Dynamic wettability evaluation of nanoparticles-coated surfaces

The present study concerns an investigation on the variation of wettability of flat aluminum plates covered with porous thin-films of nanoparticles. Since the contact angle of the obtained surfaces is small, and in many cases the deposited droplet has not achieved a static state, dynamic top-down analyses of spreading droplets were performed. Surface roughness and morphology of the deposited layers were also investigated, in order to provide additional information about the nanotextured surfaces that could be related to their wettability behavior. Aluminum oxide (20-30 nm and 40-80 nm) and silicon oxide (15 nm and 80 nm) nanoparticles were deposited on aluminum plates through a nucleate boiling process. Depositions were obtained through pool boiling of water/Al2O3 and water/SiO2 nanofluids containing 0.01%, 0.1% and 0.5% in volume of nanoparticles. According to the wettability evaluation, a change in spreading mechanism could be identified, varying from inertially-driven during the first few milliseconds to capillary-driven effects, which in some cases sustained the spreading process even after 1 s. Although deposition of nanoparticles has generally increased surface roughness, no relation between roughness and wettability was found in the present investigation. On the other hand, super-wetting behavior was related to the presence of more particles' clusters on top of the surfaces, possibly enhancing the connections through porous layers.