Wetting & Wicking Effects of Superhydrophilic Nano Structured Coatings

Abstract

Superhydrophilic NanoStructured Coatings (SHNC) were discovered during pool boiling experiments using nanofluids with alumina nanoparticles. During nucleate boiling, the nanoparticles are deposited on the heater surface, forming a uniform oxide coating. These coatings have been demonstrated to greatly decrease the liquid contact angle observed on the surfaces, both by increased surface roughness and increased surface energy. An illustration of this roughness, within 1 خ¼m thickness, can be seen in the 3D optical microscope mapping of a SHNC surface, top right. These highly wetting structures can greatly enhance macrolevel mass transfer effects, such as capillary action. The series of images on the left depict the wickability enhancement achieved by SHNC coating inside a 0.92 mm internal diameter aluminum tube. In the tube coated with SHNC, a 21 خ¼l water droplet disappeared in 183 milliseconds, resulting in an average wicking speed along the pipe of 17 cm/sec. The bare aluminum tube does not wick at all, even as it is pushed into the droplet. The bottom right sequence shows the wettability enhancement responsible for this behavior; an 8 خ¼l water droplet is dropped onto both a SHNCcoated and a bare aluminum surface from a height of 1 cm. The droplet on the SHNCcoated surface spreads instantaneously due to the high wettability of the SHNC, while the droplet on the bare aluminum remains aggregated as a hemisphere.