Hierarchical roughness is known to effectively reduce the liquid-solid contact area and water droplet adhesion on superhydrophobic surfaces, which can be seen for example in the combination of submicrometer and micrometer scale structures on the lotus leaf. The submicrometer scale fine structures, which are often referred to as nanostructures in the literature, have an important role in the phenomenon of superhydrophobicity and low water droplet adhesion. Although the fine structures are generally termed as nanostructures, their actual dimensions are often at the submicrometer scale of hundreds of nanometers. Here we demonstrate that small nanometric structures can have very different effect on surface wetting compared to the large submicrometer scale structures. Hierarchically rough superhydrophobic TiO(2) nanoparticle surfaces generated by the liquid flame spray (LFS) on board and paper substrates revealed that the nanoscale surface structures have the opposite effect on the droplet adhesion compared to the larger submicrometer and micrometer scale structures. Variation in the hierarchical structure of the nanoparticle surfaces contributed to varying droplet adhesion between the high- and low-adhesive superhydrophobic states. Nanoscale structures did not contribute to superhydrophobicity, and there was no evidence of the formation of the liquid-solid-air composite interface around the nanostructures. Therefore, larger submicrometer and micrometer scale structures were needed to decrease the liquid-solid contact area and to cause the superhydrophobicity. Our study suggests that a drastic wetting transition occurs on superhydrophobic surfaces at the nanometre scale; i.e., the transition between the Cassie-Baxter and Wenzel wetting states will occur as the liquid-solid-air composite interface collapses around nanoscale structures. Consequently, water adheres tightly to the surface by penetrating into the nanostructure. The droplet adhesion mechanism presented in this paper gives valuable insight into a phenomenon of simultaneous superhydrophobicity and high water droplet adhesion and contributes to a more detailed comprehension of superhydrophobicity overall.
In this study, a method for fabrication of high aspect ratio silicon nanopillars is presented.
The method combines liquid flame spray production of silica nanoparticle agglomerates
with cryogenic deep reactive ion etching. First, the nanoparticle agglomerates, having a
diameter of about 100 nm, are deposited on a silicon wafer. Then, during the subsequent
cryogenic deep reactive ion etching process, the particle agglomerates act as etch masks and
silicon nanopillars are formed. Aspect ratios of up to 20:1 are demonstrated. The
masking process is rapid, cheap and has the potential to be scaled up for large
areas. Three other structured silicon surfaces were fabricated for comparison. All
four surfaces were utilized as desorption/ionization on silicon (DIOS) sample
plates. The mass spectrometry results indicate that nanopillar surfaces masked
with the liquid flame spray technique are well suited as DIOS sample plates.
Titania and titania-silver nanoparticle deposits were made by Liquid Flame Spray technique, in which the liquid precursor is injected into a high temperature flame, where it will evaporate and nucleate to nanosize particles. One-step and two-step methods were used for preparation of titania-silver deposits. The amount of silver added was 1 wt%. The deposits were collected in the flame zone on steel and glass surfaces and were analyzed by TEM, EDS, XPS and SAXS. The titania deposits consisted of porous nanosized titania agglomerates of primary particles ($10 nm). With silver addition, small spherical silver metal particles ($2 nm) were detected on the agglomerates. An increase in the photocatalytic activity was verified by stearic acid decomposition and biofilm removal using Deinococcus geothermalis as the model organism.
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