Study On Fabrication Of Superhydrophobic Surfaces On Metallic Substrates

In the past few years, superhydrophobic surfaces with water contact angles larger than 150° have received a lot of research attention, due to their important applications ranging from self-cleaning materials to microfluidic devices and biomaterials. The archetype superhydrophobic surface is that of the lotus leaf, on which a water droplet apparently forms a sphere, unstably sitting on the leaf, and dirt is easily removed with a rain shower. This behavior, known as the Lotus or self-cleaning effect, is found to be a result of the hierarchical micro- and nano-structures, as well as the wax layer present on the leaf surface. Up to now, artificial superhydrophobic surfaces have been fabricated by tailoring surface topography and using techniques such as lithography, laser and plasma etching, physical and chemical vapor deposition, anodic oxidation, electrodeposition, sol-gel, electrospinning, layer-by-layer deposition, and others. However, either special equipment or complex process control is required in most cases. As a consequence, it still remains a great challenge to develop simple and convenient techniques for the fabrication of superhydrophobic surfaces.In this dissertation, we report on three simple and convenient techniques for fabricating superhydrophobic surfaces. Using these techniques, we have succeeded in the preparation of superhydrophobic surfaces on metallic aluminum, copper, zinc, and titanium substrates.(1) A dislocation etchant, which preferentially dissolves the dislocation sites in the grains, was used to produce surface roughness on three polycrystalline metals, namely aluminum, copper, and zinc. Fluoroalkylsilane was then used to hydrophobize those rough surfaces. The resultant surfaces were characterized by means of scanning electron microscopy (SEM) and water contact angle measurements. The results show that a labyrinth structure is produced on the etched aluminum surface, that crater-like pits and dispersed nanoparticles constitute a hierarchical roughness on the etched copper surface, and that microscale hillocks and fine nanogrooves form a hierarchical roughness on the etched zinc surface. These etched surfaces, when hydrophobized, exhibit superhydrophobic properties with water contact angles of larger than 150°, as well as roll-off angles of less than 10° for 8-μL drops. Also, the concept introduced here of dislocation etch pits may be helpful in the fabrication of superhydrophobic surfaces on other polycrystalline substrates.(2) Controlled surface oxidation was performed on copper substrates, using KOH aqueous solutions with an oxidant K2S2O8. The oxidized copper surfaces were then hydrophobized with fluoroalkylsilane. The resultant surfaces were characterized by means of SEM, transmission electron microscopy (TEM), X-ray diffraction (XRD), and water contact angle measurements. The results show that a film of CuO nanoflowers is produced on the oxidized copper surface. The nanoflower is formed by self-assembly of tens of nanosheets. The nanosheet is about 2 um in length, 120 nm in width, and 12 nm in thickness. After fluorination treating, the nanoflower film exhibits superhydrophobic properties with a water contact angle of about 158°, as well as a roll-off angle of about 10° for 5-^iL drops. In addition, the growth mechanism of the nanoflowers proposed here may provide some insights into the control of 3-dimensional nanostructures.(3) Chemical etching with HF and controlled surface oxidation with H2O2 were used to produce rough surfaces on titanium substrates, and fluoroalkylsilane was then employed to hydrophobize those rough surfaces. The resultant surfaces were characterized by means of SEM, atomic force microscopy (AFM), XRD, and water contact angle measurements. The results show that cater-like pits and submicron porous TiC>2 films constitute a hierarchical roughness on the titanium surface after HF etching and subsequent H2O2 treating. The hierarchical rough titanium surface, when hydrophobized, exhibits superhydrophobic properties with water contact angles of about 163°, as well as roll-off angles of about 8° for 5-uL drops. This result may be of special significance considering that the metallic titanium is a widely used type of biomaterials. In addition, the combined use of chemical etching and controlled surface oxidation offers a new strategy for the fabrication of superhydrophobic surfaces with hierarchical roughness on some metallic substrates.