The fabrication of superhydrophobic surfaces and the studies on water flow characteristics therein are of great significance to many industrial areas as well as to science and technology development. Experiments were carried out to investigate slip characteristics of water flowing in circular superhydrophobic microtubes within laminar flow region. The superhydrophobic microtubes of stainless steel were fabricated with chemical etching–fluorination treatment. An experimental setup was designed to measure the pressure drop as function of water flow rate. For comparison, superhydrophilic tubes were also tested. Poiseuille number Po was found to be smaller for the superhydrophobic microtubes than that for superhydrophilic ones. The pressure drop reduction ranges from 8% to 31%. It decreases with increasing Reynolds number when Re b 900, owing to the transition from Cassie state to Wenzel state. However, it is almost unchanged with further increasing Re after Re N 900. The slip length in superhydrophobic microtubes also exhibits a Reynolds number dependence similarly to the pressure drop reduction. The relation between slip length and Darcy friction factor is theoretically analyzed with consideration of surface roughness effect, which was testified with the experimental results.
Liquid convective heat transfer in microchannels exhibits high efficiency.However,the large pressure drop causes much concern for practical application.Water flowing in hydrophobic tubes shows low pressure drop owing to the slippage on the tube walls.Super-hydrophobic/hydrophilic micro aluminum tubes of 0.68mm inner diameter were fabricated with a two step procedure of chemical etching and then surface chemical modification.A kind of micro-nanometric hierarchical structure was formed on the surface,which could trap air serving as the slip agent.Heat transfer and fluid flow of deioned water flowing laminary in the super-hydrophobic/hydrophilic microchannels were studied experimentally.The results showed that the air-layer existing in the micro-nano hierarchical structures of the super-hydrophobic surface decreased flow resistance evidently but decreased heat transfer coefficient only a little,which was still higher than the superficial heat transfer coefficient while considering the heat conduction of stationary nanolayer of air.It was supposed that eddy flow was generated in the micro-nano bubbles by the slip flow of water on the tube wall,which enhanced the heat transfer.
