence of a heat flux, recession of the sidewall meniscus occurs, which is accompanied by a reduction in the adsorbed film thickness present in front of the meniscus. Therefore, the disjoining forces that arise from the adsorbed thin film are affected and the fluid flow in the pores of NPS plays an important role in balancing the liquid pressure profiles. The porous structure of the coating could possibly enhance its heat transfer capability in comparison to the smooth surface and generate a possible application for use as a "surface wicking" material, for e.g., in a heat pipe.