Fabrications Of Micro-and Nano-Structure Superhydrophobic Metal Substrate Surfaces And Maintaining Condensation Drops In Cassie State

Superhydrophobic stainless steel surfaces were prepared by combining two-step etching procedure or direct Beck reagent etching method, and fluoroalkylsiane decoration. The contact angle reached 159°or 156°that was examined by contact angle measure device, and their contact angle hysteresis was 5°and 9°, respectively. The two-tier micro-and nano-structures were observed by scanning electron microscoy(SEM). The direct Beck etching method showed different superhydrophobicities on stainless surface along with etching time. Two-step chemical etching was carried out with HCI and HF and orthogonal design further optimized the positive factors that promote surface superhydrophobicity, including HF concentration, HF etching period, HCI concentration and HCI etching period. Two-step etching method made nano rectangular solid on aluminum surface, and then the nano-structures stacked into micro-structure. After fluoroalkylsiane decoration, surface contact angle reached 160°.During the further condensation on the prepared superhydrophobic stainless steel and aluminum surfaces, different micro-and nano-structures showed various capacities to maintain condensation drop Cassie state, which could be divided into three categories.1) The surface could maintain drop Cassie state for a long time. The drops were very small, and left the surface by 2-dimension rolling movement and 3-dimension bouncing motion. The reuse efficiency was higher than other surfaces.2) The condensation drops were a bit big and left the surface by 2-dimension rolling movement, the reuse efficiency was high.3) The surface could maintain dropwise condensation less than 10min. The drop shapes were irregular, and merged with neighboring ones without obvious motion. Finally the drops were driven by gravity when they were getting big enough, and dropwise condensation converted to channel flowing.Through the analysis of the micro-and nano-structure on the prepared surface by SEM, it is found that nanoparticles were the key factor to maintain drops in Cassie state, and different scales and distributions of the nanoparticles could result in various drop movements.