Water harvesting capability of petal-effect hafnia films and hydrophilic-hydrophobic patterned films

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.