| In the low-temperature and high-humidity environment, condensation, icing and frosting on metal surfaces are common phenomena, and it is regarded as a tremendous safety hazard in some fields, such as aerospace, air conditioning heat pump, low temperature storage, ect. So protecting metals to avoid their condensing waterdrop, icing and frosting has become an urgent problem to be solved. It has been reported that a superhydrophobic surface with low solid-liquid interface adhesion is self-cleaning, corrosion-resistance, and prone to remove of ice and frost. Therfore, study on superhydrophobic surfaces on metal substrates is greatly needed, which is crucial academic value and important practical significance. As the keys to realizing anti-icing and anti-frosting are to inhibit or avoid the nucleation of ice crystal and frost crystal, the intrinsic nature is to inhibit the nucleation of dew drop and promote fast shedding off as-grown water drops. In addition, it has been reported that the superhydrophobic surface with composite micro-/nanostructure is prone to form water droplets with Cassie state, but the specific structure, which can be more beneficial to shed off water drops, is unknown so far. In view of the above, research on controllable fabrication, wettability and condensation properties of micro-/nano structured arrays was carried out on metal substrates, especially metal copper and aluminum, in this dissertation. The main contents and results are as follows:By immersing the substrates (copper foils and aluminum sheets) in diluted ammonia solution, grass-like micro structure, bird nest-like micro structure, nanocauliflower-like structure, and labyrinth-like nanocavity structure were obtained. After modification with heptadecafluorodecyltripropoxysilane (FAS-17), all the four structures are superhydrophobic, but their solid-liquid interface adhesion distinctly differed. Among them, especially, on the surfaces with the bird nest-like microstructure and the nanocauliflower-like structure, the static water contact angle was more than 160°, and the water roll-off angle was less than 5°. Additionally, in the synthesis process, a new finding was found that by immersing copper foil in (water and ethanol) component solvent solution of ammonia, the structure and morphology of the synthetic product is complex and novel, which entirely differs from single crystal copper hydroxide (Cu(OH)2) nanobelt fabricated by immersing copper foil in aqueous solution of ammonia. And the role of ammonia in the two kinds of solution reaction was concluded.Based on solution-phase approaches to synthesize one-dimensional (1D) ZnO nanostructure arrays, misaligned nanocone structure, well-aligned nanorod structure and honeycomb-like micro structure built from upright nanosheets were obtained. After modification with FAS-17, the three structures were non-viscous superhydrophobic for macroscopic water drops. Especially, the well-aligned nanorod structure had robust hydrophobility, where the static water contact angle was about 165.3° and the water roll-off angle was about 1.6°. Combining the three structures here with the above four structures obtained by immersing the substrates in diluted ammonia solution, the optimal structure was the well-aligned nanorod structure for macroscopic superhydrophobic property. Furthermore, a new breakthough is realized about solution-phase growth of 1D ZnO nanostructure. Large-scale misaligned hexagonal ZnO nancone arrays and well-aligned ZnO nanorod arrays were fabricated by a [Zn(OH)4]2- solution consisting of Zn(NO3)2· 6H2O and KOH on copper foil at near room temperature (～35 ℃). To the best of our knowledge, this is the first solution method for fabricating 1D ZnO nanostructure arrays on inert substrates in such mild conditions. Additionally, the detailed growth behavior of the crystals in the solution was discussed, the possible growth mechanisms were proposed.Using micro-machining technology, substrates were processed to machine different rough structures in micron scale and submillimeter scale, such as submillimeter ratchet arrays, submillimeter pyramid arrays, micro-groove arrays, micro-mesh arrays, dense micro-pillar arrays and sparse micro-pillar arrays. And then well-aligned nanorod arrays were fabricated on the six micron and submillimeter structures, which could obtain different hierarchical composite structures. After modification with FAS-17, all the six hierarchical composite structures are superhydrophobic. Except submillimeter ratchet/nanorod structure and submillimeter pyramid/nanorod structure, macroscopic dynamic hydrophobic effect of the rest of four kinds of micro-/nanostructured hierarchical composite structures was better than that of well-aligned nanorod structure. These findings indicated that for macroscopic superhydrophobic properties, rough structure in nanoscale played a leading role in surface roughness and mainly influenced water contact angle, and to a certain extent, rough structure in micron scale could increase the surface ability to capture the air which might promote the water droplets on the surface to keep Cassie state contact and influence roll-off angle.According to condensation experiments from the twelve selected superhydrophobic surfaces with different structures and common hydrophilic copper foil and aluminum sheet, obtained results and conclusions were as follows:(1) For the superhydrophobic surfaces with the single nanostructure, the superhydrophobic copper foil with well-aligned nanorod structure had the best condensation property, whose coverage rate and mass surface density of condensed drops were 10.0% and 16.6% of the hydrophilic copper foil after condensation for 30 min, respectively. For the superhydrophobic surfaces with the different hierarchical composite structures, the superhydrophobic copper foil with composite structures of (600#-1200#) micro-mesh/nanorod had the best condensation property, whose coverage rate and mass surface density of condensed drops were 7.8% and 10.4% of the hydrophilic copper foil after condensation for 30 min, respectively. Moreover, superhydrophobic aluminum sheet with honeycomb-like micro structure had lost its superhydrophobility in the the condition of water vapor condensation.(2) For the condensation properties, except the superhydrophobic surface with honeycomb-like micro structure, the other eleven kinds of superhydrophobic surfaces are much better than the common hydrophilic copper foil and aluminum sheet. Research results demonstrated that the dropwise condensation effect was mainly influenced by nanostructure morphology, and only the micro structure, whose size was not more than the mean grain diameter of shedding-off condensed microdrops, could enhance condensation effect.(3) The presence or absence of the phenomenon of self-propelling motion of condensed microdrops might become a criterion/an evidence to identify whether the superhydrophobic surface could still maintaine its stable superhydrophobility during the condensation and whether condensed microdrop could be kept stable Cassie state. Self-propelling motion of condensed microdrops should be driven by fractional redundant surface energy which was released through the coalescence among microdrops and efficiently transfered to kinetic energy. And it was governed micro structure (especially the nanostructure) morphology of superhydrophobic surface that whether the condensed microdrops could realize/conduct self-propelling motion. |
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