wettability, boiling, nanoparticle, aluminum surface
Surface wettability is known to be important in boiling, condensation, frosting/defrosting, liquid desiccant flows in falling-film devices, and a myriad of other HVAC&R processes. Research has shown that surfaces treated with nanofluid boiling nanoparticle deposition exhibit radical changes in wettability, because a layer of nanoparticle coating is formed on the metal surface during microlayer evaporation at the base of the vapor bubble inboiling. Wettability is changed because surface chemical composition, surface roughness and porosity can be modified by the nanoparticle layer. This study is focused on how to manipulate wettability by nanoparticle deposition on aluminum surfaces, since this metal is commonly used as the material for heat transfer in air conditioning and refrigeration systems. The boiling deposition process occurs under atmospheric pressure, in a reservoir large compared to the sample size. The effect of nanoparticle concentration, boiling heat flux, boiling duration and surface initial roughness are studied by varying parameters one at a time while controlling the others. Al2O3 nanoparticles of an average size of 40nm are deposited on a 20mm x 20mm aluminum surface. After the surface is created, contact-angle measurements with water are conducted using a goniometer to measure the advancing, receding, and static contact angles so as to characterize the wetting behavior. The surface morphology is characterized through scanning electron microscopy (SEM) and profilometry. It is observed that the layer of Al2O3 nanoparticle deposited on aluminum surfaces enhances the wettability on the surface. This result is explained using Wenzel’s model, in which an increasing areal roughness factor leads to a decrease in the apparent contact angle. The boiling time ranges from five to thirty minutes. It is observed that the longer the boiling time the lower the contact water contact angle. The impact of surface contamination is also investigated, and it is shown that nanoparticle deposited surfaces manifest enhanced wettability superior to that of a bare aluminum surfaces even after exposure to laboratory air.