Two types of petal-effect hafnia films and hydrophilic-hydrophobic stripe-patterned films consisting of 1-mm-wide lines and spaces were fabricated and their water harvesting capability was investigated. Petal-effect hafnia films containing glycine and glycolic acid were prepared by a sol gel technique. Hydrophilic-hydrophobic stripe-patterned films were fabricated by screen printing a hydrophobic hafnia sol as a paste onto a superhydrophilic alumina film substrate. The water harvesting capability of the films was evaluated using a custom-built water collecting apparatus. The mass of water both on the sample surface and in a petri dish set under the sample was measured to evaluate the water collecting capability. When a vapor covered the petal-effect film, small dew droplets formed on the surface and grew to large hemispheric ones, but most of the water remained on the sample surface without falling into the petri dish. When a vapor covered the patterned film surface, small droplets and large, long ones formed on the hydrophobic and hydrophilic patterns, respectively. The large droplets that grew on the hydrophilic patterns fell into the petri dish. The total amount of water that collected on the petal-effect films and the patterned films was ca. 1.6 times larger than the respective amount collected with a superhydrophilic alumina film, a hydrophobic hafnia film and a glass substrate as a control reference. The patterned films collected a larger amount of water in the petri dish than the petal-effect films did because the hydrophilic molecules of organic acids on the surface of the latter films retained a larger amount of droplets. It is concluded that both the petal-effect films and the patterned films have superior water harvesting capability from a vapor in the ambient atmosphere.
Superhydrophilic alumina films were successfully fabricated by a hydrothermal reaction of nanofibrous alumina films in a nitrilotris(methylenephosphonic acid) (NMP) aqueous solution at 180°C for 6 h. The films showed superhydrophilicity not only as prepared but also after a boiling water resistance test, indicating that they had good durability. The alumina films were phosphorylated wholly in a hydrothermal condition only at 180°C for 6 h. Phosphate moieties were present both at the surface and inside the films and a micro-structure with a serrated morphology was observed over the entire film surface. Whole phosphorylation of the alumina films resulted in good durability in the boiling water resistance test. Other films were phosphorylated partially at 160°C for 6 h and at 180°C for 4 h. The micro-structure with a serrated morphology was observed partially at the film surface. This morphology corresponded to the degree of phosphorylation. These results indicate that the phosphate moieties present on and in the films and the micro-structure of the films produced superhydrophilicity.
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